DPRG mailing list archives: [DPRG] Re: Motor stuff

Mon Apr 2 00:34:19 CDT 2007

Subhobroto Sinha wrote:
> On 3/27/07, Chuck McManis <cmcmanis at mcmanis.com> wrote:
>>
>> >>Ask yourself this question "What happens when the motor has an
>> instantaneous
>> >> load?"
>>
>> > The power supply sags momentarily ?
>>
>> > If so what are the solutions ? Optoisolation ?
>>
>> Often power supply sag can be an issue. There are several options, 
>> one is to
>> put a resistor between the power supply and the motors, this will 
>> limit the
>> current to the motors. Another is to put a diode/capacitor shunt 
>> between the
>> power supply and the logic (the capacitor powers the circuit while 
>> the power
>> supply sags, the diode prevents the capacitor from discharging back 
>> into the
>> power supply.
>
> There is a reason why I was away for over a week.
>
> I (wrongly) suspected that my SMPS were not able to supply enough
> regulation/power/power sag was taking place...
>
> A scope across the microcontroller power terminals told the story...
>
> The moment the motors started a lot of NOISE appeared in the power 
> supply...
You may want to verify that the noise is actually appearing on the power 
supply and not on your scope leads. I've falsely accused my voltage 
regulators for problems only to find that my scope probes were showing 
signals that weren't really there. The easy way to check is to switch 
your probe's impedance (e.g. from 10x to 1x) (assuming a very low 
impedance source like a power supply output) and see if the amplitude of 
the noise changes. Make sure you set your scope for the new impedance 
too (it should be obvious unless you're looking at AC).

The noise may really be there, but it doesn't hurt to verify. If it 
looks noisy on your power supply lines then it probably looks awful on 
your signal lines.

Zac