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    Actual realistic power rating of power resistor?

    Hello

    We are doing a 90-265VAC, 330W flyback with no PFC stage for guitar amplifier supply (Class D).

    It is single switch flyback, so with a 1% leakage transformer, we will get 9W of loss in the primary clamp at max load/min vin.

    Therefore we wish to put two of these 5W resistors into the RCD primary RCD clamp

    http://www.seielect.com/Catalog/SEI-VM_MVM_LVM_WVM.pdf

    ...they say they can handle 5W, but does that 5W rating depend on this resistor being soldered into a large thick copper plane to remove the heat?

    ....or could this resistor dissipate 5W continuously even if soldered to a minimum footprint?
    Last edited by grizedale; 16th June 2012 at 13:04. Reason: spelling

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    Re: Actual realistic power rating of power essitor?

    I have previously used this resistor form in a number of projects. They are disspating the power by surface convection like other ceramic wirewound resistors. The surface gets rather hot.

    You can read from the derating curve, that they are assuming a surface temperature near to 250 °C. Connection traces of sufficient width should be present to avoid self-desolder of the part. You should reserve sufficient space around the resistors to allow free convective air circulation and avoid overheating of near-by parts.



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    Re: Actual realistic power rating of power essitor?

    To emphasize what FvM says, these resistors will get very hot at 5W. Recently I tried using some 5W surface mount resistors, and found that while the resistors could withstand 5W, when they got near that limit the plastic on a nearby connector (which connects to a thick trace directly to a lead of the resistor) would start melting. The solder on the resistors could also melt, but I used glue underneath the resistors to hold them in place when this happens. So definitely give thought to how other components are going to feel about that resistor getting to >200C.



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    Re: Actual realistic power rating of power essitor?

    You can consider using a heatsink to make sure the resistors don't get too hot. In some commercial power supplies where such large resistors are used, I've seen them being attached to a heatsink.



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    Re: Actual realistic power rating of power essitor?

    Heatsink is an option. The ceramic case power resistors are designed to work without it, but a hetasink of course reduces the surface tempreature. As a drawback, you need to account for the additional power in the heatsink thermal resistance calculation.



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