35Watt resistors,do they need heatsink?

Discussion in 'General Electronics Chat' started by 64C113M Abe., Oct 3, 2016.

  1. 64C113M Abe.

    Thread Starter New Member

    Sep 13, 2016
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    As the title states, I have on the way a collection of 10,30,50 ohm resistors that look similar to TO-220, would I need a heat sink for them?

    They are rated at 35Watts.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Yes.

    Think of it this way. You can melt solder with a 15W soldering iron.
     
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  3. 64C113M Abe.

    Thread Starter New Member

    Sep 13, 2016
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    Roger that, thanks. Good thing I also included Heat-sinks in the order.
     
  4. #12

    Expert

    Nov 30, 2010
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    6,809
    In this case, it is clear that a TO-220 package can not get rid of 35 watts of heat without a heat sink.
    But there is a lesson. All resistors can remain cooler with a heat sink attached. They can also be made worse if you add shrink tubing or confine them in a box with no air flow. Many high watt resistors arrive with a flat side and some have ears with bolt holes. You can get much better performance and/or reliability by adding a heat sink to most resistors. Sometimes it is nothing more than a larger area of copper on the circuit board. Sometimes it can be liquid cooling! In between those is air flow. It can be surprising how much heat a little bit of air flow can remove.
     
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  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    OK, I'll play too.

    No, they do not need a heatsink, as long as the maximum dissipation times the thermal impedance from element to ambient is below the maximum element temperature.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    IOW, keep the current below 0.2A and you don't need heatsinks.
     
  7. schmitt trigger

    Active Member

    Jul 12, 2010
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    When in doubt, check the datasheet.
    It will tell you maximum operating temperature and its thermal resistance, case to ambient.

    With those parameters, and the maximum expected ambient temperature, you can calculate the maximum power dissipation that won't damage the resistor.
    You can exceed that power dissipation briefly. But for long term reliability, the recommendation is to give the circuit some margin, and derate the maximum power dissipation.
    A 75% derating is a good rule of thumb.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    From the datasheet of a 35W and 50W resistor in TO-220 package

    http://riedon.com/media/pdf/PF2200.pdf
    upload_2016-10-4_14-45-23.png
     
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