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Triple trouble - emitters not equally bright (self.flashlight)

submitted 2 years ago by toadiesss to c/flashlight

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Hi All,

I recently built my first 2x S2+ triples with KD SST20 MCPCBs and Convoy 5A drivers. On one of them I noticed, when at 1%, 1 emitter is noticeably dimmer than the other 2. My second triple I changed the sense resistor to provide slightly more amps and that one is fine.

Anyone experienced this? Could it be a bad mcpcb, driver/level issue?

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[–] Adair21 9 points 2 years ago (1 children)

It could be a bad reflow, but it's common for one to be brighter at moonlight because of forward voltage tolerances

[–] dmenezes 2 points 2 years ago (1 children)

common for one to be brighter at moonlight because of forward voltage tolerances

Interesting, I would never have guessed. Glad my TS10 doesn't show that even at level 1 brightness.

[–] toadiesss 3 points 2 years ago* (last edited 2 years ago)

Experience like this is why this community rocks. Intrigued by Adair21’s answer, I read around the web to understand better - I didn’t realise there could be so much tolerance in LED forward voltages!

As a result, when in parallel/at the same drive voltage, different currents drawn = different relative brightness. Outside of independent resistors, you’d need to get lucky with matched emitter tolerances or just run above moonlight so that the difference is less noticeable.

Maybe it’s a reflection of the cost of the KD boards?