Richard, just for the record, I wasn't the original poster. Good
suggestions though. I once tried all these tricks on a small DC
motor that was failing EMC compliance tests with not much luck
The answer was a series inductor in each lead to the motor and
ceramic caps to motor casing.

> Brent
> Since it only happens whaen the motor is connected I suggest you look
> again at your earthing and power supply arrangements. It sounds as if
> you're doing it nearly right with the 2 supplies. Is the motor
> supplied from the 12V also - a bit of filtering on the feed to the
> relay may assist. If you are not going to be reversing the motor a
> diode accross it may help also - particularly if it has field windings
> rather than perminant magnets.
>
> Overall, I'd suggest making sure the earths are tied at only one
> point, filtering the 12V to the relay using a 10ohm resistor and 100n
> cap and if possible filtering the motor supply with at least a small
> inductor / cap and possibly a diode as well.
>
> Richard P


Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brown@clear.net.nz

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