Motor Control without H Bridge

Discussion in 'The Projects Forum' started by secretstep, Jul 1, 2008.

  1. secretstep

    Thread Starter Member

    Jun 27, 2008
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    I'm having trouble designing a system that controls motor without using H-Bridge.
    Basically it compares two inputs (from potentiometer) and spins towards the way that has the lower voltage..
    Now, I experience weird behaviour of opamps.
    All the currents are flowing into the Vcc and Vee of the opamps, and the motor doesn't even run when the current is small.
    Since it's Power dissipitation of opamps are .5W each, as soon as the current goes over 0.1A, it starts to get heated up.
    I really don't understand what's going wrong with the circuit now.
    Very confused, any help would be nice...
    Please refer to the attachment...
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Is the op-amp getting hot? How much current does the motor draw? You should have base resistors on the transistors. As has often been mentioned before, the 741 can only get within a few volts of the supply rails and when you add this to the drop across the transistors, there may only be a maximum of a couple of volts reaching the motor.
     
  3. secretstep

    Thread Starter Member

    Jun 27, 2008
    17
    0
    yes opamp was getting hot when I touched it. The motor can draw 6V and around 400mA as a stall...
    Could you please clarify what you mean by...
    "the 741 can only get within a few volts of the supply rails and when you add this to the drop across the transistors, there may only be a maximum of a couple of volts reaching the motor."
    coz I had two power supply. first one was for opamps and potentiometers, and the second one was for transistors to draw high current. However, the first power supply tends to supply all the current and voltages for the whole circuit... leaving second power supply useless.
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    The 741 op-amp cannot output 6V (when powered by a 6V supply), it may only manage 3V. Following the output of the op-amp there are driver transistor with a Vbe of around 0.7V which will further reduce the voltage available to the motor to little more than 2V at best.

    With the output stage disconnected are the op-amps getting hot?
     
  5. secretstep

    Thread Starter Member

    Jun 27, 2008
    17
    0
    when output stage is disconnected, the opamps are fine with power supply of 6v and a small current. I wonder why the currents were going into the output stage..
     
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    368
    Have you tested the output stage on its own? Check the transistors are connected correctly and are not faulty. Due to the limited voltage available to drive the output stage I wouldn't expect a great deal of current to flow under normal circumstances.
     
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