Schematics for the LED PWM Dimmer using LM339 Picture is courtesy of:

LED PWM Dimmer using LM339

PWM dimmer using a LM339

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Generating a PWM waveform is extremely simple! This page demonstrates a circuits for doing this and discusses some of the considerations. All of the circuits are d.c. input to p.w.m. out.

The first example uses the standard op-amp oscillator circuit to generate a triangular waveform which is level-shifted and fed to a comparator (e.g. LM339) to give the PWM waveform. Most people will want to use two comparators rather than one op-amp and one comparator so that is what we show here.

The triangle wave oscillator is formed around Cp1. Consider the initial state with C1 uncharged. The comparator's - input will be low, so the comparator's output will be high (off) and C1 will charge (from the comparator's output) via R2 and R1. The comparator's +ve input will be biased to about 2/3 of the supply voltage (Vcc) since R5 is, in effect, in parallel with R3.

So, when C1 charges above 2/3 of the Vcc, the comparator will turn on, its output going low. R2 will now start to discharge C1. But R5 is now effectively in parallel with R4, so the positive input is only biased at about 1/3 of Vcc

It should be clear that the comparator will again change state when the voltage on its + input falls below 1/3 Vcc and the waveform on C1 will be a triangle waveform rising between 1/3 and 2/3 Vcc.

C2, R7 and R8 form a level shifter. Ideally the negative tip of the waveform should fall to very nearly zero volts - just enough so that a zero input will always cause Cp2 to turn fully off, so the circuit may take a bit of fiddling to get the exact levels, or you have to accept a small dead band at zero speed, which is not usually a problem.

The triangle wave is slightly non-linear because of resistive charging and discharging between 1/3Vcc and 2/3Vcc. Non linearity can be reduced by making the thresholds closer to 1/2Vc: for instance increasing R5 to 200K would reduce the triangle waveform amplitude. You would also have to alter the level shifting appropriately!


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led_pwm.gif Schematics for the LED PWM Dimmer using LM339