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Two separate caps, yes, will parallel to an additive value under DC, but under AC or RF they will each show their own relative impedance in the circuit.I just had an idea... these inexpensive digital panel meters typically don't have the robust filtering their big brother DVMs have. If you still suspect RF is going places it shouldn't, try putting a cap on the battery leads, which would also have the antenna effect you suggest.
The little cap works great, but ONLY when connected directly across the motor brushes, or within about an inch of them.If I put it a foot or so back on the wires, or back where the probes come out of the panel meter, it does not work. That is an indicator that this is an RF problem. The noise is being radiated off the wires like a transmitter, so putting the little cap across the meter inputs is useless. The noise has to be suppressed close to its source: the arcing brushes.While that works, the thing I don't like is that it's not a general purpose solution. I will have to connect a little cap across any noisy motor I measure. While I can do that, I would rather find a way to make the meter impervious to the noise.I don't think that's going to happen.
Good suggestions, Peteski.I ran the motor off a 9v battery, and hooked a simple GOW to my power supply that has the meters on it.The ammeter and voltmeter measure perfectly, rock-steady, no matter where I position the running motor.So NOW, it appears that this is NOT RF. The motor noise must be coming in through the leads to the meter.But how does that reconcile with the fact that I can put a .01 uF cap across the motor leads, and it squelches the problem completely, yet if I put that cap across the same leads back by the meter, it does nothing? If the noise were on the wires,the cap should work anywhere, no? I thought that putting it near the brushes was important to short out the high frequencynoise before it could travel down the wires and use them as antennas to "broadcast" the noise as RF?When I power the motor off the 9v, I still used 3 foot clip leads and no capacitor, so I would have expected lots of radiated noise.