Which potentiometer for LED lights?

[jwoody (Jeremy)]

Hello all
What size pot do I need to use in a homemade light box to dim them?

I am using 12v 5050 LED's
I believe I need a Linear one as opposed to Logarithmic.
Thank you
Jeremy

[bojan]

What you need is variable voltage power supply with some current capacity (200mA at least, per LED).
How many LED's you have (in parallel)?

[RobF (Rob)]

ebay has some good options. I have these 2 together and work well with 5050 red strip - you might need something more sophisticated if you expect RGB control though:

https://www.ebay.com.au/itm/Black-DC12V-0-100-PWM-Manual-Knob-Dimmer-Switch-for-LED-Strip-Light-AU/123068830365?epid=921984830&hash=it em1ca779529d


And:

https://www.ebay.com/itm/AC-110V-220....c100005.m1851https://www.ebay.com/itm/AC-110V-220....c100005.m1851

[jwoody (Jeremy)]

Thanks Bojan. I don't know yet how many LED I will use. It is a bit of a suck it and see project.

Thanks RobF, I went with that PWM dimmer so see if that does what I need.


Cheers
Jeremy

[mswhin63 (Malcolm)]

Try not to use a PWM controller for a light box. Pulse Width Modulation turns the LED's on and off for periods. This could be picked up by the camera sensor especially if the camera shutter operates quickly.

What is really required is a current limiting power supply which is more expensive but allows the LED to remain on and not switch on and off.

[jwoody (Jeremy)]

Thanks for that.
Just so I know, why can't you use a potentiometer as a variable resistor?
I have seen it in other light boxes and it seems to work ok.
Just curious.
Jeremy

[bojan]

Originally Posted by jwoody

Thanks for that.
Just so I know, why can't you use a potentiometer as a variable resistor?
I have seen it in other light boxes and it seems to work ok.
Just curious.
Jeremy

Potentiometer (unless it is high power type, wire-wound) on it's own can't handle currents involved, because voltage drop across it creates power dissipation, which may be quite high (similar to power consumption of the load - LEDs in your case).
That is why you need more sophisticated equipment.

[jwoody (Jeremy)]

Originally Posted by bojan

Potentiometer (unless it is high power type, wire-wound) on it's own can't handle currents involved, because voltage drop across it creates power dissipation, which may be quite high (similar to power consumption of the load - LEDs in your case).
That is why you need more sophisticated equipment.

Thank you.
Maybe then I should ask the question....


I want to make a simple light box that can be controlled by a potentiometer. What sort of Led's should I use.

[Stonius (Markus)]

I'm looking at this too. I think you need to get an LED driver module because the response of LED's is non-linear. The driver is essentially a (voltage? current?) regulator, I believe.

The key questions AFAIK is how many LED's arranged how (series, parallel) and what is their forward voltage and desired current.

Unfortunately, I still can't quite get there with the definitive answers yet myself.

I'm wanting to do something similar with a box with 4 lots of 4 white LED's but I can't quite make it work yet.

But I *can suggest if you're looking for a diffuser, Lee makes a very good white diffusion (216) that is much less prone to hot spotting than other things I've seen.

Best

Markus

[bojan]

Originally Posted by Stonius

I'm looking at this too. I think you need to get an LED driver module because the response of LED's is non-linear. The driver is essentially a (voltage? current?) regulator, I believe.

The key questions AFAIK is how many LED's arranged how (series, parallel) and what is their forward voltage and desired current.

Unfortunately, I still can't quite get there with the definitive answers yet myself.

I'm wanting to do something similar with a box with 4 lots of 4 white LED's but I can't quite make it work yet.

But I *can suggest if you're looking for a diffuser, Lee makes a very good white diffusion (216) that is much less prone to hot spotting than other things I've seen.

Best

Markus

Intensity of LED is proportional to current.
I am not sure why you need to consider lin/log response...

Controlled current source is all you really need for the purpose.



BTW... for flats a clear blue sky is quite OK.

[Stonius (Markus)]

Originally Posted by bojan

Intensity of LED is proportional to current.
I am not sure why you need to consider lin/log response...

Controlled current source is all you really need for the purpose.



BTW... for flats a clear blue sky is quite OK.

Well as you may have gathered, I don't know much about it, but I read that normal manufacturer's variations can lead to massive variations in current between different LED's leading to cascading failure as first one blows, then the next, etc so the current needs to be regulated either by current limiting resistors and or by limiting the input current. Have I got that right? I know it depends upon series or parallel, but there will be a mixture of both of contructing an array for lighting purposes, I figure? But what do I know?

Markus

[bojan]

White LED has 2.2V forward voltage, for 20mA.


The best is to have couple of parallel strips (with say 6 LED's in series with 100ohm resistor, as current limiter) on 12V.
Then, voltage controlled regulator will do as brightness control.


Have a look at linked webpage for details.

[peter_4059 (Peter)]

have a look at this thread

http://www.iceinspace.com.au/forum/s...&highlight=box

[jwoody (Jeremy)]

Originally Posted by peter_4059

have a look at this thread

http://www.iceinspace.com.au/forum/s...&highlight=box

Thank you Peter