LED Current Limiting Resistor

I have a LED desk light I am modifying to control by Wifi. The original Power supply is marked 12V 0.43A. I need to use a different power supply since the exiting one does not have enough current for the LEDs and the ESP32 (LEDs flicker).

I have 12V 1A Powersupply. There is a 3-ohm resistor in-line to the LED array. I don't have any info about the model or forward voltage for the LED array, but if I'm increasing the supply amp capacity, can I assume proportionally I'd need about a 7-ohm resistor? Is a 2W resistor sufficient? The existing resistor is 2W based on size (14mm), but going from 5W to 12W I'm thinking I need something bigger. The resistor heats to about 101F with the 0.43A power supply,

by jn3va3
April 24, 2022

Since you are not changing the voltage supplied, the LED and its associated circuitry will not know the difference. The amp capacity tells you what the power supply is capable of supplying and has nothing to do with the amps drawn by the load. The only caveat is that the power supply amp rating must be greater than connected load amp rating.

by Foxx
April 24, 2022

That's what I thought I understood, but when I attach the 12V 1A power supply the LEDs get noticeably warmer (not hot) and the Resistor gets up to about 185F so I assumed I needed a larger resistor.

If I put an Amp meter in the circuit when the original powersupply is attached it reads .26A When I use the 1A supply it reads 0.66A

I then added a volt meter thinking maybe that supply was putting out a much higher voltage. The voltage of the 1A supply is a little more. Its reading 13.5V (labeled 12V)

I also tried one labeled 12V 1.2A with similar results its voltage is 13.8V

What causes the increase Amp readings?

I would have expected the Amp to draw a little more with the extra voltage (0.4A) but why am I seeing 0.6A - 0.7A?

Original Supply

Labeled :: Meter Readings

12V 0.43A :: V: 12.3 A: 0.36

Other Supplies

Labeled :: Meter Readings

12V 1A :: V: 13.5 A: 0.65

12V 1.2A :: V: 13.8 A: 0.7

by jn3va3
April 24, 2022

" when I attach the 12V 1A power supply the LEDs get noticeably warmer (not hot) " This is just what I would expect. With the original power supply the voltage was lower, which caused the flickering; now that you've raised the voltage the LED current is higher so they get warmer (and brighter). And don't forget that inserting the ammeter puts in its own voltage drop which will depend on the type of ammeter. LEDS are non-linear devices so the current they draw is not a nice linear function of the voltage applied. At low voltage they have a high resistance but as you increase the voltage the resistance falls. If the resistor is getting too hot you could replace it with a higher wattage but same resistance.

by Foxx
April 25, 2022

Thank you Foxx! I'm understanding this much better now. Appreciate your help!

by jn3va3
April 28, 2022
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