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Power handling limits of resistors

Discussion in 'General Electronics Discussion' started by tonebrulee, Apr 28, 2012.

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  1. tonebrulee

    tonebrulee

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    Apr 28, 2012
    Hi folks - new to EP here and a novice in electronics.

    I need to drain the electrolytic caps in the power filter section of an old tube amp and don't have the recommended 10W 25k Ohm resistor suggested for use to avoid a spark.

    My question is about ganging together 4 100k Ohm 2W resistors. The aggregated resistance should be the target 25k Ohms, but I'm not sure if the overall power handling will be 8W (i.e. 4 x 2W) ? I would imagine it would have to, with the current being split four ways, but wanted to make sure before I go ahead.

    Also, it's typical to use a cement resistor for this application, and the 2W resistors I'm considering are metal film. Would there be any concern in not using a cement resistor here - i.e. all other things being equal, how does a resistor made of cement differ functionally from metal film?

    Thanks!
     
  2. BobK

    BobK

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    Jan 5, 2010
    Yes, paralleling resistors with the same resistance will add their power handling capability. I don't think the composisiton of a resistor matters as long as the power rating is sufficient.

    Bob
     
  3. tonebrulee

    tonebrulee

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    Apr 28, 2012
    Meant to say that the 4x 100k Ohm resistors would be in parallel, but I guess I could get the same effect with 4x 6.5k Ohm resistors in series, right? In that case would the power distribution give the aggregate 8W of power handling?
     
  4. tonebrulee

    tonebrulee

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    Apr 28, 2012
    Thanks Bob! Could you answer my follow up question re: resistors in series?
     
  5. BobK

    BobK

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    Jan 5, 2010
    As long as the resistance is the same, they would divide the voltage equally and again, the power would be divided equally and hence the total power would just add up.

    On the other hand, if you used different value resistors, either in parallel or in series they would divide the power unequally and you could not simply add up the power handling.

    Bob
     
  6. tonebrulee

    tonebrulee

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    Apr 28, 2012
    Thanks again BobK. As it turns out, I'm going to use both parallel and series b/c I've got 4x 22k Ohm 2W resistors and will combine them (22k--22k) || (22k--22k) to get the resulting 22k Ohm 8W needed.
     
  7. TedA

    TedA

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    Sep 26, 2011
    tonebrulee,

    Note that each resistor's power is generally rated with the resistor mounted to something that will draw heat from the resistor, not another hot resistor. If you just twist together the leads of several small resistors, then run them at maximum power, they may overheat.

    To realize their full power rating, you will need to spread the resistors apart, and provide a terminal strip or some other means of getting rid of heat conducted out the leads.

    Ted
     
  8. D_Hambley

    D_Hambley

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    Sep 1, 2011
    Are these resistors active only during discharge or, dissipating power continuously?
     
  9. davenn

    davenn Moderator

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    Sep 5, 2009
    disapating power contineously.... as long as current is flowing through them

    D
     
  10. tonebrulee

    tonebrulee

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    Apr 28, 2012
    They are used during the full time to discharge (approx 20 seconds) but note that the voltage will drop by 2/3 in the first 5 seconds, so the period with the highest voltage is brief.
     
  11. tonebrulee

    tonebrulee

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    Apr 28, 2012
    Really? Hmmm, that seems surprising - I would imagine the rating has to assume a stressful condition, rather than an ideal one. If that's not the case, how can the rating be relied upon really?
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Because if you look at the specifications for the resistor it will tell you all about how you need to de-rate its dissipation when the ambient temperature varies from some nominal value.
     
  13. tonebrulee

    tonebrulee

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    Apr 28, 2012
    Ahhh, the data sheet, OK that makes sense - thanks. I guess that's why a 10W resistor is suggested for this application - probably overkill by some large factor but spec'd that way to handle unexpected excessive heat.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,401
    2,777
    Jan 21, 2010
    The resistor specified for almost any use will typically be rated for a power dissipation in excess (sometimes well in excess) of what you expect it to be called on to dissipate. This helps keep things cool(er) and extends the life of your circuit by operating things under less stress.
     
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