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