noob-(ish) PWM Question

with everyone talking about noticeable vs. unnoticeable pwm, why hasn’t someone stuck a semi-big capacitor on the led leads, and have that smooth out the waves a little? space constraints aside.

tag on question- is there a thread/excel sheet floating around that lists flashlight/ replacement driver combos?

Listing combos has a measure of futility because often people will make adapter rings or use contact boards to fit smaller drivers in bigger holes. LCK-LED and CNQualitygoods both carry a wide selection of drivers of different diameters. Or you could tell us what light you have and we can offer advice. :slight_smile:

As to the capacitor…I dunno. Expense? Packaging? Try it and post results. I know though that the whine associated wit PWM won’t go away because of a capacitor on the LED leads.

Sticking a capacitor on the output won’t help. The dimming is not caused by an averaging of power. It is actually caused by the light being on and off for a certain period of time and your brain makes you think it’s dimmer. If you were to take a very high speed shot equivalent in shutter speed to the PWM on time you’d notice on high or on low the instantaneous light output is the same.

The problem with adding a capacitor would be large current drains during the charge cycle, and since the peak voltage is the same, if you got a large enough capacitor to cancel out the rippling all you’d end up doing is keeping the same brightness on full but lowering the efficiency due to internal power losses due to high peak currents.

I understand that it would not turn the output completely smooth, but I was sort of thinking it would knock off the highs and lows of the wave and even it out (a little). Would adding a (small) capacitor across the pwm line smooth it out (a little)? I am annoyed that even very high frequencies can be picked up by my eye by flicking my eye around (if that makes sense). I guess what I am trying to do is even out those rapid on-off so that it sort of fades on-off , however small. it would still be square waveish, but less so, and more sine-waveish. So I wouldn’t see a flickering during a rapid eye movement, but more of a pulse.

krabs, it took me a minute to figure out that you meant modes. I love that fact because it allows me to take photographs of fast moving objects, aka mosquitoes, with so-so results, or fan speeds. I only like pwm when I have a use for it (aka not in a flashlight, unless it is a disco mode)

Do da math.: I = C * dV / dT. C = I * dT / dV. Assume dT = 1/200 Hz, dV = 0.1V, I=3 amps -> C = 150,000 uF

You ain’t gonna get a 150,000 uF cap into a light…

regardless of what the math says, capacitors help even if a little. I stuck a 1000uF capacitor in a shadow bike light that had a moderately annoying flicker and it improved to a ‘not that annoying anymore’ state. :slight_smile:

(I recently upgraded said light with a Dr Jones Mobydrv :smiley: )

http://www.alliedelec.com/search/productdetail.aspx?SKU=70316416#tab=overview

?

There was something with a limited amount of charges/discharges of these ultracaps equivalent to a few minutes flashlight runtime. And there has been a thread about this question before.

just found it

Interesting thread.

Unfortunately with today’s technology, storing that potential energy between duty cycles isn’t plausible.

It might work when you’re dealing with very high frequencies at many MHz, but you won’t notice it at all even without a cap.

Those caps are designed for memory backup and cannot supply significant currents without damage…

Instead of trying to smooth out the PWM with a capacitor which would offer little if any real world benefit, it’s better to just get a high frequency board. The NANJG linear boards like intl-outdoors has work amazingly well. Invisible PWM only really visible if you’re shining it on a fast rotating fan and even then it’s subtle. If even that bothers you I’m surprised you’re not constantly complaining of flickering LCD monitors and TV’s. But that’s the better way of dealing with PWM, raise the frequency vs trying to shoehorn a bandaid solution to make work.