How to wire a potentiometer for use dimming multiple LEDs?

Thread Starter

birckcmi

Joined Jan 1, 2018
135
Hello, guys n' gals, I am trying to salvage an LED light panel, which includes an unknown number of LEDs, but is rated on the back at 5 volts, 3 watts, meaning 1.7 Amps (yes, you know that). The crappy off-on-dimmer switch on the panel never really worked, so I got a 25-ohm 5 watt pot to use as a dimmer. I bypassed the switch circuitry to wind up with two wires, red and black, supposedly going to the panel's LEDs. My questions are: will this pot work as a dimmer for the panel, or do I need a pot with a different capacity, or just a rheostat, and if the pot will work, how do I wire it to the two wires coming out of the panel and to the wall-wart?
 

KeithWalker

Joined Jul 10, 2017
1,307
Hello, guys n' gals, I am trying to salvage an LED light panel, which includes an unknown number of LEDs, but is rated on the back at 5 volts, 3 watts, meaning 1.7 Amps (yes, you know that). The crappy off-on-dimmer switch on the panel never really worked, so I got a 25-ohm 5 watt pot to use as a dimmer. I bypassed the switch circuitry to wind up with two wires, red and black, supposedly going to the panel's LEDs. My questions are: will this pot work as a dimmer for the panel, or do I need a pot with a different capacity, or just a rheostat, and if the pot will work, how do I wire it to the two wires coming out of the panel and to the wall-wart?
If you connect the 5VDC directly across the LED panel, you will probably blow all of the LEDs. The current through them must be limited to a safe value. This is usually done using series resistors but unless you can tell us how the LEDs are connected together and whether there are any resistors built into the display, we can only guess at a solution.
The potentiometer does not sound like a good solution and you may destroy the LEDs if you experiment with it.
Regards,
Keith
 

OBW0549

Joined Mar 2, 2015
3,566
Got that backwards. 3W ÷ 5V ≠ 1.7A
I'd love to know how he ever figured that current equals voltage divided by power...

We're going to want to see a wiring diagram of how you are thinking to hook this stuff up.
Plus, it's important to know whether the salvaged panel contains only the raw LEDs, or the LEDs plus some sort of active or passive current limiting or regulating circuitry. Depending on the answer, putting a rheostat in series with the panel may or may not give acceptable results.
 

Bernard

Joined Aug 7, 2008
5,686
Your pot connected as a rheostat may give about 60 % dimming, connected as a pot. just pot dissipation is about 1 W & all dimming in first
90 deg. , but if you add a resistor of around 39 ohms, bottom of pot to common, loss about 4/10 W & full rotation. If you had a 100 ohm pot to sub. for R then could select light cutoff R. Keep new pot or replace with closest value.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
135
I'd love to know how he ever figured that current equals voltage divided by power...


Plus, it's important to know whether the salvaged panel contains only the raw LEDs, or the LEDs plus some sort of active or passive current limiting or regulating circuitry. Depending on the answer, putting a rheostat in series with the panel may or may not give acceptable results.
OK, I guess I've stuck my foot in it already. Thanks for correcting my math. Duh. At last now I know. I can't tell you how the LEDs are wired, but I was able to disassemble the panel a bit more and get a shot of them. I hope you can tell something from this; I can't tell if the LEDs have attached resistors, or even whether in series or in parallel. I hope this tells you something.LEDs_1.jpgLEDs_1.jpg
 

Thread Starter

birckcmi

Joined Jan 1, 2018
135
Your pot connected as a rheostat may give about 60 % dimming, connected as a pot. just pot dissipation is about 1 W & all dimming in first
90 deg. , but if you add a resistor of around 39 ohms, bottom of pot to common, loss about 4/10 W & full rotation. If you had a 100 ohm pot to sub. for R then could select light cutoff R. Keep new pot or replace with closest value.
Let me see if I can restate that: I should be able to achieve 60% dimming with this 25-ohm pot as opposed to a rheostat, but all the dimming will only use the first 90˚ of rotation. If I add a ca. 39 ohm resistor, connected from the wiper contact to terminal 1 (right), there would be some power loss, but I would get full pot rotation. If I used a 100 Ohm pot instead of the current 25 ohm pot, I would be able to turn the whole thing all the way off, rather than just dimming it (?) Is that correct?
 

sghioto

Joined Dec 31, 2017
1,562
The effective resistance of the panel is 8.33 ohms. A 25 ohm pot set at 25 ohms in series would drop 1.25 volts across the panel so the LEDs should be off and dissipating .56 watts across the pot. Probably work OK but a 10 ohm 3 watt pot would be better. Also depends on the color of the LED strip.
 
Last edited:

Bernard

Joined Aug 7, 2008
5,686
You might try using the 25 ohm pot as a rheostat,+ 5 V to one end, panel
+ to wiper. I tried a 10 ohm R in parallel with a LED & 100 ohm R, a poor substitute for panel, but it indicated full dimming.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
135
Let me see if I can restate that: I should be able to achieve 60% dimming with this 25-ohm pot as opposed to a rheostat, but all the dimming will only use the first 90˚ of rotation. If I add a ca. 39 ohm resistor, connected from the wiper contact to terminal 1 (right), there would be some power loss, but I would get full pot rotation. If I used a 100 Ohm pot instead of the current 25 ohm pot, I would be able to turn the whole thing all the way off, rather than just dimming it (?) Is that correct?
You might try using the 25 ohm pot as a rheostat,+ 5 V to one end, panel
+ to wiper. I tried a 10 ohm R in parallel with a LED & 100 ohm R, a poor substitute for panel, but it indicated full dimming.
Is that " I tried a 10 ohm rheostat-or resistor (?) in parallel with an LED & 100 ohm resistor-or rheostat? So far, the +5V to one end, panel + to wiper looks fine, but I want to make sure what else is possible. I'll post wiring diagrams before I plug anything in.
 

Bernard

Joined Aug 7, 2008
5,686
I retried post # 6 with a 3W LED. 100 ohm pot connected from bottom of 25 ohm pot to common gave full cutoff of light when set to 20 ohms. 25 ohm pot then gives full range of dimming. Results are slightly skewed from the large number of clip leads. First light @ 2.3 V, 3.2 V at 500 ma, losses too great to reach full 600 mA.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
135
The LEDs look yellow like a white LED, that has a forward voltage of about 3.2V.
OK, here's a schematic, at figure A, of a circuit I found on YouTube that uses a 20K-ohm pot preceded by a 510-ohm resistor to control the brightness of one LED. Figure B is my proposed parallel circuit, based on fig. A, to control the LEDs in the light panel. Working from Bernard's comments, I included the resistor at omega-?. In the YouTube example it was-I think- 510 ohms, but here Bernard has suggested a 100-ohm pot instead. I don't know how that should be wired. Any comments, drawings and suggestions will be appreciated. lightbox LED circuit.jpglightbox LED circuit.jpglightbox LED circuit.jpglightbox LED circuit.jpg
 

Bernard

Joined Aug 7, 2008
5,686
Almost there. Interchange #1 & #3 in figure A. A & B are the same #3 is 25 ohm pot, #1 to be determined, around 20 ohms.
.
 

ElectricSpidey

Joined Dec 2, 2017
1,336
If you are going to use a pot to control LED brightness, I would suggest using an Emitter Follower circuit and let a transistor take the heat instead of the pot.

That way there is no need to worry about the pot wattage, and a complete range is available.

If you are interested tag my screen name and I will post a schematic.
 

sghioto

Joined Dec 31, 2017
1,562
First I think you should go ahead and connect the panel to a 5 volt supply and see what color the LEDs are when lit. Then try the pot in series with the panel. Connect one end of the pot to 5 volts and the middle terminal to the panel. This should work but you probably won't get the best results .
 

Bernard

Joined Aug 7, 2008
5,686
A quick sketch to confirm your schematic. If you have a second pot of 25 to 100 ohms for #1, adjust #3 for minimum light, adjust # 1 for light off .
Keep #1 or measure resistance & sub, a 1/2 W R, Prob. 18, 20 or 22 ohm. Pots are rear view.Scan.jpg
 

Thread Starter

birckcmi

Joined Jan 1, 2018
135
First I think you should go ahead and connect the panel to a 5 volt supply and see what color the LEDs are when lit. Then try the pot in series with the panel. Connect one end of the pot to 5 volts and the middle terminal to the panel. This should work but you probably won't get the best results .
The light panel did function for a while, and the LEDs are white.
A quick sketch to confirm your schematic. If you have a second pot of 25 to 100 ohms for #1, adjust #3 for minimum light, adjust # 1 for light off .
Keep #1 or measure resistance & sub, a 1/2 W R, Prob. 18, 20 or 22 ohm. Pots are rear view.View attachment 220099
Bernard, thanks for the schematic. I will do a front-view version for your approval to see if I have the right idea.
 
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