1722RE: [rsa] Re: Help Needed! Chips are in, but 1 questions still
- May 3, 2005Hi Terry,
> Sorry for what may seem a simple (dumb) question,I think you may be confusing diodes and capacitors. Diodes only allow current
> but from what I know capacitors are basically a 1
> way flow control valve in DC circuits, and a
> variable frequency cut off depending on the
> capacitors rating.
to flow in one direction.
Some capacitors will only work properly if they're connected with the correct
polarity (electrolytics and tantalums). The capacitors that you connect
across the motor are normally monolithic or ceramic, and these types of
capacitors work equally well with either polarity.
A capacitor is basically like a little storage device that can hold charge.
In the case of the motor, it absorbs the charge which is created by a current
spike, which is noise.
If you're familiar with modern plumbing, there's a device called a water
hammer arrestor they often put on your water lines before going to a washing
machine. These are basically performing the same function as a capacitor on
the motor. When the washing machine opens the valve quickly there is a sudden
inrush of water. The water hammer arrestor absorbs the initial shock. The
capacitor on the motor is doing the same thing when the PWM is switching on
If your motor has a metal case, then adding additional capacitors from the
metal case to each motor lead helps as well. The metal case will absorb the
RF trasnmitted by the brushes arcing, and the capacitors will allow it to be
Vancouver, BC, Canada
From: email@example.com [mailto:firstname.lastname@example.org] On Behalf Of Terry
Sent: Tuesday, May 03, 2005 10:25 AM
Subject: RE: [rsa] Re: Help Needed! Chips are in, but 1 questions still
Thank you again! Just got the chips in and am going to play with it today.
Just one thing I am not clear on -
putting a cap _across_ the motor leads.
I understand from one current direction it will help ground out noise
(reverse EMF?) or cut out AC frequencies, but as I am going to be using this
motor in both directions and polarities, isn't that going to just create a
parallel loop with the motor in one of the directions/polarities and cause
and from both the leads to ground will do the same(creat a closed circuit in
one direction), correct? So the motor would just be a resistive loop attached
in parallel to a close circuit?
Terry Kremin, Ph.D.
Ernest Gallo Clinic and Research Center, UCSF
"If the misery of the poor be caused not by the laws of nature, but by our
institutions, great is our sin." -- Charles Darwin
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