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1722RE: [rsa] Re: Help Needed! Chips are in, but 1 questions still

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  • Dave Hylands
    May 3, 2005
      Hi Terry,

      > Sorry for what may seem a simple (dumb) question,
      > 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.

      I think you may be confusing diodes and capacitors. Diodes only allow current
      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
      and off.

      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

      Dave Hylands
      Vancouver, BC, Canada

      -----Original Message-----
      From: sfrsa@yahoogroups.com [mailto:sfrsa@yahoogroups.com] On Behalf Of Terry
      Sent: Tuesday, May 03, 2005 10:25 AM
      To: sfrsa@yahoogroups.com
      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.
      Postdoctoral Fellow
      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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