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Topic: pwm a lm317? (Read 2300 times) previous topic - next topic

winner10920

Hey I just wanted to see if anyone has pwm' d a lm317 as a way to dim a highpower led, I imagine the lm317 setup as a normal current regulator, with a fet switching the power to the lm317
or is it better to switch the power at the led? Or the entire ground?

Grumpy_Mike

Yes you can switch the power to it with a p-channel FET or switch the ground with an n-channel logic level FET.

winner10920

Awesome, so it doesnt mind the fast frequency at all?

Grumpy_Mike

I didn't say that but by and large no.

pwillard

Probably best if you run PWM through an Resistor Capacitor  Filter.

Grumpy_Mike

Quote
Probably best if you run PWM through an Resistor Capacitor  Filter.

In this configuration I would say not because the LM317 is trying to make the current constant so if you applied a variable resistance in the circuit (which is what a smoothed PWM would be) the LM317 would up it's voltage to try and maintain the constant current.

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