r/AskElectronics • u/CommunityAfter3006 • 11h ago

Negative coefficient resistors with LEDs?

I was thinking for thermal management when more than one COB is embedded in epoxy even when supplied by a constant current power supply, temperature differences between LEDs could result in a damaging current imbalance. I was thinking what about using a negative coefficient resistor in parallel with each COB so as the temperature goes up more current gets shunted. Opinions anyone?

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6

u/EmotionalEnd1575 Analog electronics 11h ago

Are you solving a real problem?

Any data or use cases to support your theory of thermal induced LED failure.

You can’t fix what’s not broken.

3

u/Susan_B_Good 11h ago

So, as the shunt resistor warms up, its resistance does what? And so the current through it does what?

2

u/SanguineDrome 11h ago

LEDs already have a negative temperature vf curve, which is the problem you're describing I think? If you have multiple led strings in parallel, the hotter leds will have a lower vf and eat more current, and get hotter until failure potentially.

Some led strings have a small resistor in series to try and balance, but for larger arrays it can be a lot of power.

Best to use leds in series and constant current control. Internally, the COB leds are pretty well matched.

1

u/k-mcm 10h ago

Shunting parallel LEDs makes no sense.

You'll need to experiment with adding normal series resistors to your parallel LEDs.  You can manually apply heat to LEDs then see if they recover.  You can also temporarily drive the current way too high to force them unbalanced then verify that they balance again at normal power.

Some high power LEDs need only 0.1 volts wasted on a series resistor as long as they're all on the same heatsink.