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Power rating of WEL resistor

**Rafael**

Apr 27, 2015
16
Joined
Apr 27, 2015
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Hi there!

I am repairing a 1 HP TB Woods AC drive. After taking it apart, I noticed a couple of flat 100 Ohm capacitor-precharge-resistors blown.

I did a Google search to find out power rating but none was found. Thinking that they probably were around 2 or 3 Watts, and as they are in parallel, I installed a 5W 47 Ohm standard type power resistor.

Drive worked but resistor overheats. So drive is in the back burner while I find out right power rating. Can physical dimensions be used to determine actual power rating? Resistor measures 25.4mm long, 12.7 mm high and 1 mm thick.

Thanks in advance for any help you guys can provide.

Rafael
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
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What voltage is on the capacitors?

Use the formula P = V*V/R to determine the actual dissipation.

If the voltage is 25V, then the power dissipated is 25*25/100 Watts. That's 6.25W. in this case of use a 10W, or maybe even a 15W resistor. Always leave plenty of margin of you can.

Also beware that resistors need either some airflow or heatsinking (depending on the style) to permit anything near their full dissipation.
 
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