Dimming Leds and Potentiometers

Looking for a formula;

I have a 24v 1.7 amp power supply ( http://www.allelectronics.com/make-a-store/item/PS-2417/24VDC-1.7A-POWER-SUPPLY/1.html ) to light 2 - 24v led strips ( http://www.allelectronics.com/make-a-store/item/LBR-24/24-WHITE-LED-LIGHT-STRIP-24VDC/1.html ). I want to be able to dim the led's from full power to completely off, but don't know how to calculate what size of potentiometer would give me the right ratio for dimming. IE, I don't want the LEDs to be completely off when the pot is only halfway turned. The urls are links to the exact products I purchased.

Also, the power supply seems kind of large, and I still have to enclose it, so if anybody knows of a better and cheap path to get 24v that involves a little less potential for death, I'm all ears.


lemonie7 years ago
You want to run the power through a suitable transistor (probably heat-sunk), and control that with the potentiometer.
24V won't kill you. Maybe if you bridged the terminals with a metal watch-strap it might give you a horrid-burn...


bctour7 (author)  lemonie7 years ago
Thanks! This led me in the right direction for sure, but I'm running in to another problem.

I'm obviously a new hobbyist, and I haven't worked with transistors until now. I did as much research as I could, and ended up with the LM317 with a 5k potentiometer from adjust to ground and a 470 ohm resistor between the adjust and the output, and it's dimming fine, but if I turn the power on while the potentiometer is at the dimmest point, my leds flash on and off rapidly, and my power supply makes a clicking sound that it usually makes when there's a short somewhere (I assume that's a safety feature). Once I turn the potentiometer full or close to full on, the LEDs go solid, and I can then turn it back down to dim them normally, without any flashing.

Any ideas?

Thanks again
lemonie bctour77 years ago
You'd be right in thinking the PSU cutting in & out is a protective feature. As far as I understand voltage regulators, they're not designed for this - a small-ish voltage drop, but not 50% or more.

bctour7 (author)  bctour77 years ago
Sorry, forgot to add that in addition to my 5k potentiometer, i have another 6280 ohms of resistance. I'm sure this isn't the best way to go about it, and probably where my issues are, but I tried several other combination and this seemed to get me the correct dimming and everything, except for when I first flip on the power.