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transistor array, more power wanted

Discussion in 'Electronic Basics' started by Julia Goolia, Feb 2, 2004.

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  1. Julia Goolia

    Julia Goolia Guest

    hi!

    i am trying to pump more power through a transistor array
    (8-darlington high-current(500 mA) driver array, part no:ULN2803A) for
    a step motor. i don't understand why more current will not go
    through... i am plugging in a 12V, 200mA power supply (from a
    transformer plugged directly into the wall) to the COMMON power supply
    of the IC. then I turn on the inputs with a basic stamp, but very
    little power gos through the outputs.

    Is it better to ground the COMMON and plug the load wires into the +
    of the power source? How can I get more power through a transistor?

    Thanks!
    juliabean
     
  2. Have you read the data sheet?
    http://www.controlord.fr/CONTRIB/uln2803.pdf

    The positive side of the supply should connect to all the loads and to
    the diode common (com). The other side of each load should connect to
    one output. The negative side of the supply connects to the chip's
    ground which also has to be connected to the logic ground of the
    signal source driving the inputs.
     
  3. One other detail: If you are driving center tapped step motor
    windings, you connect the center taps to the positive supply (possibly
    through current limiting resistors) and the diode common to a zener
    diode cathode. The zener diode anode connects to the positive supply
    rail. The zener should have a breakdown voltage about equal to the
    positive supply voltage. This is needed, because when one driver
    pulls one end of a center tapped winding to ground, the other end of
    the winding pops up to twice the supply voltage, like a see saw. It
    wastes power to clamp the swing lower than that. But there is an
    additional positive voltage spike generated by any end when it is
    first turned off, and you want only that clamped by the internal
    diode, the zener and the supply all in series.
     
  4. Julia Goolia

    Julia Goolia Guest

    John,

    Thank you so much for your reply! Sure enough, I had it totally
    backwards... so now, thanks to you, I have a working step motor!
    Still, the amount of power that I get is small. It doesn't work with
    a 9V battery. It only works with my 12V 200mA transformer that I plug
    directly into the wall. The output is only about 4V and about 100mA
    on each pulse. But at least the motor turns!

    thanx again!
    juliabean
     
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