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Discuss Working out resistor calculations... in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. bilbo_88
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    bilbo_88 Regular EF Member

    Location:
    London
    Late at night....tearing my hair out....need help with working out resistor calcs.

    230V supply....R1=70...R2=90

    What is the value of a resistor which when connected in parallel with 70 ohm resistor will cause a total current of 2.4 amps to flow in the circuit?


    any guidence to push me in the right direction greatly appreciated
     
  2. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

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    Rt =(R1 x R2)/ (R1 + R2)
     
  3. bilbo_88
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    bilbo_88 Regular EF Member

    Location:
    London
    thanks, the questions thrown me a bit, do I in anyway need to involve the 2.4 amps in the calculations, or is it just there as information
     
  4. bilbo_88
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    bilbo_88 Regular EF Member

    Location:
    London
    scrap that, ive just read the question properly.....time for bed
     
  5. MarkieSparkie
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    MarkieSparkie Guest

    Let the unknown parallel resistor = Rx
    The total circuit resistance (Rt) that would cause 2.4A to flow:
    Rt = V / I
    The equivalent resistance of the parallel resistors (R1 & Rx):
    Req = Rt - R2
    The unknown parallel resistor Rx:
    1/Rx= 1/Req - 1/R1

    Rx= 6.36Ω (to 2DP)
     
    Last edited by a moderator: Feb 23, 2012
  6. Knobhead
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    Knobhead Guest

    If you PM me I’ll send an Excel spread sheet. It originally set off as transformer and motor calculations, but it’s got a bit like Toppsy it just keeps growing. Parallel resistance calculations, unbalanced neutral currents and all kinds of things have found there way in there.
     
  7. bilbo_88
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    bilbo_88 Regular EF Member

    Location:
    London
    hi mark
    struggling a bit with the calculation as im getting a defferent answer to you.
    my working out is as follows....
    230/2.4=95.83ohms
    95.83-90(R2)=5.83
    (1/5.83)-(1/70)=0.17-0.014
    1/0.156=6.41
    Rx=6.41

    anyone able to tell me where i've cocked up? cheers
     
    Last edited: Feb 23, 2012
  8. MarkieSparkie
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    MarkieSparkie Guest

    See the corrections above, hope this helps, Mark.
     
    Last edited by a moderator: Feb 24, 2012
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