Ways to dim leds without using PWM

Maybe there are other threads on this topic, if so, feel free to point me at them.

My question is how can I dim say..oh...an array of 100 5mm leds. Would preferably be controlled by a small pot that could swept from 0-100% output.

The flickering that comes from pulse width modulation is an undesireable effect that would render the device somewhat useless for the intended needs.

Also, the leds would have to somehow not shift color at lower output levels.

To say it can't be done is not true, as I've personally handled a device capable of it...Just couldn't take it apart.

Any ideas feel free to shout it out!

If you can find a potentiometer that works with the input voltage that should work fine.

if the PWM rate is high enough you wont notice flickering, or you can build a current limitator , but for 100 leds its going to be a little more complicated. These leds are in series? are you grouping them?

For the LM 317 in curent limiting the schematic is pretty simple:

Just hook up the input to the power supply and put a resistor from the adj pin and the output, this resistor sets the desired current using this simple formula: I=1.2/R where I is the current in amps and R is the resistance in ohms.

Keep in mind that the R you use will need to take the heat for the current is flowing thru it, so to be sure, if your current is too high, use hefty resistors.



Have fun


Pablo

PEU said:

if the PWM rate is high enough you wont notice flickering

Click to expand...

what's the max frequency you can use with (5mm) leds?
if it's for photographic purposes, that's far more critical than what you notice with your eye
for stable exposures i'd say you need a frequency of at least 10 times faster than the exposure time
for an exposure of 1/2000s, that would need a frequency of 20,000Hz, would that be possible?

stillphoto, what's the device you have handled?

An adjustable constant current power supply?

EDIT: Sorry, double posting...

PANZERWOLF said:

what's the max frequency you can use with (5mm) leds?

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>10 MHz.

I don't see any objective reason to avoid PWM. It's easy, cheap, efficient, and the best way to control brightness with precision (without tint shift). I think any frequency between 200Hz and 1000Hz shoud be safe for the eyes and for LEDs (they have a typical ton, toff <1µs).

PWM still has tint shift, it is a fallacy that PWM doesn't cause tint shift. The shift is just less.




Efficiency increases with current dimming, but it does not with PWM.






In light of that, PWM is actually less efficient.

Awesome info so far guys, and yes its for photographic/cinematography purposes.

As far as the device I've played with, check out litepanels.com. That is essentially what I want to try to build. Not the big 1x1 foot version, but the smaller brick styled light. It would appear they have some serious stuff going on in that little unit of theirs, or at least they better for nearly $1000.
Previously (over a year ago) I had posted about finding out if anyone could make something like that for me, but now that I'm a bit more wise, and a bit more skilled in soldering, and have a friend that understands circuitry better than myself, I am willing to take a stab at this sorta thing on my own. The only thing I wouldn't be able to do is make it look that slick (especially with the snap on battery pack) but oh well.

You could arrange your LEDs in 10 groups of 10 and switch 10 in at a time to get the desired brightness. You could run each group at a constant current to reduce tint shift.

Paul

Interesting..., very interesting...

chimo said:

You could arrange your LEDs in 10 groups of 10 and switch 10 in at a time to get the desired brightness. You could run each group at a constant current to reduce tint shift.

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the problem is that, together with the output, this changes the quality and softness of the shadows, which are very sensitive to the pattern and spacing of multiple light sources

i remember litepanels.com, i looked into them when they first came out, but lost interest due to the horrendous price
but interesting that they claim "no color shift when dimming" and no one here knows how this could be achieved?
i remember brilliant reviews in photo magazines, so their claim can't be that far from the truth either ...

PANZERWOLF said:

the problem is that, together with the output, this changes the quality and softness of the shadows, which are very sensitive to the pattern and spacing of multiple light sources

Click to expand...

These issues could be partially mitigated by:

- using a less directional LED (think cut-down SMJLED)
- careful spacing of the LED groupings
- adding a diffuser in front of the array

chimo said:

These issues could be partially mitigated by:

- using a less directional LED (think cut-down SMJLED)
- careful spacing of the LED groupings
- adding a diffuser in front of the array

Click to expand...

enough overlap of the led beams is mandatory for something like this anyway, but less directional leds are still the same size single light sources
the softness of shadows doesn't depend on the beam angle of the light source(s), but only on the size and shape of the emitting area and its distance
when using 100 leds, the 100 single shadows will be visible as one, so the size of the light source can be equalled to the whole array
but when you dim down to let's say 1/16 (a range of 4 f-stops) by turning off individual leds, you only have 6 left, which at closer range will produce 6 overlapping, but still distinguishable single shadows, regardless of how you space them in the array, without changing the overall size

the diffuser would work, but with a loss of about 50% output, and an additional loss of throw
on the other hand, when lugging around heavy camera gear, some spare batteries don't make much difference, so the output factor isn't that important ...

and there is still the mysterious litepanels method ...
maybe they use a special wave for PMW?
or their own special leds?

As you can tell by looking at the price, I haven't really had anyone I know say oh sure go ahead and open mine up!
Yeah Panzer is correct about turning off banks of them being a good way to dim but also a great way to get noticeable beam patterns to show up, which is no bueno. Litepanels has definitely enjoyed the fruits of being essentially the first mover in this market to make a quality product. As is usual with bringing a new idea to market, first there were skeptics and not as many people buying them. Now, on the other hand, you'll see these on lots of film sets, the gaffer usually having a few hanging from his/her belt so that if some sort of soft fill is needed immediately...poof... in the time it takes to grab a light from the truck and have power run to it, the litepanel is already in place.
I'm anxiously awaiting some photos from a friend who just got to try out a new led light system for film from a company called led-z (which aparently has no web presence yet). He says they are small and round, have only 3 leds, and put out as much light as a litepanel. This would lead me to believe they are probably luxeon based. They come in both 3200k and 5500k versions and have dimmers built in, but also the ability to be plugged into an external dimmer. Like the panels, they run off of ac and dc power. The plot thickens..

I looked at the Litepanels web site.

They explicitly state that they do not use PWM.

Their brochure also states "virtually no shift in color" wrt dimming. This could indicate that for their output range a colour shift exists but it is negligible. If you run variable regulated constant current but shorten your dimming range you will may get acceptable colour shift.

Your hardest part may be getting LEDs that are from the same tint bin.

I think a solution with high-power LEDs would be better than 5mm LEDs, althought diffusion optics may be harder to find.

chimo said:

I looked at the Litepanels web site.

They explicitly state that they do not use PWM.

Their brochure also states "virtually no shift in color" wrt dimming. This could indicate that for their output range a colour shift exists but it is negligible. If you run variable regulated constant current but shorten your dimming range you will may get acceptable colour shift.

Your hardest part may be getting LEDs that are from the same tint bin.

I think a solution with high-power LEDs would be better than 5mm LEDs, althought diffusion optics may be harder to find.

Click to expand...

I recently built a hardware PWM dimmable RGB light. It's a basic 555 timer+comparator circuit and PWM's the LEDs at 15Khz. Works flawlessly and there's absolutely NO flicker...ever.

I would go for a similar approach and use 1W luxeons (same bin) with 45° optics.

You can even use a LM317 current source between your PWM output and the LEDs to make sure you always get 350mA to your LEDs .

Originally Posted by PANZERWOLF
what's the max frequency you can use with (5mm) leds?


>10 MHz.

Agreed. 5mm LEDs are used in giant video displays and those have to be pulsed really fast to be able to display full motion video without motion artifacts.

now we only need Goldserve to chime in
I don't know that much, but from this thread I'm learning quite a bit !