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2N3507 driving motor

Discussion in 'Electrical Engineering' started by Peter, Mar 14, 2006.

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  1. Peter

    Peter Guest

    I have a question:


    Tonight for laughs I built a triangle wave generator (floating above
    ground) to the input of a unity gain op-amp which is in a closed loop
    system.


    On the op-amps output, I have a 100ohm resistor connected to the base of a
    2N3507, 9 volts on the collector and the emitter is fedback to the
    inverting input (for the closed loop system). The emitter's output is also
    connected to a minature fan that draws about 0.5 amps at full on.

    My idea was to slow the fan and then speed it up with the triangle wave
    generator (changing the ramp time by RC). But I ran into a problem: the
    transistor gets as hot as a pistol.

    The datasheet has an Ic of 3A and yet I'm only driving (let's even call it
    1A) not even half the maximum rating.

    Should this be connected to a heat sink and if yes, how would I have known
    this by reading the datasheet?


    Thanks in advance
     
  2. ehsjr

    ehsjr Guest

    The problem is that you are not driving the transistor into
    saturation with the triangle wave, you are ramping it through
    the linear region. You're also driving the transistor with an
    op amp, which may not be able to provide enough current
    to drive the 2N3507, even if you use a square wave.

    If you *must* use a triangle wave, get a bigger transistor and
    put it on a heatsink. Better yet, use a square wave PWM. And
    drive a darlington power transistor, like a TIP120 so that the
    amount of base current neede for saturation is real small.
    Or use a 555 as the square wave source - it can provide up to
    200 mA into the base of your transistor and a darlington won't
    be needed.

    Ed
     
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