Series/parallel resistor maximum power rating

Discussion in 'The Projects Forum' started by jackferz, Jul 12, 2011.

  1. jackferz

    Thread Starter New Member

    Jul 12, 2011
    3
    0
    hi

    if i have a supply of 460vac with three resistors (250Ω, 100Ω, and 40Ω) of different values but same power rating say 300W.

    I would like to connect them in series or parallel so that each resistor is used at maximum power rating.

    if i connect them in series, the lower resistors is not going to be used at max power : following P=I^2R.

    please help me with this
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    In series the total resistance is 390Ω so the current is 1.18A and power dissipation is 543W (348W+139W+56W) but the circuit fails since the wattage rating of the 250Ω resistor is exceeded by 48W.

    In parallel the dissipations would be 846W, 2116W and 5290W with all three resistors disappearing in a burst of flame and smoke.

    There are 3 possible combinations of 1 in series and 2 in parallel and 3 of 2 in series and 1 in parallel but all of them would exceed the wattage rating of the resistors.

    The conclusion is that if you can't protect the highest ohmic value resistor from failure with all resistors in series, then you have no chance of any other combination working.
     
    Last edited: Jul 12, 2011
  3. jackferz

    Thread Starter New Member

    Jul 12, 2011
    3
    0
    i have already tried all the possible combinations.

    What i should have said earlier that since its a three phase supply, i even have a possiblity of conencting them in Delta, Wye, three wire configuration or mixing delta-wye. But still it does not help.

    do you have any thoughts on that
     
    Last edited: Jul 13, 2011
  4. jackferz

    Thread Starter New Member

    Jul 12, 2011
    3
    0
    i actually have available resistors in large quantity for each of them(250Ω, 100Ω, and 40Ω).
     
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