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transistor driver problems

Discussion in 'The Projects Forum' started by popto, Jun 6, 2009.

  1. popto

    Thread Starter Active Member

    May 1, 2009
    I decided to build a simple (I thought) circuit that when I pushed one temporary push button an electric motor would go forward and when I let off the motor would stop. Then when you pressed the second push button the electric motor would go backwards.

    I decided to use four transistors to do the job. I tested all of the transistors that I have scavenged off of old circuit boards and found four of them that were the same. I am not sure whether they are PNP or NPN, but when I apply current to the base, current flows from the emitter to the collector.

    I set up the circuit so that the first push button connects the positive to the bases of two transistors. then I connected the positive to the emitter of the first transistor, then from the collector to the electric motor. Then I connected from the negative of the motor to the emitter of the second transistor, then from the collector of the second transistor to the negative.

    When I tested this first part of the circuit nothing happened. Using my voltmeter I measured the voltage across the motor leads and when there was power applied to the circuit but the button was not pushed it measured 0.3 volts. when I pushed the button It went to 0.4 volts.

    I discovered that when I bypassed the second transistor (between motor and negative) the circuit worked. then I replaced the second transistor with another one but it didn't work either.

    After that I decided to reconnect the second transistor and bypass the first one. when I tested that, the transistor started smoking, melting and stinking.

    When I first thought of this Idea it sounded really good and I thought It would work, but apparently I don't quite understand how transistors work.

    Any help is appreciated, and if there is a better or easier way to reverse power to an electric motor I would really like to know about it.

  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Search this forum for "h-bridge" and "h bridge" (without the quotation marks). Do the same search in Google.
    The type of transistors you use is important. Don't try to do it by salvaging parts, until you know what you are doing.
  3. popto

    Thread Starter Active Member

    May 1, 2009
    ok, Thanks. I'll try that.
  4. hobbyist

    AAC Fanatic!

    Aug 10, 2008
    It's best to drive a motor with power mosfets.
    They handle more current.