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LED driver based on LM317 gives more voltage than required? Answered

Hi, I tried to run a 1 watt led with the LM317 in constant current mode. Since the led required 350ma currrent, I used a 3.3+1 ohm resistor in series across the Vout and ADJ. So my resistor series was about 4.3 ohm which should deliver about 290ma current to the led.

The driver was based on https://www.instructables.com/id/Super-simple-high-power-LED-driver/

When powered up, the current across led was around 290ma but the voltage across the led was 4.33v whereas it should be between 3.0-3.6v according to the led datasheet.

What am I doing wrong? Is it safe to run the led at 4.33v ? It was getting hot.


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steveastroukBest Answer (author)2012-08-07

You're doing nothing wrong, and everything right. That's EXACTLY what it should do. You'll find these big LEDs DO get hot - that's why they emit so much light !! make sure its got a heatsink on it.

As the LED temperature increases the voltage across it will change, but the current won't.

Steve

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ethanol47 (author)steveastrouk2012-08-07

Thank you for quick response! Also one more question, is the small star shaped aluminium heatsink that came fixed behind the leds enough? Is that even a heatsink?

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steveastrouk (author)ethanol472012-08-07

No, its a "transfer pad" - it MUST be screwed to a heatsink. We fooled about and made one from an old processor fan !

Steve

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