On 9/8/05, Wouter van Ooijen <wouter@voti.nl> wrote:
> I am PWMing two DC motors, 24V 100mA and 24V 250 mA (unloaded currents).
> I know a capacitor directly at the motor connectors is a good idea, but
> which value, and can it be just any capacitor or does it need to be a
> special one?

C may create resonance in parallel with the motor L. As long the
resonance will not affect (increase) the current sourced from the
bridge will not be danger. On the other hand using a big capacitor
value will decrease the bridge efficiency by increasing the rising and
falling edges. I suppose a capacitor near resonance (before getting
the resonance) will be the best choice. You must knew at least approx.
the frequency and the motor inductance to compute the initial value of
C and then measuring the current sourced to tune (increase) slowly the
C and found the resonance. Before getting resonance you'll freeze the
capacitor value. The warning is you may get mechanical resonace of the
motor for some loads (whistles).

Vasile

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