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Reply: Voltage Regulator Question

Discussion in 'General Electronics' started by default, Nov 6, 2003.

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  1. default

    default Guest

    Five amps is a fair amount of current. Dropping 9 volts in the
    regulator will mean generating 45 watts of heat to provide 15 watts of
    power to the motor. No self respecting designer would do that.

    You have two better options.

    Chop the power (modulate on and off) rapidly with a variable pulse
    width - "on" time varies. The motor probably won't care that the
    power is switching on and off; its inertia is too great. Plenty of
    simple 555 timer circuits to provide PWM and the output of the 555 is
    great enough to drive a pass transistor (large transistor carrying the
    current to the motor). These types of circuits are very efficient and
    you would waste 3 watts or less (small heat sink)

    If you are more flexible regarding the power supply, use an SCR
    (Thyristor) speed control. see "basic version of the SCR throttle
    circuit" I use that
    circuit with some modifications to control four DC fans I use for
    ventilation at home. With an SCR type controller you need AC into the
    input and the SCR varies the firing angle (portion of the AC sine wave
    that gets sent to the motor) The big advantage of this circuit is
    that the motor sees nothing but smooth DC voltage. Waste power 3
    watts in the control element and another 3 in the rectifier (something
    that would happen inside your 13.8 supply anyway).

    The problem to you: you already have a DC supply - but you could tap
    into the stepped down voltage on the internal transformer (assuming it
    has a large power transformer and isn't a switching supply) Or just
    buy a power transformer and use that instead of the DC supply.

    The chopper circuit can be further modified to supply smooth DC power
    by adding an inductor to store energy while the transistor is off -
    otherwise known as a switching power supply - but it isn't necessary
    for an ordinary DC motor - but would be for a "brush less"
    (electronic) DC motor. Waste power 3 watts or less. A little more

    check out robotics web sites - those guys are always dealing with just
    what you want to do - model railroad sites ditto. There are tons of
    info on the web already.

    All the good solutions require more work than just using a three
    terminal regulator. (incidentally there are 3 term regs that can
    carry five amps if you have a heat sink large enough to get rid of the
    waste heat) See the LM338 regulator: 5 amps and 1.2 to 32 volt. has a simple PWM motor
    speed control links to more
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