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How to increase the current?

  1. Jul 27, 2012 #1
    How to increase the current?!!!

    I'm designing a drive circuit for a 12V DC motor
    I'm using a (2N3055 BJT) as a switch to control the output, and I'm using a (74121 monostable oneshot) to control the BJT switch.
    So, the output of the 74121 is fed into the base of the BJT.

    Here is the schematics for my circuit
    http://img594.imageshack.us/img594/7411/etpcircuit.jpg [Broken]

    According to the datasheet of the BJT, β=100. Which means that the current supplied to the base should be at least 1/100 of the current that is supposed to run through the collector. The motor needs about (2-3) Amps to run at a proper speed, meaning that the base current should be at least (20 - 30 mA).
    The problem is that the output current of the 74121 ranges from (4 - 16 mA).

    Is there anyway to increase the current?
    I can sacrifice some of the voltage
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jul 27, 2012 #2


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    Re: How to increase the current?!!!

    You could add a base drive circuit like in the attachment.

    Or you could just replace the 2n3055A with a suitable n channel logic level mosfet and you wouldn't need any auxiliary drive components.

    Attached Files:

    Last edited: Jul 27, 2012
  4. Jul 27, 2012 #3
  5. Jul 27, 2012 #4

    jim hardy

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    Gold Member

    Re: How to increase the current?!!!

    indeed the good ol' 3055 is not the greatest for current gain. hfe may drop to more like 10 in the Ic=amperes range.

    TTL sinks better than it sources so your design is needing a tweak or two

    uart's MOSFET is one solution, so is his darlington

    a 555 in between the '121 and the '3055 is another, but the 555 inverts so use the '121's Qnot output.
    Tie TRIG and THRESH together and 555 becomes a schmitt trigger buffer with substantial drive capability. That's a handy trick .

    glad you're experimenting , it's the best teacher.

    old jim
    Last edited: Jul 27, 2012
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