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A project log for Make a cheap bike light better!

Hardware and software improvements to a £10 LED cycle light

RobGRobG 11/20/2018 at 16:280 Comments

Took the old inductor off and measured it on an LCR bridge at about 49uH. I then wound a new one using thicker wire on a slightly larger core, again aiming for 49uH. I now realise that the inductor isn't getting warm, it's the MOSFET. Using a thermal camera I could tell that the MOSFET case was hitting 90°C and rising after about 20 secs on full power.

I've increased COFF (by adding 39pF in parallel with the 33pF that was there) to halve the switching frequency. This means the MOSFET spends half as much time in its not-quite-off-not-quite-on state where it dissipates heat. Seems to have helped, as it's now starting to level out at about 70°C. However, the mean LED current (whilst on) is also slightly down now at about 2.25A, plus duty is 47%.

From Cree datasheet, that puts the average light output at about 1300lm. A bit better...

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