Need to power a Bi-colour LED strip with VOLTAGE-based dimming.

Thread Starter

-Ty-

Joined Feb 5, 2017
83
Hello again everyone,

I am hoping to build an LED panel for use in photography. As such, I need to be able to control the brightness of both colours of LED chips on the strip.

I'm looking at the following LED strip.

The "claimed" specs are:
  • Voltage: 24V DC
  • Current: 4A/roll (@ 600 LEDs per roll = 6.6 mA per 5050 LED??)
  • Power: 96W /roll
I need some way of driving this LED strip. While there is a pre-made driver available, I believe it uses PWM dimming (since in one of the product images, it shows the brightness control button a being "Speed/Brightness"), which isn't suitable for photography.

Now, I know (roughly) how to build a voltage-dimming circuit for a SINGLE colour led strip, based around amazon-available voltage regulating boost converters. However, the dual-colour nature of this setup is throwing me off, because I don't know how to set up a potentiometer to gradually shift the delivery of power from one led colour to the other.

I have an understanding of basic circuits and terminology, but I am no electrical engineer, so :p

Thanks in advance for any help!
-Ty
 

wayneh

Joined Sep 9, 2010
16,102
You still want to use PWM dimming, but with the addition of a low-pass filter. That will average out the current applied to the LEDs so they will glow continuously. You may be able to find a pre-made driver with this built in.

But, are you certain that PWM dimming won't work? Some schemes use high frequency, say 100kHz. Even if your exposure time is 1/1000 of a second, the LEDs will flash 100 times during your exposure. You might get 99 or 101, but is that really a problem?
 
Last edited:

Sensacell

Joined Jun 19, 2012
2,450
Analog dimming of LED strips is rather difficult - if you expect linear control over a wide range of intensity.
It seems simple - "just vary the voltage" but there are many subtle pitfalls that make this a crappy way to dim LED's.

I agree that choosing a PWM frequency higher than the fastest exposure time is the way to go.
 

Thread Starter

-Ty-

Joined Feb 5, 2017
83
I REALLY don't like the idea of any flicker, but I've been convinced to use PWM by reading your comments, and looking at some other sources.

Now though, I need to figure out what kind of PWM controller to use.

I do NOT want to hear even the faintest buzz out of the dimmer, so I need to stay about the 25 khz PWM range, correct?

In that case, all i seem to be able to find is something Like This, or the much cheaper style of PWM dimmer, Like This and Like This.

That being said, the video I'm basing my design off of used a 12khz dimmer, and he didnt mention anything about buzz... but just because he didnt mention it, doesnt mean its not there, right?
 

dendad

Joined Feb 20, 2016
3,025
If you don't want PWM, use a variable constant current supply, not variable voltage.
An analog constant current regulator can be made for those power levels but will no doubt run pretty warm.
 

wayneh

Joined Sep 9, 2010
16,102
That being said, the video I'm basing my design off of used a 12khz dimmer, and he didnt mention anything about buzz... but just because he didnt mention it, doesnt mean its not there, right?
That frequency would not be described as a buzz. If you can hear it at all (I cannot, nor can anyone over 50 or so), it's more of a high pitched hiss. But the problem with 12kHz, compared to a higher frequency, would more likely be the flashing. A short exposure might catch, say, 3 flashes one time and 4 the next. That's 33% more light on the second photo and you will likely see that in the results. The difference between 10 or 11 flashes (because it's flashing faster during the exposure window) is less of a problem.

One thing about analog, constant current control you should be aware of: The color produced by an LED changes slightly with current. One advantage of PWM is that the LED is always "on" at the same current and brightness, and this produces the same color even as the "on" time is varied.
 

dendad

Joined Feb 20, 2016
3,025
One thing about analog, constant current control you should be aware of: The color produced by an LED changes slightly with current. One advantage of PWM is that the LED is always "on" at the same current and brightness, and this produces the same color even as the "on" time is varied.
That is a good point I had not considered.
Maybe having the LEDs synced to the camera would be a way to go?
 

Thread Starter

-Ty-

Joined Feb 5, 2017
83
The colour shift of non constant-current power supply to the LED's doesn't matter at all though, given that I would be dialing in the colour to exactly what I want it to be. After all, I'm using a bi-colour strip with two dimmers so that i can mix the cool white and the warm white to get the overall colour that I want. If the colours shift as the current drops with an analog solution, that doesn't matter, because I would just stop dimming it once it reaches a colour and brightness im happy with.

Also, yes, granted, its a whine, not a buzz, but i can definitely hear it clearly in many of the electronics around me, even from their ghost-power consumption when they're off.

at 25 khz, thats 25000 flashes per second, If I used a 1/500 shutter speed (which is much faster than i would ever actually), i would have 50 strobes in that 1/500th of a second. If it was 51 or 49, that's only a light difference of 2%, which will be imperceptible to the camera.

So, of the 25 khz pwm dimmers I linked, which would be the best?
 

wayneh

Joined Sep 9, 2010
16,102
The colour shift of non constant-current power supply to the LED's doesn't matter at all though, given that I would be dialing in the colour to exactly what I want it to be. After all, I'm using a bi-colour strip with two dimmers so that i can mix the cool white and the warm white to get the overall colour that I want. If the colours shift as the current drops with an analog solution, that doesn't matter, because I would just stop dimming it once it reaches a colour and brightness im happy with.

Also, yes, granted, its a whine, not a buzz, but i can definitely hear it clearly in many of the electronics around me, even from their ghost-power consumption when they're off.

at 25 khz, thats 25000 flashes per second, If I used a 1/500 shutter speed (which is much faster than i would ever actually), i would have 50 strobes in that 1/500th of a second. If it was 51 or 49, that's only a light difference of 2%, which will be imperceptible to the camera.

So, of the 25 khz pwm dimmers I linked, which would be the best?
Since the cheapest one ought to work, I’d probably give it a try. You might learn that it doesn’t hold up or could be better in some way, but on the other hand it might be fine.
 
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