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1721RE: [rsa] Re: Help Needed! Chips are in, but 1 questions still

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  • Terry Kremin
    May 3, 2005
    • 0 Attachment
      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 problems?
      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?
      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. 


      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


      From: sfrsa@yahoogroups.com [mailto:sfrsa@yahoogroups.com] On Behalf Of rtstofer
      Sent: Sunday, April 24, 2005 5:52 PM
      To: sfrsa@yahoogroups.com
      Subject: [rsa] Re: Help Needed! Not sure relays will work.

      Ok, there are other considerations...

      Try the 754410 driver chip.  It is good for about 800 mA per side at
      a motor supply voltage of as low as 4.5V.  By the time you account
      for losses through the chip, your 3V motor will probably be just

      Now, each chip has 2 drivers so you can parallel them by simply
      wiring similar functions in parallel - that ought to handle your

      Here is a datasheet on the chip

      You can get heatsinks here:

      You can breadboard it on just about anything.  I tend to use a lot
      of Radio Shack 276-168 boards - search by number at
      www.radioshack.com but most stores stock them.

      You will also need to put an inverter on the mercury switches - it
      might as well include Schmitt triggering so the 74LS14 should work. 
      If you look at the driver truth table you will see that to run one
      direction, 1A should be high while 2A should be low.  The opposite
      polarity (1A low, 2A high) runs the motor the other direction.  So
      run each mercury switch into an inverter to clean it up and run one
      output to 1A and the other to 2A - ground the other 4 inverter
      inputs (or use them for something else).  Tie the enable pins high
      or, perhaps, switch them for overall control.  Ordinarily, the pulse
      width modulated speed signal is injected on the enable pin.  You can
      get a nice logic high from the output of one of those grounded

      There is no problem when both switches are in the same position - it
      will apply braking to the motor.

      If the braking is a problem, you will need to change the logic ahead
      of the 1A and 2A inputs - post back.

      One of the nice things about the 754410 is the kick-back diodes are
      internal.  You should still put 0.1 ufd capacitors between each
      motor lead (at the motor) and ground and ground the motor housing. 
      Also, put a 0.1 ufd capacitor across the motor leads.  This will
      help with your noise issues.

      SF robot meetings are at 7PM on the first Wednesday of the month at SFSU's Science bldg - 1600 Holloway, room SCI 256.


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