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[DPRG] Overload protection on a motor not working

Subject: [DPRG] Overload protection on a motor not working
From: Benoit Provost provost at ee.tamu.edu
Date: Sat Jul 21 00:07:28 CDT 2001

Hi,

  I would like to have your opinion/advice on a problem I have.  In order to protect the driving motor of my robot, I have built a small circuit to monitor the current going to it.  The circuit (see at http://amesp02.tamu.edu/~provost/tmp/i_monitor.gif) amplifies the voltage drop in a 0.25 ohm resistor and compares the result with a threshold value.  If it is higher than the threshold, the output goes low and the LED turns off, indicating that the motor is overloaded (actually, I wanted to get the LED to turn on with an overload, but I made the PCB with the comparator inverted...).

  The circuit works fine when the motor is directly driven by the power supply.  However, when I use it on my robot with the H-bridge ( I use a 754410), the output from the amplifier is always very low.  R1, which controls the gain of the amplifier, has almost no effect on the output of the amplifier.  The circuit is limited to monitoring the current in one direction only, but I tried the two directions and the results are the same.  One possibility that I suspect is that I use PWM, so the current is continuously pulsing, which could be too fast for the amplifier to respond.  So I tried the current monitor at the power supply of the motor controller instead of directly at the motor, same results.  

  Any idea why the H-Bridge would prevent this circuit from working?

  Thanks!
  Benoit
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