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Case Study: Incredibly short life cycle of LEDs, requesting advice

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billbyrd1945:
I bought these cheap flashlights from Amazon (a ten pack for $19 (with batteries)). I bought them purposely so I could use their innards to make a 3d printed light source that would be powered by USB. The 3-pack of alkaline batteries measures 4.9v. My USB adapter outputs 4.8 volts. The flashlights have an array of 9 LEDs in a circle with one in the middle. After only about 48 hrs of use, all but two LEDs have burned out. Are these simply inferior LEDs, or, is there something about my design that's killing them? What can I do to extend the usability of my light source? Do I need a resistor? What value? I appreciate any help.

TimFox:
If your USB voltage source is "stiff" (relatively low source resistance), there is nothing to limit the current in the LEDs, which will increase with temperature at a constant applied voltage.
The internal resistance of alkaline batteries is perhaps sufficient to not kill the LEDs, but is not a particularly elegant design.
See  https://www.rohm.com/electronics-basics/leds/led-characteristics  They state that the forward voltage drops with increasing temperature at a constant current:  that means that the current at constant voltage increases.  Normally, LEDs are never driven from a constant voltage.

james_s:
A lot of these very cheap flashlights do rely on the internal resistance of the batteries to limit the current. The solution is to buy better quality flashlights that use a single LED and a proper driver circuit.

mikerj:
They are simply garbage. I've had a had a couple of the same design flashlights (though aluminium case) and the LEDs are low quality and very overdriven, both of mine had several failed LEDs after very little use.  Even whilst the LEDs were working the grounding between the PCB and the aluminium case was very poor and caused flickering on mine.

BrokenYugo:

--- Quote from: james_s on January 17, 2022, 07:47:47 pm ---A lot of these very cheap flashlights do rely on the internal resistance of the batteries to limit the current. The solution is to buy better quality flashlights that use a single LED and a proper driver circuit.

--- End quote ---

This, even if there is a resistor it will be sized with the droopy, probably carbon-zinc battery power in mind.

I'll hazard a guess that these cheap flashlights are the same blueish white LEDs that you buy from China for like $0.01/each in a bag of 100 or 1000 or whatever, probably rejects/seconds, unsuitable for anything serious.

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