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Low resistance

Discussion in 'Electronic Basics' started by PW, Nov 14, 2008.

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  1. PW

    PW Guest

    I need to measure the resistance of some motorcycle alternator coils
    They should be about 1.5ohm
    I have a cheap DVM, which is of little use at this value
    Is there a simple way to do this keeping in mind that I have Zero
    electoral knowledge.
  2. ian field

    ian field Guest

    The coils will have higher "resistance" (reactance) when AC current is
    applied, if you can find a wall wart with AC output (possibly a dial-up
    modem wall-wart) with approx' 12V output - hook it up with a dial or
    indicator bulb in series, a gross difference in brightness between any two
    identical windings would indicate a problem. You can also make use of your
    meter on its AC voltage range for a more precise indication.

    If you really do need an actual DC resistance measurement, the easiest way
    is to apply a regulated (and known) current to the winding and then measure
    the voltage developed, from there its a simple Ohm's law calculation: V/A=R.

    This topic comes up frequently on so the regulars
    have had plenty of practice giving advice.
  3. ian field

    ian field Guest

    The childish PHucker troll escaped from .
  4. Jamie

    Jamie Guest

    I bet you voted for Obama!"
  5. Jasen Betts

    Jasen Betts Guest

    use Ohms law.

    pass a known current like 100mA through the coil
    (eg use a 120 ohm resistor and a 12V supply) and measure the current
    using your DMM

    +12V ---[120]---[coil]---[DMM]---0V

    then disconnect the DMM and set the circuit up again and measure the
    voltage across the coil.

    (view with fixed pitch font - eg cut and paster into notepad)

    +12V ----[120]---[coil]---------- 0V
    | |

    then divide current by voltage eg 150mV/100mA=1.50 ohms.
  6. For the DC measurement, if you have not a regulated current source, and
    if your DVM can not measure current, try the following:
    - buy a resistor between 1.5ohm to 15ohm, let's say R;
    - put it in serial with the coil;
    - put some DC voltage across the two resistors;
    - measure the voltage around the coil (Vc) and around the resistor (Vr)
    - compute coil resistance: Rc = (Vc)/(Vr)*R

    Warning: depending the DC voltage and the value of R, you have to
    correctly select the power of R. Do not put too much DV voltage. For
    example, if R = 4.7ohm and if you apply 5V, then power =
    4.7*(5/(1.5+4.7))^2 = 3 watts

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