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Old 16th June 2012, 01:00 PM   #1
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Default Actual power rating of 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?
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Old 16th June 2012, 01:25 PM   #2
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
No, standing in free air with Ta as specified, Ta=55ēC, not 70ēC.
But @ 100% duty cycle they will run hot, hotter than you can finger touch.
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Last edited by AndrewT; 16th June 2012 at 01:30 PM.
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Old 16th June 2012, 01:27 PM   #3
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As a general rule if a component requires a copper plane for a heat sink it will be specified in the spec sheet along with the area they needed for cooling.

With that said, for long life and if the enclosure has no fan, do not run the resistor more than 50% of its power rating. Dead air heats up fast.

There is a power derating curve in the pdf, but you have to build it and measure the operating temperature to know if you are all right. Its faster and easier to just derate the resistor 50%.
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