[sdiy] PWM noise in analog ground in Optigan-like system

[Tom Wiltshire]

Hi Derek,

Am I right in thinking that the motor is powered off this same power  
supply?

If you've got the TIP120 switching the motor on and off at the PWM  
rate and the motor is on the same power supply as everything else,  
I'm not surprised you've got noise. It's a high(ish) current load,  
and switching it rapidly on and off is going to put spikes on the  
power rails. I don't think the problem is specifically the PWM, since  
I've used PWM lots and not had problems with it bleeding through to  
the output - rather the problem is the load.

I've seen references to spikes caused by the inductance of motors  
when the power is turned off, and diodes across the motor to help  
prevent this. I understand the gist of it - a motor is dynamo once  
the power is turned off, so if it's still moving (which it is) it'll  
generate voltage, but you'd need someone better versed than I with  
motor control electronics to tell you if that is the issue.

Could you try an oblique strategy, like change the PWM frequency to  
be outside the audio range? If you could get it up to 30KHz, all your  
problems might magically disappear. That'd be my (typically hacky)  
approach!

HTH,
Tom


On 29 Mar 2010, at 12:27, Derek Holzer wrote:

> Hi folks,
>
> the system I am building uses a PWM motor controller (Arduino 
> +TIP120) as well as an op-amp buffer for a phototransistor. Think  
> of it a bit like an Optigan... technical details here: http:// 
> www.umatic.nl/tonewheels_technical.html
>
> I've been running the PWM controller and the op-amp buffer from  
> separate wall-warts for some time now, but this week I wanted to  
> combine everything into one box with one power supply. And, of  
> course, I can hear the PWM coming through the + and ground lines to  
> the op-amp.
>
> I tried using a separate LM7809 voltage regulator with decoupling  
> caps (100 uF between supply and ground, 100nF between regulated and  
> ground) for the audio portion, but enough noise still comes through  
> the ground alone to be heard (noise can be heard with +  
> disconnected). And it still fails to remove the noise from the +  
> side as well!
>
> I also tried small ferrite beads on the + and ground lines. No  
> noticeable difference.
>
> Short of plugging two different wall warts into the same box, one  
> for the PWM and one for the audio, what can I do to remove this noise?
>
> How far back in the power path do I need to separate the powers and  
> grounds of the two systems? For example, at the AC/DC conversion  
> stage?
>
> Thx+best!
> Derek
> -- 
> ::: derek holzer ::: http://macumbista.net :::
> ---Oblique Strategy # 8:
> "Accretion"
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