PWM Motor Audio Hum

[mooginnyc]

Hello,
I found a circuit that works well for 12v motor control using PWM driven by an LM324, but I'm now hearing the pulse wave on the audio output.

The motor controller is for a tape player to get variable speed. However, I'm hearing a variable pulse width around 85Hz almost as loud as the audio on the tape.

Putting a diode in series feeding the + of the motor helps, but not enough. A diode and capacitor across the motor doesn't help the hum at all.

I've heard of the pulse being audible in the motor itself, but can't figure out how it's making its way to the output. Could it be because the audio circuit and motor circuit share the same ground and + supply? If so, is there a way to isolate?

I suppose I could try a notch filter, but I don't know how well that would work.

Thanks!

 

[mooginnyc]

A little update.. If I use a separate 12v power supply for the motor and the pwm circuit, it eliminates the hum completely. If I share the grounds, the hum is there, but only slightly.

I have just one 12VDC input for the entire thing, and I have a regulated 9v rail coming off of that for other circuits. I also tried powering the motor and PWM circuit off that 9v rail (used LM317 to get 9v), but the hum is there just as bad.

Is there a magical way to isolate 12VDC from 12VDC?

The other idea, would a notch filter adequately eliminate a single 85Hz variable pulse wave? Or would there be too many harmonics along with it?

 

[Resqueline]

The noise seems to be induced in the ground tracks, so you'll need to read up on "star" (one point) grounding theory and carefully review your connections and layout.
It's not an easy matter to solve without hands-on.
A notch filter is not a good idea for many reasons. Varying overtones among them.

 

[mooginnyc]

Thanks for the reply. I do have the motor control circuit grounded to the chassis for now with an alligator clip. Maybe grounding it directly to the audio circuits or output?

If that doesn't help, would I be able to get an isolated supply by sending 12VAC into, and creating 2 separate rectifier stages? Hopefully I won't have to do that, as it will take a lot to rewire.

 

[mooginnyc]

I tried grounding to the audio circuit, but no luck. Shot in the dark.. could the ceramic cap factory installed on the motor (not sure it's value right now) cause that signal to mess with the ground causing the noise? I suppose I could just snip it and see what happens...

no luck

 

[Resqueline]

There's also the possibility of the noise being induced into the audio preamp due to the pwm wires being routed in different paths going to & from the motor etc..

There are small dc-dc converters with galvanic isolation from 1W & up available on the market, costing 5-10 bucks I imagine. That could be one way of solving the issue.