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