Power dissipation across multiple resistors

Discussion in 'General Electronics Chat' started by tom66, Jun 22, 2009.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    If I have a 100 ohm/0.25 watt resistor in my circuit I can dissipate 0.25 watts safely across the resistor \frac{{V^2}}{R}. But what if I have two resistors... I now have 50 ohms, but how much power can I dissipate? What about multiple resistors of different values...?

    Any help appreciated. This is not homework, this is for a project to make my own oscilloscope.

    Tom
     
  2. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Hi.
    Two 0.25W resistors in parallel can dissipate 0.5W. Two 0.25W resistors in series can dssipate 0.5W.
    Many resistors in parallel, or in series, can dissipate the sum of their rated power.
    Miguel
     
  3. gerty

    AAC Fanatic!

    Aug 30, 2007
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    If you put 2 50Ω in series, it's still going to be .25 watt, since the current remains the same in a series circuit. Is this what you had in mind?


    Externet, you beat me by 2 min!!
     
  4. Externet

    AAC Fanatic!

    Nov 29, 2005
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    :rolleyes: Yes, but not trying to beat... And our responses differ :) The difference can be in the word 'can'.
    Miguel
     
  5. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Okay, sounds great. I understand. But what about different values for each resistor?
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Are you going to put them in series or parallel?
    If you post your schematic we can help you without guessing.
     
  7. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I'm just curious what each configuration would allow me to do. There is no schematic at the moment.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    It is good practice to use resistors which are rated for double the power expected in the circuit. If you expect 1/4W power dissipation, then use 1/2W resistors.

    If you use 1/4W resistors with 1/4W actual power dissipation across them, it may cause the value of the resistors to change over time.

    If you expect to dissipate 1/4W across a 100 Ohm resistor rated for 1/4W, then you have a few options:
    1) Increase the rating of the resistor to 1/2 Watt.
    2) Use two 200 Ohm 1/4W resistors in parallel.
    3) Use two 50 Ohm 1/4W resistors in series. The closest standard value is 51 Ohms.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    A resistor operating at its max power rating is extremely hot. It will melt insulation and might char a pcb. It will burn you if you touch it. Don't operate a resistor at its max power rating.
     
  10. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I know... I have burnt myself on a resistor before.

    What I am asking is if it is possible to create a 1/2W resistor from two resistors at 1/4W. This is theoretical, I am not going to run them at 1/2W instead probably 3/4 the total rating (3/8W).
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Yes. Double the resistance and connect in parallel, or half the resistance connected in series.
    The "rule of thumb" still applies. I strongly suggest that you use resistors rated for at least twice the expected power dissipation.

    Resistors are very cheap. No sense in ruining a board or project because you used under-rated resistors.
     
  12. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Yes, either in series or in parallel.
     
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