L293d motor driver current enhancement?

Discussion in 'The Projects Forum' started by Archaeus, Sep 14, 2016.

  1. Archaeus

    Thread Starter New Member

    Sep 10, 2016
    12
    0
    Good day. I was hoping someone might be able to let me know if this circuit would: 1. Work. 2. Provide the estimated current gain hoped for. I am about to work with an lm293d motor driver (I know there's better, but it's what I have), and it only has a wimpy 600ma output for a DC motor. People rave about overheating issues and voltage drops. I am still a new hobbyist, so I was wondering if I can get a 2a available output using 2 transistors in an emitter follower. I think it will still allow the alternating high/low pins to operate a 12v reversable motor, but I may be overlooking crucial variables. The bjt's will add to the voltage drop I realize, but the current gain would be perfect for start up and stall currents. Feel free to rip it apart with any feedback. I estimate total voltage drop to leave approx. 9v. Thanks again.
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  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,912
    379
    The transistors with the 450Ω resistors connected to their bases don't seem to be doing anything useful, just shorting out the 12V supply when they are turned on.

    The other two transistors can pull either end of the motor positive, but then the motor current must flow the resistors connected to the transistor emitters. So the 2A motor current will drop some voltage across these resistors - let's say 1V. Then we can calculate the value of those resistors as 0.5Ω. Now when either transistor is conducting your estimated 9V output is across that resistor drawing an extra 18A from the supply in addition to the 2A motor current. That is not going to work.

    You could do it by connecting the circuit below between the '293 output and each end of the motor (2 circuits required) but note that this will drop at least another 2 diode drops from your available voltage and you will need to add the 4 catch diodes to protect the transistors. I think it would make a lot more sense to use something like the L298.

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