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Simple Current Limiter

Discussion in 'Electronic Components' started by John Weall, Aug 27, 2003.

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  1. John Weall

    John Weall Guest

    I know this is a pretty basic question, but I can't remember for the life of
    me what the answer is.
    I've put a car stereo in an mdf box with a car battery, and I need to
    recharge the thing. I was thinking I could plug it into my car cigar
    lighter, and recharge it from there, but the cigar lighter is fused at 10A.
    Therefore, I need to put something in the wire to stop the current rising
    above...say, 9A or something. Also, if it's easy to do, i'd like to have
    something to stop it charging with enough current left to start my car, so I
    can leave it plugged in without the engine running.
    Thanks for any help,
  2. René

    René Guest

    12 Volt lamp (~50W? ~ 5 amp current limit )
    limits current, has low impedance when cold (PTC effect), prevents
    also exessive current to flow the wrong way if you start the car.
    And a visual charge indication for free :)
  3. sci.electronics.basics is a good group for questions like this, I think. In
    any event:

    To limit the current, you could just use a resistor. As current increases,
    voltage across it increases, meaning the battery being charged sees less
    voltage, meaning it draws less current. Use a value that drops 12V at 9A,
    so that even if the battery being charged behaves like a short circuit, it
    draws only 9A. By Ohm's law, R = E/I, so 12V / 9A is 1.25 ohms. It'll be
    dissipating 12V * 9A = 108W, so you might want to use four 5-ohm 50W power
    resistors in parallel with each other. It'll get pretty hot, but it's a
    very simple easy solution. You can also build a fancier current limiter
    from an LM317 and some power transistors in parallel, but it's going to do
    functionally the same thing in this case. (And it's going to produce just
    as much heat.)

    As for the second point, respectfully, charging a car battery from another
    car battery is probably a bad idea. You really want to be charging it from
    a running engine. Your car battery is not intended to start your car with
    half its charge.
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