DPRG mailing list archives: [DPRG] Motor backlash / Back EMF

Mon Jan 7 14:50:38 CST 2008

At 12:11 AM 1/7/2008, DeltaGraph at aol.com wrote:
>OK, Chuck,
>I think I am getting clued in.
>
>So really more than IR drops due to loads we are talking inductive 
>drops or increases of voltage V=Ldi/dt depending on increasing or 
>decreasing current.
>
>So for an instant, when relay opens and current in motor circuit 
>decreases very rapidly , Scott's ground lead looks like a battery 
>supplying at least a few volts to the ground wire -- got it.
>
>If per chance it supplied 5 volts, the voltage supply might drop to 
>zero -- assuming no capacitance at processor.
>
>This brings up a question. Could a reasonable capacitor say 10 uF 
>tied to processor (or real close)  protect processor from the bounce?
>
>Ron

Sometimes. Often a better ground plane design in connection with 
decoupling capacitors will do what you need. The decoupling capacitor 
on a bouncing ground will bounce right along with it and keep the 
voltage to your CPU constant, but this can put you into another 
problem which is that the voltage regulator for the CPU may see an 
"overvoltage" condition on its output try to compensate. (this is 
where the good ground plane design comes in). Basically if you have a 
linear regulator then its too stupid to care but if you're using a 
switching topology, especially a boost switcher, then the sudden 
apparent rise in the positive rail can cause the switcher to 
mistakenly think it is responsible and crowbar itself to prevent 
damage to the circuit. When I was building my big motor controller I 
managed to turn off the HP 1.5KW power supply this way several times 
before I figured out what was going on.

As Triffid mentioned if your steering motor ground and your processor 
ground come together at a the nexus of a "star" ground plane then 
there is much reduced chance of bounce.

--Chuck