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

[cheater cheater]

On Mon, Mar 29, 2010 at 18:01, Tom Wiltshire <tom at electricdruid.net> wrote:
> 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!

I think this would still generate sub-harmonics

Cheers
D.

> 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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