On Thu, 14 Jan 2010 07:16:21 -0800, John Larkin
wrote:

On Thu, 14 Jan 2010 06:53:35 -0600, John Fields
wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:



If you use 14 volts to run a 3.4 volt LED, with a dissipative system,
the efficiency is always around 24%. Different circuits just move the
heat around.

With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.

---
Yes, but if one fails shorted the second one will follow soon after if
its If(max) is exceeded, while if one fails open two will go out so
you're trading efficiency for reliability.

JF

Sure, except that LEDs rarely fail.

John

Those used for lighting have a higher failure rate. For a panel, they
will likely never fail.

Just convert the 14V down and waste nothing with a silly dissipative
circuit. Drive each directly, and use a dedicated current limit resistor
on each. Brightness consistency becomes accurately repeatable, and there
are no failure modes that would cause others to fail as a result.

On Thu, 14 Jan 2010 18:45:36 -0500, Spehro Pefhany
wrote:

On Thu, 14 Jan 2010 11:11:03 -0600, the renowned John Fields
wrote:

On Thu, 14 Jan 2010 07:16:21 -0800, John Larkin
wrote:

On Thu, 14 Jan 2010 06:53:35 -0600, John Fields
wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:



If you use 14 volts to run a 3.4 volt LED, with a dissipative system,
the efficiency is always around 24%. Different circuits just move the
heat around.

With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.

---
Yes, but if one fails shorted the second one will follow soon after if
its If(max) is exceeded, while if one fails open two will go out so
you're trading efficiency for reliability.

JF

Sure, except that LEDs rarely fail.

---
"Rarely" ain't "never", bucko.

It's kinda the same thing as "a long time" ain't "forever".

JF

So "love you long time" is different from "love you forever"?


Best regards,
Spehro Pefhany


'love you long time' is more truthful, and more intimate.

'Love you forever' sounds like a lie before the statement is even
finished.

On Thu, 14 Jan 2010 16:39:53 -0800, Archimedes' Lever
wrote:

On Thu, 14 Jan 2010 07:16:21 -0800, John Larkin
wrote:

On Thu, 14 Jan 2010 06:53:35 -0600, John Fields
wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:



If you use 14 volts to run a 3.4 volt LED, with a dissipative system,
the efficiency is always around 24%. Different circuits just move the
heat around.

With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.

---
Yes, but if one fails shorted the second one will follow soon after if
its If(max) is exceeded, while if one fails open two will go out so
you're trading efficiency for reliability.

JF

Sure, except that LEDs rarely fail.

John

Those used for lighting have a higher failure rate. For a panel, they
will likely never fail.

Just convert the 14V down and waste nothing with a silly dissipative
circuit. Drive each directly, and use a dedicated current limit resistor
on each. Brightness consistency becomes accurately repeatable, and there
are no failure modes that would cause others to fail as a result.

Excuse me, but that makes no sense. A schematic would help.

John

John Larkin wrote:
On Thu, 14 Jan 2010 12:39:35 -0500, ehsjr
wrote:


John Fields wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:





If you use 14 volts to run a 3.4 volt LED, with a dissipative system,
the efficiency is always around 24%. Different circuits just move the
heat around.

With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.


---
Yes, but if one fails shorted the second one will follow soon after if
its If(max) is exceeded, while if one fails open two will go out so
you're trading efficiency for reliability.

JF

I have a problem - my lack of knowledge - with running the
LEDs in series. I don't know aircraft rules/regs, or whether
there is ever a time you would want individual on/off control
of instrument illumination. So I want each LED to be able
to be switched on/off to accomodate that possibility, which
rules out series connection.


His posted schematic didn't have switches.

Yes, I saw that. Not knowing aircraft, I didn't/don't
know if that is an oversight or whether all instruments
are required to be illuminated all the time.




My next problem is the same that you have in mind - if one
LED fails you lose two lights. Not understanding what I'll
call "cockpit dynamics", I would be concerned about that series
design. If I'm a passenger on that plane, I want the pilot
to have 100% of his instrumentation available, 100% of
the time.


Panel backlights usually have overlapping so that any region is lit by
at least two lamps. If one goes out, a zone may be a bit dimmer but
not invisible. If led's are used in series, just locate them so that a
string failure doesn't blank any region, just dims some.

One common structure is a clear plastic plate with a white reflective
layer on both sides, then a black layer on top. Lettering is engraved
through the black on top. Multiple lamps are recessed into the back
side, and they scatter light all over the place, so no area is lit by
just one lamp.

Ah - that makes sense, and eliminates most concerns I had.
It also means that all the instruments will be lit so
switching individual LEDs on/off is not a factor. Thanks!

Ed



Actually, I'd prefer two complete systems - regulators,
PWM, whatever with two LEDs per instrument, one fed by
system A and one fed by system B. That way, it would take
two failures for an instrument's lighting to fail completely.
I'm guessing that if the 14V bus goes down, you have bigger
problems. Maybe I'm just being a nervous Nelly because
I have no knowledge of flying a plane.


A flashlight is the backup.

Do small planes still use dual magneto ignitions?

John

On Mon, 11 Jan 2010 21:06:32 -0500, Charlie Smith wrote:



Been reading the thread. A combination of switching to a 7809 regulator and using
two LEDs in series may save some power/reduce heat. (Ed and Speff IIRC)

You also need to protect the input of the regulator just like automobile applications,
able to withstand up to 100 v spikes in either direction for 5 ms duration. Also for
30 to 40 V for 200 ms.

If you go with PWM you may wish to consider a lower resistor on the base of the 3055,
typical Beta is about 50 at 500 mA IC.

I also suggest some isolation between the regulator and the LED block, say a lossy
inductor with about 1 ohm resistance. Inductor size should be premised on discontinuous
conduction at low brightness and continuous conduction at max brightness. A couple more
diodes may be appropriate.

On Thu, 14 Jan 2010 06:53:35 -0600, John Fields wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:



If you use 14 volts to run a 3.4 volt LED, with a dissipative system,
the efficiency is always around 24%. Different circuits just move the
heat around.

With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.

---
Yes, but if one fails shorted the second one will follow soon after if
its If(max) is exceeded, while if one fails open two will go out so
you're trading efficiency for reliability.

JF

Seeing as how we are talking of 10 or 20 parallel strings losing 10% or less
of total illumination does not constitute complete failure.

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:

Oh, for the love of Christ, can't you people learn how to SNIP????

Jim



With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.

A switching regulator can approach 100%, but is probably not worth the
hassle here.

John

On Mon, 18 Jan 2010 23:24:03 -0800, RST Engineering
wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:

Oh, for the love of Christ, can't you people learn how to SNIP????

Jim



With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.

A switching regulator can approach 100%, but is probably not worth the
hassle here.

John



We'll start a fund to buy you some scroll bars.

John

On Mon, 18 Jan 2010 23:24:03 -0800, RST Engineering
wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:

Oh, for the love of Christ, can't you people learn how to SNIP????

Jim



With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.

A switching regulator can approach 100%, but is probably not worth the
hassle here.

John



And why can't you learn how to bottom post?

John

John Larkin wrote:

On Mon, 18 Jan 2010 23:24:03 -0800, RST Engineering
wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:

Oh, for the love of Christ, can't you people learn how to SNIP????

Jim



With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.

A switching regulator can approach 100%, but is probably not worth the
hassle here.

John



We'll start a fund to buy you some scroll bars.


Don't forget a sense of humor, too.


--
Greed is the root of all eBay.

On Tue, 19 Jan 2010 08:36:47 -0800, John Larkin
wrote:

On Mon, 18 Jan 2010 23:24:03 -0800, RST Engineering
wrote:

On Wed, 13 Jan 2010 19:15:56 -0800, John Larkin
wrote:

Oh, for the love of Christ, can't you people learn how to SNIP????

Jim



With pairs of LEDs in series, efficiency becomes 6.8/14 = 48%.

A switching regulator can approach 100%, but is probably not worth the
hassle here.

John



And why can't you learn how to bottom post?

John


He doesn't think that there should be any need for a difference between
the way one answers an email, and the way one replies in a chrono based
forum as Usenet. That, and he blatantly acts inconsiderately because he
doesn't think that any of us should be bothered by it, or hassle him for
it. In other words, being a perpetual asshole about it with no time to
put aside for actually complying with the decades old
convention/courtesy.

The convention I ignore is that I cuss. I think his is worse.