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Measure 100 amps with 10 amp meter ?

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  • sdold
    replied
    Re: Measure 100 amps with 10 amp meter ?

    I forgot:

    power = E x I

    So the 0.1 ohm resistor would have to dissipate 100amps x 10V, = 1000W.

    It would be an impressive resistor!

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  • sdold
    replied
    Re: Measure 100 amps with 10 amp meter ?

    There's an easier way to do it, just find a shunt resistor with a known (small) value of resistance, say 0.1 ohms. Then when you have current flowing, you measure the voltage across the shunt, with the meter in the volts position (not in the amps position).

    E = Voltage
    I = current
    R = resistance

    E=IR
    R=E/I
    I=E/R

    So, to find current, if you read 10V on the voltmeter (example), the current is 100A (if the shunt is 0.1 ohms).

    Steve

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  • seanc
    replied
    Re: Measure 100 amps with 10 amp meter ?

    eprigge:
    If the second shunt resistor has 1/9th the resistance of the meters shunt, the meter will read 1/10th the actual current flowing.
    Your values are off.

    The formula for parallel resitance is: (R1 X R2)/(R1 + R2)

    In order to read 10x the original amperage, you need 1/10 the original shunt resistance. Your numbers produce just under 1/5 the resistance, and the meter would be way off.

    The correct value for the second resistor would be @ 0.11 x the first value

    MotoChrome:

    The problem is that the meters on the market can only measure 10 amps. I have heard it is possible to do using resistors. Does anyone know how to do this ?
    You first have to determine the internal resistance of the meter, which may or may not be easy.

    Is it a digital or analog meter?

    All digital meters are shunt type, but some are sealed so that you can't take them apart to find the resistance.

    Analog meters are either shunt or series.

    A series analog meter does not use a shunt resistor, you have to determine the resistance of the internal coils. Not easy. But the resistance MIGHT be marked on the face plate.

    A shunt analog meter may have an external or internal shunt. If it's external, disconnect it and measure its resistance. If it's internal, you will have to disassemble the meter to get at it.

    Once you've determined the original resistance, find a resistor of 0.11 x that value, and use it in parallel with the meter. You want a precision resistor, of 1% tolerance or better, which is temperature stable.

    The wattage will depend on your maximum PS voltage. As eprigge said, the new resistor will carry 90 amps. Multiply that by your max voltage, and that will give you the MINIMUM wattage for the resistor.

    eg. if you'll be plating @ 12v, then 12x90= 1080 watts. That would be a HUGE and EXPENSIVE resistor.

    Sean

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  • moto-chrome
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Thanks a million, that's exactly the answer I needed.

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  • eprigge
    replied
    Re: Measure 100 amps with 10 amp meter ?

    Your meter has an internal shunt (resistor) which the meter uses to measure the current. What you need to do is attach a second shunt resistor in parallel with the meter. If the second shunt resistor has 1/9th the resistance of the meters shunt, the meter will read 1/10th the actual current flowing. And this second shunt would have to be rated for 90A or more.

    Leave a comment:

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