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Math Help - Shunt Resistance.

  1. #1
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    Shunt Resistance.

    Hi all,

    I need to work out the required value of a parallel shunt resistor to shunt a meter.

    I have been given the maximum incoming current of 200A (I), the resistance of the meter (Rm) which is 0.1 Ohm and been told that the maximum the meter can read is 1mA (Im).

    I've gone down this route:

    Voltage = I x Rm = 200 x 0.1 = 20V

    Then worked out the current that the meter is pulling = 20V/Rm = 200A

    Correct me if i'm wrong, but doesn't that mean that the shunt resistor (Rs) is effectively 0? So the shunt current (Is) be 20/0 = 20A?

    Any pointers would be appreciated.

    Thanks.
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  2. #2
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    Quote Originally Posted by becca View Post
    Hi all,

    I need to work out the required value of a parallel shunt resistor to shunt a meter.

    I have been given the maximum incoming current of 200A (I), the resistance of the meter (Rm) which is 0.1 Ohm and been told that the maximum the meter can read is 1mA (Im).

    I've gone down this route:

    Voltage = I x Rm = 200 x 0.1 = 20V

    Then worked out the current that the meter is pulling = 20V/Rm = 200A

    Correct me if i'm wrong, but doesn't that mean that the shunt resistor (Rs) is effectively 0? So the shunt current (Is) be 20/0 = 20A?

    Any pointers would be appreciated.

    Thanks.
    Post the exact wording please.

    (The 200 amps does not flow through the 0.1 Ohm resistance. 0.001 Amp does)

    CB
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  3. #3
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    Quote Originally Posted by CaptainBlack View Post
    Post the exact wording please.

    (The 200 amps does not flow through the 0.1 Ohm resistance. 0.001 Amp does)

    CB
    Hi,

    Thanks for your reply.

    Ok, I take it you mean the wording in the question.

    "Determine the required value of the shunt resistance if the maximum value of the current I is 200A. The meter can read a maximum of 1mA and has a resistance of 0.1Ohm."

    There is a diagram which basically consist's of a meter and a shunt resistor in parallel.

    So, 0.001 x 0.1 = 0.1mV

    Therefore, Rtotal = 0.1mV/200A = 0.0000005 Ohm

    Rmeter - Rtotal = 0.999995 Ohm?
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  4. #4
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    Actually, would it be more like this:

    200A - 1mA = 199.999A

    0.0001V / 199.999A = 0.0000005000025?

    I always thought that in a parallel circuit like this, the total resistance is a lot less than any of the component resistances?

    Thanks.
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  5. #5
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    Quote Originally Posted by becca View Post
    Actually, would it be more like this:

    200A - 1mA = 199.999A

    0.0001V / 199.999A = 0.0000005000025?

    I always thought that in a parallel circuit like this, the total resistance is a lot less than any of the component resistances?

    Thanks.
    that would be true, and it serves the purpose of reading current well since the ammeter is placed in series with the circuit ... the ammeter is essentially "transparent" to the circuit whose current it is measuring.
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  6. #6
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    Thanks for your reply Skeeter.

    Am I correct in my math?

    Thanks.
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  7. #7
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    the numbers are correct ... I'd use scientific notation to express them, however.
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  8. #8
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    Thank you.

    Yes, I purposely didn't include notation.

    Are there any programs you recommend that would enable me to write equations on the computer and copy them into the forum etc?

    Sorry about my lack of knowledge, I'm a complete noob to all of this.

    Thanks!
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  9. #9
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  10. #10
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    Thank you very much!
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