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Calculating the power consumed by an astable multivibrator

  1. Sep 16, 2011 #1
    I modified an astable multivibrator to send an alternating current across a load (square wave centered at 0V), given a DC power supply. The multivibrator is connected to to the load through an H-bridge and the multivibrator alternates which switches are on and off via transistors. The point is that the multivibrator saps out a certain amount of power and the rest is left to flow across the load.

    My problem is that if the resistance of the load is too small, square wave deteriorates. I am positive this is because the device not getting enough power because too much current is flowing across the load.

    I am not sure how to calculate the power consumption of an astable multivibrator. I know how to calculate power consumption of simple LRC circuits, but my background and experience in circuitry is limited to the electronics/circuits section of some introductory physics courses.

    Here is one of MANY links to the multivibrator my circuit is based on:
    http://upload.wikimedia.org/wikipedia/commons/6/6a/Transistor_Multivibrator.svg

    PS: What is the significance of R1 and R4 in this design--there was not a SINGLE article that discussed that!
     
  2. jcsd
  3. Sep 16, 2011 #2
    Imagine Q1 is in saturation so the voltage across R1 is ~+V. When Q1 turned on, C1 transferred the 0V at the collector of Q1 to the base of Q2, turning it off. Now C1 is charging through R2 and eventually the voltage on the right side of the capacitor will be enough to turn on Q2. When that happens the falling voltage on the collector of Q2 will be transferred to the base of Q1, turning it off. R1 and R4 develop the voltage that turns on or turns off the other transistor.

    The total current of the circuit will be very close to +V/R1 and the power would be +V^2/R. It seems to me your problem is that your power supply cannot supply enough current to power the load. Could you run your load from a different supply than that for your multivibrator?
     
  4. Sep 16, 2011 #3
    Thank you very much. As I followed your description on the circuit simulator, I could see everything you described happening.

    I suppose I could come up with a way to have the multivibrator connect to a different power supply.

    However, this begs the question (which perhaps should be a new thread):
    How can I optimize the multivibrator to use the minimum power necessary to function?

    I guess, since most of the current is flowing through R1 and R4, I would make them as close to R2 and R3 as possible--but how close I do not know. I presume calculating the optimum values of R2 and R3 would require going beyond the approximation that "all" current goes through each of them anyway.

    Any thoughts--or perhaps I should start a new thread for that?
     
  5. Sep 16, 2011 #4
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