Multivibrator Drive Current

Discussion in 'The Projects Forum' started by JohnDiggs80, Nov 16, 2014.

  1. JohnDiggs80

    Thread Starter New Member

    Nov 16, 2014
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    I'm trying to design a transistor-based multivibrator as a PWM driver for my DC motor. Whenever, I attempt to add a load to one side of the multivibrator, it completely throws off the PWM signal and I don't seem to have enough current to drive the load transistor. Does anyone have any suggestions for a very cheap solution to fix the problem?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Make R8 at least 10k and add another transistor to form a Darlington pair with Q4 (or else replace Q4 with a Darlington).
    How much stall current does your motor draw?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Q4 has a 100 ohm base resistor which nearly shorts the 2.2k ohm Q1 collector resistor and obviously has a serious effect on the multivibrator operation.
    Also how do you expect to drive a 0.5 ohm load when you have a 0.4 ohm resistance in the power supply Vcc? :confused: Why is that resistance so high?

    The easiest solution for a 3V supply is to use a logic-level MOSFET for Q4 (one that fully turns on with a Vgs = 3V) and lower the power supply resistance.
     
  4. JohnDiggs80

    Thread Starter New Member

    Nov 16, 2014
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    Thanks Alec and Crutshow for the suggestions. I switched to the darlington configuration and switched out the load transistor with a mosfet type. I'm getting better waveforms all around.

    Stall current is 1.5A. Typical draw during operation is 700mA. To model motor, I'm doing a simple worst case resistance. I also corrected the battery resistance to something average for 2xAA. Inrush current is dropping the voltage down to around 1.3-1.5V. Any suggestions for keeping the PWM circuit stable during this time besides maybe a decoupling cap?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    You can apply the power to the multivibrator through a Schottky diode and add a large capacitor to ground to keep the voltage relatively constant on the multivibrator when the motor starts up (the diode isolates the multivibrator from the momentary battery voltage drop). For this to work the voltage must be applied to the multivibrator prior to applying voltage to the motor.
     
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