Furqan Tariq

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10 years, 1 month ago.

PWM range reduced after optocoupler

I am using STM32F4 to generate a PWM signal to control a DC motor. I have attached an optoisolator/optocoupler to isolate my MCU and the driving circuit. the optocoupler (4N35) is attached to H bridge (L293D) to drive the DC motor.

My simulated design on proteus works fine (I have just provided a pulse input to the optocoupler...no controller used in simulation) as I can observe motor speed decreasing as modulation is decreased. However, my practical implementation has me baffled. Somehow my PWM range has been reduced, varying the modulation in the region of 3% to 6% the DC motor speed varies from 0 to full.

However, if i directly connect my microcontroller to the H bridge, the modulation is as expected.

Is there a flaw in my design? (I do not know why simulation is working). Also, my PWM signal is of 20kHz...is that causing the optoisolator to behave funny?.

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1 Answer

ceri clatworthy
10 years, 1 month ago.

you might have a SLOW opto-optocoupler,

Assuming you have fitted these values, it should be OK,

you could try putting the opto on the Hi-Side (in place of R2)

dont forget you have basically INVERTED the signal from micro ..

led = on, -> Transistor = on, pulling Collector LOW

Hope this helps

Ceri

Accepted Answer

It helped! My range of pulse modulation has increased but there is still a little problem...at low modulation like 12.5% or so the DC motor stops and starts generating a high pitched noise.

posted by Furqan Tariq 09 Apr 2014

the high pitch noise is most likely not enough current (from voltage).. sensor resistance too high?

or too high PWM frequency. Ceri

posted by ceri clatworthy 09 Apr 2014

I actually decreased the frequency around 5kHz.

posted by Furqan Tariq 09 Apr 2014