Which potentiometer?

[JLKDS]

Hello all,

I am trying to find the correct potentiometer to dim LEDs. I am aware that PWM should be used for efficient and linear dimming, but in the name of speed and simplicity I would like to just use a potentiometer.

The LEDs will draw a maximum of 90W and are 12V. Will I need a potentiometer with a power rating of 100W? What should the maximum resistance of the potentiometer be to give dimming over the full range?

Many thanks,

Peter

 

[JLKDS]

In other words, how would this one work?

http://www.lightsworld.co.uk/p/Saxb...iliate=gbase&gclid=COS_osOTrrUCFabLtAodQFgA8A

 

[Skogsgurra]

That one is either for a lot less power than you need (housing is too small for the needed dissipation) or it is PWM - which I think is the case.

I cannot find any specifications in that ad. Nor any connection diagram. There are too many terminals for a simple series potentiometer. You will have to dig out the specs before buying.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JLKDS]

Hi Skogsgurra,

Thank you for your reply. I have found a PDF with the product spec

http://www.saxbylighting.com/instructions/40648.pdf

 

[JLKDS]

And I agree, I think it uses PWM which I imagine is far more involved for me to replicate than can be done in a day. Anyone have any suggestions?

Cheers

 

[Skogsgurra]

How about buying one of those?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LiteYear]

If you really want to go down the potentiometer route, it's going to be tricky with such a high power LED. Here's what you need anyway. First double check the "forward" voltage drop of the LEDs. 12V sounds very high, but then again, 90W is huge for a LED. Most likely there's quite a few LEDs bundled together. Once you're very sure of the voltage, V, and power, P, of the LED, calculate the current at full brightness as I = P/V. Using your numbers that gives I = 7.5A.

Now take your power supply voltage, say 18V, subtract the LED voltage and divide by your max current to find your resistance at maximum brightness. Eg. R = V/I = (18-12)/7.5 = 0.8 ohms. Now to figure out the power rating of your potentiometer, take the square of the max current and multiply it by the resistance: P = I*I*R = 7.5*7.5*0.8 = 45W. Adjusting the potentiometer to a higher resistance will dim the LED (though it wont necessarily be particularly linear!).

If you're still keen to proceed, consider a few practical matters:

1) a 45W resistor will be very large (at least as big as your hand) and get very, very hot
2) trying to make a variable resistor that is accurate down to 0.8 ohms is tricky - this is a pretty specialist area that might be hard to find off the shelf
3) as resistors get hot, their resistance (usually) goes up. While this good from a stability point of view, is does mean your brightness will tend to wander.
4) at such low series resistances, the internal impedance of your power supply will likely be significant. Might just mean your potentiometer needs to go a bit lower than the simple calculation above gives.
5) Using the numbers above, you'll need at least a 135W power supply. That's a very big, low voltage DC power supply.

I appreciate you may have your own motivations and requirements, and it's certainly possible to do it, but don't neglect Skogsgurra's suggestion - it has some fairly serious practical advantages!

 

[mcgyvr]

100W pot... ha ha ha
how silly and a great way to turn "green" into "black"...
Buy a driver that accepts a simple 100k pot across a dimming input on the driver..

Here "speed and simplicity" just means you don't know what the heck you are doing.

Please tell me you are at least using a constant current driver?
How many LED's?
Forward voltage of LED's?
Current rating of LED's?
Series or parallel strings?
Got a schematic?

 

[VEBill]

"...The LEDs will draw a maximum of 90W and are 12V. ..."

I wonder if this is for off-road LED light bar; ([sub]probably not[/sub]) coincidentally 90 watts seems to be a common product spec for such off-road LED light bars. If so, then being Green is not a requirement.

Still, using a potentiometer is not the right approach.