In search of a non-PWM dimmer for DIY LED video lights

AlexGroove

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(I'm new to decent LED modding, so hope you can bear with a noob haha)
Dear all,

After some research on CPF and video-related forums I got intrested into making my own led floods.
Like the title states I'm looking for a non-PWM dimmer (PWM will cause banding on video) and I found out it will have to be a pot-dimmer. Since I'm not the most skilled in piecing together my own driver I would like to use a regular DC-DC converter. I'm aiming at a 100w led (10x10watt) running off of a LI-ION batterypack (voltage to be determined; anything between 12v or 14.4v/14.8v or 24v would be great)

Hope you guys can help me out (you most likely can haha)

-Alex Groove (a camera guy)
 
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Conte

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Since you are using 10 leds, instead of dimming, just have switches to turn on how many you need.
That way you could use single mode drivers that often will not need to use PWM when running full power.
Or you could go direct drive, maybe.

I think your bigger challenge is going to be keeping all those high power LED's cool.
 

AlexGroove

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Thanks Lucca I'll check in to that a bit more!

Conte, It really has to be dimming as it will for video-purposes. So different banks and switches won't be a viable option.

I'll check into direct drive as I have no clue of what it is haha. The cooling wont be a "big" challenge as I have no problem implementing a fan system, I just have to find a quiet one.

Thanks for the responses so far!
 

Conte

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You'll need more than fans, or no fans at all. A big copper chunk of sort to stick them to. A big enough open air heatsink would work without the need of fans.
This depends on what kind of emitter you go with and how hard you're going to push it.

I'm not sure why you'd need dimming for video.
I got my idea from the lighting I saw when once working on a movie set.
Can't say they would dim there lights, always need as much as you can get.
They had rigs that were exactly that, a bank of lights with a set of switches. I'm the guy who had to climb 30 feet up the latter to turn more lights on, hah.

It makes sense to me anyway, you need more light, you turn on more emitters, you need less, you turn some off.
Meanwhile, you maintain a consistent colour temperature . . . that's more of an incan issue tho'.
At least it makes sense if you're making a single point array.

Direct drive is when you run the LED without the use of a driver. In some cases just a power resistor to bias the LEDs so as to flow the correct amount of current to them. Cree makes a 6v led called the MT-G2.

The problem with non-pwm dimming systems is they are often very inefficient. The power you throttle away from the load gets eaten by the dimmer anyway and burned up in heat. You end up with a dimmer that also needs to be kept cool and wastes your battery power.
 

AlexGroove

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I'm aware I'll need a decent sized heatsink to cool the leds and I have seen some designs of others who have used the whole design of the lights as the heatsink, similar to the CREE truck/quad lights on eBay.

And I know what you mean with the lighting systems without a dimmer, those are halogen/tungsten lights. Dimming in leds is very very VERY common in videoleds, I'll see if I can post the circuitry of my 160-led lights. Small system that uses a 7.4v 18650 pack that works on cameras aswell, it has a rotary dimmer and I know for a fact it's not-pwm because there is no banding whatsoever.
 

AlexGroove

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This is the circuitry of the store-bought light.
At the bottom left you see the while that is (i guess) a potentiometer that is the dimmer.
a9asu5ej.jpg

denazyhu.jpg


(Sorry for the crappy-phone pic, but I have no macro-lenses to do this otherwise.)
 
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kychas

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have you tried to put a capacitor across the LED?
the square wave will be smoothed to a lower RMS voltage and be a saw tooth shape on the top.
try a 5v 10 micro farads cap and see what happens.
 

AlexGroove

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Kychas, the pics are not the led I'm going to build, it's a store-bought led that functions as it should, it's just for reference.

It has to be a linear dimmer, so adding caps to change the PWM to a sawtooth waveform is not a viable option.
Linear dimmers have been done (and are still done) by major manufactors and even custom builder, I think one of the people I took inspiration from is a member here aswell (or a partner of the group they do it in). I just have to find out how they do it (sound like I should pitch this to the Discovery Channel show haha).

I'm planning to do a custom 100w flood with 10x10w leds, and don't have any parts as of yet.
 

DIWdiver

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It is possible to build a highly efficient dimmable switchmode driver with DC output (no significant PWM or other AC waveform in the output). Unfortunately most drivers, especially inexpensive ones, are not built this way.

SOME designs can have a capacitor added at the output to smooth the PWM (some will cease to work properly if you do this). The output ripple would be inversely related to capacitance times frequency, so it is at least theoretically possible to reduce the ripple to a sufficiently low level that it would not produce visible artifacts in video. However, the capacitor may be prohibitively large at the power levels you are talking about.

You might ask George at Taskled if any of his drivers are suitable, with or without additional caps. Before you do though, it might be polite to search around CPF and see if you can find the answer yourself.

If you get desperate, I make a linear dimmable driver that could be used. PM me if interested.
 

DIWdiver

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...However, the capacitor may be prohibitively large at the power levels you are talking about...

I did a quick calculation, assuming 10V, 10A output switched at 50% Duty Cycle (I think this is worst case) with 1000 Hz PWM frequency. If you want to reduce the peak-to-peak ripple current to about 1% of average current, it works out you need around 50,000 uF of capacitance. This assumed a 2 ohm resistive load. Since an LED is not a simple resistive load, this is only a ballpark calculation.

Still, a 100,000 uF 16V capacitor with a ripple current rating of >11 Amps can be had for under $10, in a 35mm dia. x 80mm size. So if you had the right driver, this isn't a totally unworkable solution.
 
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