Group,

Many thanks for all the innovative and informative posts on the LED
lighting scheme. Many of you have been quite generous with your time
and knowledge and I am most appreciative.

Certainly there are many different ways to put this together and each
has a particular strength. I would prefer to stay with PWM techniques
for dimming versus voltage changes. With 20 (and perhaps more) LEDs in
the circuit, some are certain to begin to shut off before others when Vf
starts falling too low. I have taken as many of your suggestions as I
could understand (Sorry JosephKK, but for me, yours was grad school.
Beta sounds like a coefficient of some sort. Perhaps you could post a
primer for me.) and put them into a modified design. I have de-rated
the LEDs somewhat and switched to a low noise regulator that uses a trim
pot to adjust Vout. This way, should I need to go to two or even three
LEDs in series, I can accommodate this in the design.

Here I have calculated the total power consumption of the circuit at
4.56W with the LEDs consuming 1.23W (27% efficient). At 14V, it should
require 0.33A to run. The most critical part is the junction temp of
the regulator. It seemed to me to run too hot as is so I put in a
series resistor (12 ohms, 7W) to absorb some (3.075W) of the power. I
now estimate Tj to be 31.5°C hotter than ambient with an allowable Tmax
of 125°C on the chip. Safe even in the desert.

I also fixed a couple errors on the original schematic. As always, I
would appreciate some experienced eyes looking at this and letting me
know if I have seriously screwed something up somewhere. I'm not at all
familiar with linear low-noise regulators.

Thanks,
Charlie

On Fri, 15 Jan 2010 21:02:27 -0500, Charlie Smith
wrote:

Group,

Many thanks for all the innovative and informative posts on the LED
lighting scheme. Many of you have been quite generous with your time
and knowledge and I am most appreciative.

Certainly there are many different ways to put this together and each
has a particular strength. I would prefer to stay with PWM techniques
for dimming versus voltage changes. With 20 (and perhaps more) LEDs in
the circuit, some are certain to begin to shut off before others when Vf
starts falling too low. I have taken as many of your suggestions as I
could understand (Sorry JosephKK, but for me, yours was grad school.
Beta sounds like a coefficient of some sort. Perhaps you could post a
primer for me.) and put them into a modified design. I have de-rated
the LEDs somewhat and switched to a low noise regulator that uses a trim
pot to adjust Vout. This way, should I need to go to two or even three
LEDs in series, I can accommodate this in the design.

Here I have calculated the total power consumption of the circuit at
4.56W with the LEDs consuming 1.23W (27% efficient). At 14V, it should
require 0.33A to run. The most critical part is the junction temp of
the regulator. It seemed to me to run too hot as is so I put in a
series resistor (12 ohms, 7W) to absorb some (3.075W) of the power. I
now estimate Tj to be 31.5°C hotter than ambient with an allowable Tmax
of 125°C on the chip. Safe even in the desert.

I also fixed a couple errors on the original schematic. As always, I
would appreciate some experienced eyes looking at this and letting me
know if I have seriously screwed something up somewhere. I'm not at all
familiar with linear low-noise regulators.

Thanks,
Charlie


I wonder if a series choke in line with the final drive bus to the LED
resistor bank would make the 'brightness control' "smoother" than harsh
square waves would.

Also, would it not be a good idea to control pulse width as well as the
'duty cycle' or 'off time'? Or are you controlling the final drive
current average going to them?

On Fri, 15 Jan 2010 21:02:27 -0500, Charlie Smith
wrote:

Group,

Many thanks for all the innovative and informative posts on the LED
lighting scheme. Many of you have been quite generous with your time
and knowledge and I am most appreciative.

Certainly there are many different ways to put this together and each
has a particular strength. I would prefer to stay with PWM techniques
for dimming versus voltage changes. With 20 (and perhaps more) LEDs in
the circuit, some are certain to begin to shut off before others when Vf
starts falling too low. I have taken as many of your suggestions as I
could understand (Sorry JosephKK, but for me, yours was grad school.
Beta sounds like a coefficient of some sort. Perhaps you could post a
primer for me.) and put them into a modified design. I have de-rated
the LEDs somewhat and switched to a low noise regulator that uses a trim
pot to adjust Vout. This way, should I need to go to two or even three
LEDs in series, I can accommodate this in the design.

Here I have calculated the total power consumption of the circuit at
4.56W with the LEDs consuming 1.23W (27% efficient). At 14V, it should
require 0.33A to run. The most critical part is the junction temp of
the regulator. It seemed to me to run too hot as is so I put in a
series resistor (12 ohms, 7W) to absorb some (3.075W) of the power. I
now estimate Tj to be 31.5°C hotter than ambient with an allowable Tmax
of 125°C on the chip. Safe even in the desert.

I also fixed a couple errors on the original schematic. As always, I
would appreciate some experienced eyes looking at this and letting me
know if I have seriously screwed something up somewhere. I'm not at all
familiar with linear low-noise regulators.

Thanks,
Charlie

Get rid of the 555 PWM thing! It accomplishes nothing.

John

On Fri, 15 Jan 2010 20:01:25 -0800, John Larkin
wrote:

On Fri, 15 Jan 2010 21:02:27 -0500, Charlie Smith
wrote:

Group,

Many thanks for all the innovative and informative posts on the LED
lighting scheme. Many of you have been quite generous with your time
and knowledge and I am most appreciative.

Certainly there are many different ways to put this together and each
has a particular strength. I would prefer to stay with PWM techniques
for dimming versus voltage changes. With 20 (and perhaps more) LEDs in
the circuit, some are certain to begin to shut off before others when Vf
starts falling too low. I have taken as many of your suggestions as I
could understand (Sorry JosephKK, but for me, yours was grad school.
Beta sounds like a coefficient of some sort. Perhaps you could post a
primer for me.) and put them into a modified design. I have de-rated
the LEDs somewhat and switched to a low noise regulator that uses a trim
pot to adjust Vout. This way, should I need to go to two or even three
LEDs in series, I can accommodate this in the design.

Here I have calculated the total power consumption of the circuit at
4.56W with the LEDs consuming 1.23W (27% efficient). At 14V, it should
require 0.33A to run. The most critical part is the junction temp of
the regulator. It seemed to me to run too hot as is so I put in a
series resistor (12 ohms, 7W) to absorb some (3.075W) of the power. I
now estimate Tj to be 31.5°C hotter than ambient with an allowable Tmax
of 125°C on the chip. Safe even in the desert.

I also fixed a couple errors on the original schematic. As always, I
would appreciate some experienced eyes looking at this and letting me
know if I have seriously screwed something up somewhere. I'm not at all
familiar with linear low-noise regulators.

Thanks,
Charlie

Get rid of the 555 PWM thing! It accomplishes nothing.

John


Did you forget about the dimming function?

On Fri, 15 Jan 2010 20:11:04 -0800, Jupiter Jaq
wrote:

On Fri, 15 Jan 2010 20:01:25 -0800, John Larkin
wrote:

On Fri, 15 Jan 2010 21:02:27 -0500, Charlie Smith
wrote:

Group,

Many thanks for all the innovative and informative posts on the LED
lighting scheme. Many of you have been quite generous with your time
and knowledge and I am most appreciative.

Certainly there are many different ways to put this together and each
has a particular strength. I would prefer to stay with PWM techniques
for dimming versus voltage changes. With 20 (and perhaps more) LEDs in
the circuit, some are certain to begin to shut off before others when Vf
starts falling too low. I have taken as many of your suggestions as I
could understand (Sorry JosephKK, but for me, yours was grad school.
Beta sounds like a coefficient of some sort. Perhaps you could post a
primer for me.) and put them into a modified design. I have de-rated
the LEDs somewhat and switched to a low noise regulator that uses a trim
pot to adjust Vout. This way, should I need to go to two or even three
LEDs in series, I can accommodate this in the design.

Here I have calculated the total power consumption of the circuit at
4.56W with the LEDs consuming 1.23W (27% efficient). At 14V, it should
require 0.33A to run. The most critical part is the junction temp of
the regulator. It seemed to me to run too hot as is so I put in a
series resistor (12 ohms, 7W) to absorb some (3.075W) of the power. I
now estimate Tj to be 31.5°C hotter than ambient with an allowable Tmax
of 125°C on the chip. Safe even in the desert.

I also fixed a couple errors on the original schematic. As always, I
would appreciate some experienced eyes looking at this and letting me
know if I have seriously screwed something up somewhere. I'm not at all
familiar with linear low-noise regulators.

Thanks,
Charlie

Get rid of the 555 PWM thing! It accomplishes nothing.

John


Did you forget about the dimming function?

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.

John

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It all
depends on how much heat the OP is willing to let the dimmer dissipate.


--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It all
depends on how much heat the OP is willing to let the dimmer dissipate.

PWM is no more efficient than resistive dimming the way he did it.
There are no inductors in his circuit. All this sort of PWM can do is
move the heat around.

John

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat. For a low Vsat device you
get a pretty low heat dissipation even without inductors. The mark/space
ratio doesn't matter as the output is always either saturated or cut off.
It's not ideal, admittedly, but it's a lot better than using linear
output devices.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

On 17 Jan 2010 16:54:58 GMT, the renowned mick
wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat. For a low Vsat device you
get a pretty low heat dissipation even without inductors. The mark/space
ratio doesn't matter as the output is always either saturated or cut off.
It's not ideal, admittedly, but it's a lot better than using linear
output devices.

The overall efficiency (watts input from the +14 rail for a given
light output) is not improved by using PWM.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.

That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John

On Sun, 17 Jan 2010 10:23:00 -0800, John Larkin
wrote:

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.

That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John


You take a few of these and drive four sets of LED pairs per chip.

That's $0.50 per LED for constant current, precise control.

Might as well go all out. Beter than any use specific "LED driver" chip
out there.

http://www.edn.com/article/CA6702709.html

http://a330.g.akamai.net/7/330/2540/...6702709XLG.jpg

On Sun, 17 Jan 2010 11:22:23 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 10:23:00 -0800, John Larkin
wrote:

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.

That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John


You take a few of these and drive four sets of LED pairs per chip.

That's $0.50 per LED for constant current, precise control.

Might as well go all out. Beter than any use specific "LED driver" chip
out there.

http://www.edn.com/article/CA6702709.html

http://a330.g.akamai.net/7/330/2540/...6702709XLG.jpg

Yikes! It's horrendously complex, needs an I2C interface to set
brightness, and is constant-voltage, not constant current. And it will
generate lots of EMI, bad news in an airplane.

All he needs is an LM317, a pot, and a few more passives.

John

On Sun, 17 Jan 2010 11:43:13 -0800, John Larkin
wrote:

On Sun, 17 Jan 2010 11:22:23 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 10:23:00 -0800, John Larkin
wrote:

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.

That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John


You take a few of these and drive four sets of LED pairs per chip.

That's $0.50 per LED for constant current, precise control.

Might as well go all out. Beter than any use specific "LED driver" chip
out there.

http://www.edn.com/article/CA6702709.html

http://a330.g.akamai.net/7/330/2540/...6702709XLG.jpg

Yikes! It's horrendously complex, needs an I2C interface to set
brightness, and is constant-voltage, not constant current.


You did not look close enough. It will work in either mode.

And it will
generate lots of EMI, bad news in an airplane.

All he needs is an LM317, a pot, and a few more passives.

I know how to make a simple feedback controlled driver loop, Johnny. I
don't need a primer. That wasn't the point.

On Sun, 17 Jan 2010 11:56:32 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 11:43:13 -0800, John Larkin
wrote:

On Sun, 17 Jan 2010 11:22:23 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 10:23:00 -0800, John Larkin
wrote:

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.

That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John


You take a few of these and drive four sets of LED pairs per chip.

That's $0.50 per LED for constant current, precise control.

Might as well go all out. Beter than any use specific "LED driver" chip
out there.

http://www.edn.com/article/CA6702709.html

http://a330.g.akamai.net/7/330/2540/...6702709XLG.jpg

Yikes! It's horrendously complex, needs an I2C interface to set
brightness, and is constant-voltage, not constant current.


You did not look close enough. It will work in either mode.

And it will
generate lots of EMI, bad news in an airplane.

All he needs is an LM317, a pot, and a few more passives.

I know how to make a simple feedback controlled driver loop, Johnny. I
don't need a primer. That wasn't the point.

The point is that you're AlwaysWrong.

John

On Sun, 17 Jan 2010 13:46:36 -0800, John Larkin
wrote:

On Sun, 17 Jan 2010 11:56:32 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 11:43:13 -0800, John Larkin
wrote:

On Sun, 17 Jan 2010 11:22:23 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 10:23:00 -0800, John Larkin
wrote:

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.

That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John


You take a few of these and drive four sets of LED pairs per chip.

That's $0.50 per LED for constant current, precise control.

Might as well go all out. Beter than any use specific "LED driver" chip
out there.

http://www.edn.com/article/CA6702709.html

http://a330.g.akamai.net/7/330/2540/...6702709XLG.jpg

Yikes! It's horrendously complex, needs an I2C interface to set
brightness, and is constant-voltage, not constant current.


You did not look close enough. It will work in either mode.

And it will
generate lots of EMI, bad news in an airplane.

All he needs is an LM317, a pot, and a few more passives.

I know how to make a simple feedback controlled driver loop, Johnny. I
don't need a primer. That wasn't the point.

The point is that you're AlwaysWrong.

John


And like I stated once before... you are not worth responding to.

You immature, pathetic little *******, dweeb wanna be jerk.

flipper wrote:
On Sun, 17 Jan 2010 11:43:13 -0800, John Larkin
wrote:

On Sun, 17 Jan 2010 11:22:23 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 10:23:00 -0800, John Larkin
wrote:

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip
There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.


PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.
PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.


I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.
That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John

You take a few of these and drive four sets of LED pairs per chip.

That's $0.50 per LED for constant current, precise control.

Might as well go all out. Beter than any use specific "LED driver" chip
out there.

http://www.edn.com/article/CA6702709.html

http://a330.g.akamai.net/7/330/2540/...6702709XLG.jpg
Yikes! It's horrendously complex, needs an I2C interface to set
brightness, and is constant-voltage, not constant current. And it will
generate lots of EMI, bad news in an airplane.

All he needs is an LM317, a pot, and a few more passives.

John


After looking at all the suggestions made I tend to agree. Put some
load dump protection on the front end and be done with it.

The only difference is I tend to favor distributing heat to the limit
resistors but it works just fine either way.


Sorry to see the conversation has degenerated to name calling. Its the
primary reason I dislike usenet and I really wish you wouldn't.

Thanks for the suggestions but I think we need to refocus a bit. The
question here is not how to dim the lights. That decision is made; it
will be with PWM as I have yet to come across a single reference that
would suggest that dimming more than one LED by changing voltage is good
practice. They are constant current devices and must be treated that
way. And I mean by dimming to suggest variable dimming as you would
have with a normal car instrument panel lights. Not dimmed to one
constant, less than full-on level.

Perhaps a constant dimmed level could be a situation where voltage
manipulation could be made to work but not with variable dimming. There
just does not seem a viable alternative to PWM as all the LED drivers I
have found so far use a clock. I really do wish it were that simple but
if it were, everybody would just use an appropriate DC voltage and a pot
and there would be no need for drivers. And true, this approach wastes
about 75% of the 5 watts it takes in. But consider that 5 watts is a
little more than one incandescent instrument panel bulb would use. Not
all that bad folks. I can easily live with that.

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.

Thanks to all for their input.
Charlie

On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:


Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.

What does your DAR think of it?

On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

flipper wrote:
On Sun, 17 Jan 2010 11:43:13 -0800, John Larkin
wrote:

On Sun, 17 Jan 2010 11:22:23 -0800, Archimedes' Lever
wrote:

On Sun, 17 Jan 2010 10:23:00 -0800, John Larkin
wrote:

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip
There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.


PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.
PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.


I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.
That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John

You take a few of these and drive four sets of LED pairs per chip.

That's $0.50 per LED for constant current, precise control.

Might as well go all out. Beter than any use specific "LED driver" chip
out there.

http://www.edn.com/article/CA6702709.html

http://a330.g.akamai.net/7/330/2540/...6702709XLG.jpg
Yikes! It's horrendously complex, needs an I2C interface to set
brightness, and is constant-voltage, not constant current. And it will
generate lots of EMI, bad news in an airplane.

All he needs is an LM317, a pot, and a few more passives.

John


After looking at all the suggestions made I tend to agree. Put some
load dump protection on the front end and be done with it.

The only difference is I tend to favor distributing heat to the limit
resistors but it works just fine either way.


Sorry to see the conversation has degenerated to name calling. Its the
primary reason I dislike usenet and I really wish you wouldn't.

Thanks for the suggestions but I think we need to refocus a bit. The
question here is not how to dim the lights. That decision is made; it
will be with PWM as I have yet to come across a single reference that
would suggest that dimming more than one LED by changing voltage is good
practice. They are constant current devices and must be treated that
way. And I mean by dimming to suggest variable dimming as you would
have with a normal car instrument panel lights. Not dimmed to one
constant, less than full-on level.

Your PWM thing will work, although it doesn't provide any regulation
against 14-volt changes, and it's more complex than necessary.

An LM317 or LM1117 regulator, with a pot, could generate a regulated
voltage from, say, 4 to 10. That could drive a number of strings, each
one resistor and two LEDs. It would regulate and continuously dim, and
would be simple and reliable.

John

On Sun, 17 Jan 2010 10:23:00 -0800, John Larkin
wrote:

On 17 Jan 2010 16:54:58 GMT, mick wrote:

On Sun, 17 Jan 2010 08:35:12 -0800, John Larkin wrote:

On 17 Jan 2010 16:01:35 GMT, mick wrote:

On Fri, 15 Jan 2010 23:02:08 -0800, John Larkin wrote:

snip

There's a pot on the linear regulator; it's in plain sight. And that's
all this thing needs. The PWM accomplishes nothing.



PWM dimming is more efficient than linear dimming - by a long way. It
all depends on how much heat the OP is willing to let the dimmer
dissipate.

PWM is no more efficient than resistive dimming the way he did it. There
are no inductors in his circuit. All this sort of PWM can do is move the
heat around.



I couldn't see his circuit on my server, but I assumed that he was
varying the mark/space ratio. In that case power dissipation in the
output device is always low, depending on Vsat.

That just moves the heat into the series resistors. Overall efficiency
is always the same for a dissipative (inductor-free) regulator.

With a linear regulator and properly chosen series resistors, you can
balance the regulator heat distribution versus dimming level.
Regulator power dissipation versus output voltage is sort of
parabolic... low at low illumination, low at max illumination, peaking
somewhere between.

It's easier to heatsink a voltage regulator than a lot of small
resistors.

Another advantage of a voltage regulator is that it regulates.

John

Of course, John, as I posted _4_days_ago_....

Message-ID:

You have me plonked, yet post that I don't contribute circuits to the
group.

If you didn't have your ego stuck firmly up your posterior orifice,
you'd knock off the crap, stop feeding Slowman, and actually post a
circuit (rather than a vague description that prevents criticism ;-)

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.

---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF

"John Fields" wrote in message
...
On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.

---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF


Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

Thanks.
Charlie

On Mon, 18 Jan 2010 15:30:55 -0500, "Charlie Smith"
wrote:


"John Fields" wrote in message
.. .
On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.

---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF


Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

That's impossible. The brightness is proportional to the current. If
current remains constant the brightness will not change.

When you PWM all your doing is changing the average current the peak
current remains the same.

As everyone said lose the 555 and just use a POT to adjust your output
voltage thereby adjusting the current through your LED's. If your
worried about not having sufficient voltage to drive your LED's then
just select the POT with a resistor so it cant be adjusted below a
certain level. Add a fuse and some TVS to.

If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

Thanks.
Charlie

On Mon, 18 Jan 2010 14:40:13 -0600, flipper wrote:

snip

Has anybody asked what the minimum input voltage requirement is, since I
presume the 14V has a battery?



Just a guess, but I suspect that it will be a 12v battery on continuous
charge, making a nominal 13.8v under normal circumstances (referred to as
14v - 7x2V cells would be silly for a battery). I'm willing to be wrong
though! That would give a useful end point of about 11v, but things may
already be in trouble by then.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

On Mon, 18 Jan 2010 15:30:55 -0500, "Charlie Smith"
wrote:


"John Fields" wrote in message
.. .
On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.

---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF


Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

---
If you want the peak current to remain the same but the brightness to
vary, then you're describing PWM, which isn't what I offered.

My circuit would be more like:


BRIGHTNESS CURRENT
% mA
------------+---------
100 20
75 15
50 10
25 5
0 0

Not _exactly_ like that though, because the brightness VS current curve
isn't totally linear, but it's a dimmer, for goodness' sake, so you
crank it until it's bright or dim enough for you and who cares about
lumens per degree???
---

If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

---
What you seem to be missing is that the varying control
voltage/resistance is used to vary the current through the lamps and,
therefore, their brightness.

Here's a simple example to illustrate the concept: (View in Courier)


+14V-+-------+
| |
| [R1]
| |
| [LED]
| |
| C
[POT]--B NPN
| E
| |
GND--+-------+

With the pot's wiper at the GND end of the element, there'll be 0V on
the base of the transistor, so the transistor will be cut off and only
leakage current will flow through R1, the LED, and the c-e junction of
the transistor, keeping the LED off.

As the wiper is rotated more and more toward +14V, however, a point will
be reached where the voltage on the base will rise enough for current to
be injected into the base, causing the current through R1, the LED, and
the c-e junction to increase as well, eventually illuminating the LED.

Then, with a fixed supply, since the transistor is causing the current
through the LED to vary as the voltage on the base is varied, it's
acting like a voltage-variable resistor with the entire system, BTW,
dissipating no more and no less power than a PWM controlled dimmer.

The ball's in your court; what do you want to do?

JF

John Fields wrote:
On Mon, 18 Jan 2010 15:30:55 -0500, "Charlie Smith"
wrote:

"John Fields" wrote in message
...
On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.
---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF

Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

---
If you want the peak current to remain the same but the brightness to
vary, then you're describing PWM, which isn't what I offered.

My circuit would be more like:


BRIGHTNESS CURRENT
% mA
------------+---------
100 20
75 15
50 10
25 5
0 0

Not _exactly_ like that though, because the brightness VS current curve
isn't totally linear, but it's a dimmer, for goodness' sake, so you
crank it until it's bright or dim enough for you and who cares about
lumens per degree???
---

If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

---
What you seem to be missing is that the varying control
voltage/resistance is used to vary the current through the lamps and,
therefore, their brightness.

Here's a simple example to illustrate the concept: (View in Courier)


+14V-+-------+
| |
| [R1]
| |
| [LED]
| |
| C
[POT]--B NPN
| E
| |
GND--+-------+

With the pot's wiper at the GND end of the element, there'll be 0V on
the base of the transistor, so the transistor will be cut off and only
leakage current will flow through R1, the LED, and the c-e junction of
the transistor, keeping the LED off.

As the wiper is rotated more and more toward +14V, however, a point will
be reached where the voltage on the base will rise enough for current to
be injected into the base, causing the current through R1, the LED, and
the c-e junction to increase as well, eventually illuminating the LED.

Then, with a fixed supply, since the transistor is causing the current
through the LED to vary as the voltage on the base is varied, it's
acting like a voltage-variable resistor with the entire system, BTW,
dissipating no more and no less power than a PWM controlled dimmer.

The ball's in your court; what do you want to do?

JF

You are correct, I was missing your point regarding base current on the
transistor. I can wire this up here on the breadboard and get a feel
for how it works. My main concern is that somewhere between 50 and 25%,
a part of the panel will switch itself off and part will stay on. And
what goes off will depend on manufacturing variation on the LEDs,
exactly how much voltage the regulator is putting out and the
temperature in the cockpit. The practical consideration for me has to
be how often will I be operating in that envelope. Maybe never. Maybe
all the time. I won't know until I build the thing and see how well it
works. You are also correct in that what I described is PWM. Be fair.
You did ask what did it bring to the table and what it brings is the
ability to dim an LED at constant current.

I really have no care at all as to how efficient the circuit is. I do
care about how much total power it pulls. Either system will be
efficient enough for me; by a far cry. Let me look at this alternate
set-up here over the next day or so. Either system will have to work on
5-8 volts regulated power so thats just a matter of resistors. If the
alternator dies, I want the panel to work as long as possible and the
3080 chip will function until well below voltage the LEDs will work at
and the load the lighting is taking off the battery will be minimal.

Thanks for the suggestion.

Charlie

Hammy wrote:
On Mon, 18 Jan 2010 15:30:55 -0500, "Charlie Smith"
wrote:

"John Fields" wrote in message
...
On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.
---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF

Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

That's impossible. The brightness is proportional to the current. If
current remains constant the brightness will not change.

When you PWM all your doing is changing the average current the peak
current remains the same.

As everyone said lose the 555 and just use a POT to adjust your output
voltage thereby adjusting the current through your LED's. If your
worried about not having sufficient voltage to drive your LED's then
just select the POT with a resistor so it cant be adjusted below a
certain level. Add a fuse and some TVS to.
If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

Thanks.
Charlie


It is not impossible as it is the sole basis for LED drivers. The clock
cannot partially turn the circuit on as it causes the generation of a
square wave. It is either on or off. When it is on, the circuit is
energized to 3.4V and 18 mA will flow through the LED. When its off,
its off. Nothing in between (0 or 1). But because it is doing this at
almost 400 Hz, only your eyes see the average. The LED certainly does
not see any average. It lives in the intervals just fine, thank you.

On Mon, 18 Jan 2010 18:00:46 -0500, Charlie Smith
wrote:

Hammy wrote:
On Mon, 18 Jan 2010 15:30:55 -0500, "Charlie Smith"
wrote:

"John Fields" wrote in message
...
On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.
---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF

Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

That's impossible. The brightness is proportional to the current. If
current remains constant the brightness will not change.

When you PWM all your doing is changing the average current the peak
current remains the same.

As everyone said lose the 555 and just use a POT to adjust your output
voltage thereby adjusting the current through your LED's. If your
worried about not having sufficient voltage to drive your LED's then
just select the POT with a resistor so it cant be adjusted below a
certain level. Add a fuse and some TVS to.
If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

Thanks.
Charlie


It is not impossible as it is the sole basis for LED drivers. The clock
cannot partially turn the circuit on as it causes the generation of a
square wave. It is either on or off. When it is on, the circuit is
energized to 3.4V and 18 mA will flow through the LED. When its off,
its off. Nothing in between (0 or 1). But because it is doing this at
almost 400 Hz, only your eyes see the average. The LED certainly does
not see any average. It lives in the intervals just fine, thank you.


Really how do you propose to alter the brightness with a constant
18mA?

If you say switch 18mA on/off at a varying duty cycle this will
change the average current and it will be below the peak current which
in this case is 18mA. This is dimming.

Example

25% duty the average current is ideally 4.5mA
50% ' '8mA
75% ' '13.5mA


If you maintain 18mA average current you can't change the brightness.
That is what you said is it not? Change the brightness without
changing the current?

The LED most certainly does see the average current that's how you
determine power dissipation in it unless you like to cook your LED's.

On Mon, 18 Jan 2010 18:30:43 -0500, Hammy wrote:



Really how do you propose to alter the brightness with a constant
18mA?

If you say switch 18mA on/off at a varying duty cycle this will
change the average current and it will be below the peak current which
in this case is 18mA. This is dimming.

Example

25% duty the average current is ideally 4.5mA
50% ' '8mA
75% ' '13.5mA


If you maintain 18mA average current you can't change the brightness.
That is what you said is it not? Change the brightness without
changing the current?

The LED most certainly does see the average current that's how you
determine power dissipation in it unless you like to cook your LED's.

whoops the 8mA should be 9mA at 50% duty.

On Mon, 18 Jan 2010 17:47:44 -0500, Charlie Smith
wrote:

John Fields wrote:
On Mon, 18 Jan 2010 15:30:55 -0500, "Charlie Smith"
wrote:


Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

---
If you want the peak current to remain the same but the brightness to
vary, then you're describing PWM, which isn't what I offered.

My circuit would be more like:


BRIGHTNESS CURRENT
% mA
------------+---------
100 20
75 15
50 10
25 5
0 0

Not _exactly_ like that though, because the brightness VS current curve
isn't totally linear, but it's a dimmer, for goodness' sake, so you
crank it until it's bright or dim enough for you and who cares about
lumens per degree???
---

If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

---
What you seem to be missing is that the varying control
voltage/resistance is used to vary the current through the lamps and,
therefore, their brightness.

Here's a simple example to illustrate the concept: (View in Courier)


+14V-+-------+
| |
| [R1]
| |
| [LED]
| |
| C
[POT]--B NPN
| E
| |
GND--+-------+

With the pot's wiper at the GND end of the element, there'll be 0V on
the base of the transistor, so the transistor will be cut off and only
leakage current will flow through R1, the LED, and the c-e junction of
the transistor, keeping the LED off.

As the wiper is rotated more and more toward +14V, however, a point will
be reached where the voltage on the base will rise enough for current to
be injected into the base, causing the current through R1, the LED, and
the c-e junction to increase as well, eventually illuminating the LED.

Then, with a fixed supply, since the transistor is causing the current
through the LED to vary as the voltage on the base is varied, it's
acting like a voltage-variable resistor with the entire system, BTW,
dissipating no more and no less power than a PWM controlled dimmer.

The ball's in your court; what do you want to do?

JF

You are correct, I was missing your point regarding base current on the
transistor. I can wire this up here on the breadboard and get a feel
for how it works.

---
It won't work very well at all because it's common emitter, so its going
to have a very narrow dimming range, probably about 5 or 10 degrees.

But then, it's not supposed to, [work well] since it's just an example
designed to demonstrate voltage controlled resistance.
---

My main concern is that somewhere between 50 and 25%,
a part of the panel will switch itself off and part will stay on. And
what goes off will depend on manufacturing variation on the LEDs,
exactly how much voltage the regulator is putting out and the
temperature in the cockpit. The practical consideration for me has to
be how often will I be operating in that envelope. Maybe never. Maybe
all the time. I won't know until I build the thing and see how well it
works. You are also correct in that what I described is PWM. Be fair.
You did ask what did it bring to the table and what it brings is the
ability to dim an LED at constant current.

I really have no care at all as to how efficient the circuit is. I do
care about how much total power it pulls.

---
At full brightness with a 14V supply and 20 LEDs drawing 20mA each,
either system will draw 400 mA from the supply and dissipate 5.6 watts
plus whatever the quiescent circuit requirements happen to be.

With the lights dimmed to off, the power dissipated by either system
will be zero watts plus whatever the miniscule quiescent circuit
requirements happen to be, so the point is that since either system
dissipates the _same_ amount of power for equal dimming, that's a wash,
and the circuit topology will depend on other factors.
---

Either system will be
efficient enough for me; by a far cry. Let me look at this alternate
set-up here over the next day or so. Either system will have to work on
5-8 volts regulated power so thats just a matter of resistors. If the
alternator dies, I want the panel to work as long as possible and the
3080 chip will function until well below voltage the LEDs will work at
and the load the lighting is taking off the battery will be minimal.

Thanks for the suggestion.

---
You're welcome, but don't use that circuit to seriously evaluate
_anything_ since it won't be part of the final circuit.

I'll put something together either tomorrow or Wednesday, and I'll post
it here when it's done. In the meantime take a look at Jim's
circuit...

Not bad for an old guy! :-)


JF

On Mon, 18 Jan 2010 15:39:05 -0600, John Fields
wrote:

On Mon, 18 Jan 2010 15:30:55 -0500, "Charlie Smith"
wrote:


"John Fields" wrote in message
. ..
On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.

---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF


Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

---
If you want the peak current to remain the same but the brightness to
vary, then you're describing PWM, which isn't what I offered.

My circuit would be more like:


BRIGHTNESS CURRENT
% mA
------------+---------
100 20
75 15
50 10
25 5
0 0

Not _exactly_ like that though, because the brightness VS current curve
isn't totally linear, but it's a dimmer, for goodness' sake, so you
crank it until it's bright or dim enough for you and who cares about
lumens per degree???
---

If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

---
What you seem to be missing is that the varying control
voltage/resistance is used to vary the current through the lamps and,
therefore, their brightness.

Here's a simple example to illustrate the concept: (View in Courier)


Or do this:


+14V-+-------+
| |
| [R1]
| |
| [2 LEDs]
| |
| +--------etc
| |
| E
[POT]--B PNP
| C
| |
GND--+-------+


which actually solves the whole problem.


John

On Mon, 18 Jan 2010 19:11:55 -0600, flipper wrote:

On Mon, 18 Jan 2010 16:43:07 -0800, John Larkin
wrote:

On Mon, 18 Jan 2010 15:39:05 -0600, John Fields
wrote:

On Mon, 18 Jan 2010 15:30:55 -0500, "Charlie Smith"
wrote:


"John Fields" wrote in message
m...
On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.

---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF


Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

---
If you want the peak current to remain the same but the brightness to
vary, then you're describing PWM, which isn't what I offered.

My circuit would be more like:


BRIGHTNESS CURRENT
% mA
------------+---------
100 20
75 15
50 10
25 5
0 0

Not _exactly_ like that though, because the brightness VS current curve
isn't totally linear, but it's a dimmer, for goodness' sake, so you
crank it until it's bright or dim enough for you and who cares about
lumens per degree???
---

If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

---
What you seem to be missing is that the varying control
voltage/resistance is used to vary the current through the lamps and,
therefore, their brightness.

Here's a simple example to illustrate the concept: (View in Courier)


Or do this:


+14V-+-------+
| |
| [R1]
| |
| [2 LEDs]
| |
| +--------etc
| |
| E
[POT]--B PNP
| C
| |
GND--+-------+


which actually solves the whole problem.


John

Or put R1 under the original NPN emitter.

Yes, that removes any concern about variation of LED voltage drops
affecting low-end brightness. On the other hand, you'll need a
transistor per led (or pair of led's)

JFs circuit has the virtue of destroying the pot and/or transistor at
maximum pot rotation.

I still like the LM317 feeding multiple LED+resistor branches. It has
regulation, current limiting, thermal limiting, and is bog simple.

John

On Mon, 18 Jan 2010 19:09:45 -0800, John Larkin
wrote:

On Mon, 18 Jan 2010 19:11:55 -0600, flipper wrote:

On Mon, 18 Jan 2010 16:43:07 -0800, John Larkin
wrote:

On Mon, 18 Jan 2010 15:39:05 -0600, John Fields
wrote:

On Mon, 18 Jan 2010 15:30:55 -0500, "Charlie Smith"
wrote:


"John Fields" wrote in message
om...
On Sun, 17 Jan 2010 22:55:31 -0500, Charlie Smith
wrote:

Unless somebody has any modifications to suggest to V2, then the board
will be built as submitted.

---
I suggest you ditch the PWM and use either Jim's circuit or mine in
order to get the variable dimming that you want.

Why?

Because the PWM circuit, as JL stated is doing nothing.

In truth, it's worse than that because no matter how you slice it the
PWM circuit will generate EMI, which is potential trouble that goes away
completely with a linear dimmer.

In addition, you save no power using a PWM dimmer.

Consider:

If you're using a 14V supply to light up 20 LEDs to full brightness with
20mA going through each of them, then the total power dissipated by
either a linear or a switched supply system running at 100% duty cycle
will be: 14V * 0.4A * 1s = 5.6 watts.

Now, (using a switched supply and assuming a 50% duty cycle will get us
1/2 brightness) if we drive the LEDs with 20mA for half the time, the
system will dissipate: 14V * 0.4A * 0.5s = 2.8 watts.

In the same vein, if we assume that half the current through the LEDs,
all the time, will give us half brightness, then for the linear supply
the dissipation will be: 14V * 0.2A * 1s = 2.8 watts.

If you're interested in blowing off the PWM stuff, I'd be happy to
rework my design to provide input transient protection and supply you
with a schematic, a BOM, and a simulation.


JF


Thanks for the offer John and I am interested. Here is what I would like
your circuit to do:

At 100% brightness, make the current in the LED's be 18 mA.
At 75% brightness, make the current in the LED's 18 mA.
At 50% brightness, make the current in the LED's 18 mA.
And at 25% brightness, make the current in the LED's 18 mA.

---
If you want the peak current to remain the same but the brightness to
vary, then you're describing PWM, which isn't what I offered.

My circuit would be more like:


BRIGHTNESS CURRENT
% mA
------------+---------
100 20
75 15
50 10
25 5
0 0

Not _exactly_ like that though, because the brightness VS current curve
isn't totally linear, but it's a dimmer, for goodness' sake, so you
crank it until it's bright or dim enough for you and who cares about
lumens per degree???
---

If it can do this then I am absolutely interested. I agree, there is the
risk of noise in the circuit since nobody that I know has done this that I
am aware of. No data exists. I plan to build it and place my hand-held
within 6" of the board and see if it talks to me. If it does, then its time
for Plan B. If not, then I am not so interested if it turns 3 or 4 Watts
into heat. That's roughly the amount of energy one incandescent instrument
bulb would consume. Insignificant. The heat rejection is built into the
board to handle that much and more. But if you have a better way, I really
would seriously review it. I am not married to any one approach. I just
want the best compromise. Right now from my perspective, varying voltage
does not seem to be it.

---
What you seem to be missing is that the varying control
voltage/resistance is used to vary the current through the lamps and,
therefore, their brightness.

Here's a simple example to illustrate the concept: (View in Courier)


Or do this:


+14V-+-------+
| |
| [R1]
| |
| [2 LEDs]
| |
| +--------etc
| |
| E
[POT]--B PNP
| C
| |
GND--+-------+


which actually solves the whole problem.


John

Or put R1 under the original NPN emitter.

Yes, that removes any concern about variation of LED voltage drops
affecting low-end brightness. On the other hand, you'll need a
transistor per led (or pair of led's)

JFs circuit has the virtue of destroying the pot and/or transistor at
maximum pot rotation.

I still like the LM317 feeding multiple LED+resistor branches. It has
regulation, current limiting, thermal limiting, and is bog simple.

John

You could use the op amp and its feedback loop to push the transistor
drivers too. More parts but better control when failure modes take
place. 2n2222s are cheap. Heck a few more parts and you could eliminate
the op amp too. Make your own control loop(s).

On Mon, 18 Jan 2010 22:19:11 -0600, flipper wrote:


Or put R1 under the original NPN emitter.

Yes, that removes any concern about variation of LED voltage drops
affecting low-end brightness. On the other hand, you'll need a
transistor per led (or pair of led's)

Yep, that's the downside.

You also don't need to worry about 'low bat' and regulator dropout,
though.

Make a trimmable Vref for the top end of the pot and size the
resistors for 18mA at max and you have 0-18mA irrespective of 'LED
drop', temperature drift, etc.

JFs circuit has the virtue of destroying the pot and/or transistor at
maximum pot rotation.

I still like the LM317 feeding multiple LED+resistor branches. It has
regulation, current limiting, thermal limiting, and is bog simple.

Yeah, those are it's advantages.

It strikes me we're, pardon the pun, shooting in the dark a bit not
knowing just how 'low' it needs to go.

Another point is the eye's response is logarithmic so it might be best
with a log pot (same goes for PWM) and that might mean the individual
transistor approach works best for the low end. Either that or custom
shape a linear pot with a resistor from wiper to one end.

Really need to do some testing to see what the desired parameters
really are.

I admit I haven't tried linear current dimming white LEDs so I don't
know how much the color temp shifts.




Not to change the subject, NO!, but I tested some super-efficient
T1-3/4 Agilent green LEDs to see the minimum current that would make
light. Dark adapted, staring straight into the LED, the visual
threshold was just under 1 nA. These things are decently bright in
room light at 1 uA.

John

In article ,
flipper wrote:

[snip]
The LED most certainly does see the average in power dissipation. You
could, for example, pulse higher currents through the LED as long as
the duty cycle meets the power dissipation curves.

That's only true of LEDs designed for pulsing. (Some laser diodes
operate from 30 Amp 100 ns pulses.)

Illumination LEDs may or may not be able to use current pulses higher
than the normal current rating. A lot of them will perform poorly or
physically degrade beyond 100%, even if the duty cycle is too low to
cause heating. The pulses should be smoothed to a constant current
where efficiency matters. Phosphor LEDs may require pulses at a
specific current when constant color quality matters.
--
I won't see Google Groups replies because I must filter them as spam

On Fri, 15 Jan 2010 20:01:25 -0800, John Larkin
wrote:

Did you really need to quote 40+ lines for a one line response?

Jim

Get rid of the 555 PWM thing! It accomplishes nothing.

John

Why is EMI bad news in an airplane as opposed to anywhere else?

Jim

On Sun, 17 Jan 2010 11:43:13 -0800, John Larkin
wrote:

Yikes! It's horrendously complex, needs an I2C interface to set
brightness, and is constant-voltage, not constant current. And it will
generate lots of EMI, bad news in an airplane.

All he needs is an LM317, a pot, and a few more passives.

John

"RST Engineering" wrote in message
...
Why is EMI bad news in an airplane as opposed to anywhere else?

Jim


Hi Jim.

I will be building the PWM board and will simply test it. As you have often
said, there is a reason why we put that one word promanently in our
aircraft. If it looks and sounds fine on my workshop bench, I'll take it to
an airplane and fire it up there. If that's ok, its going into the plane.
And though I am not at all a fan of varying the current through LEDs, I will
take a look at the variable current version. 317s are easy enough to get at
RadioShack, they're plenty cheap and there's lots of time in which to do the
work.

Charlie

On Mon, 18 Jan 2010 16:43:07 -0800, the renowned John Larkin
wrote:


Or this (blue or white LEDs)

Or do this:


+14V-+-------+
| |
| [R1]
| |
[2 mo LEDs] [2 LEDs]
| |
| +--------etc
| |
| E
[POT]--B PNP
| C
| |
GND--+-------+


which actually solves the whole problem.


John



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Mon, 18 Jan 2010 23:31:17 -0800, the renowned RST Engineering
wrote:

Why is EMI bad news in an airplane as opposed to anywhere else?

Jim

Because if things go badly wrong in an airplane, people die.

An airplane's natural state is on the ground, in pieces.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Mon, 18 Jan 2010 19:09:45 -0800, John Larkin
wrote:


JFs circuit has the virtue of destroying the pot and/or transistor at
maximum pot rotation.

---
Geez, and here the thread was going so nicely...

Why is it you have such a hard time keeping a civil tongue in your head
and what is it you don't understand about:

"Here's a simple example to illustrate the concept:" ?

JF