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What is the high end temperature for a tp42C Transistor?

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electrocub

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I built a 9.6 volt battery charger that uses a PNP TP42C transistor. I am using a cheap heat sink for the transistor that looks like a piece of black channel iron with slots cut in it. The charging amperage to the battery is .4 amps. The transistor with the heat sink is operating between 175-190F degrees. The circuit seems to be working and the battery charging, but is this too hot for the transistor? Also it is out in the open. Also wouldn't it need some vent holes drilled in the enclosure to keep it cooler when I go about enclosing it?

Thanks

Carroll
 

audioguru

Well-Known Member
Most Helpful Member
I can't find a TP42C transistor listed anywhere. Maybe it is a TIP42C?

The datasheet for the TIP42C lists its max allowed internal temperature at 150 degrees C. Its max allowed dissipation without a heatsink is 2W and your tiny heatsink is nearly useless. If you use a pretty big finned heatsink then it could dissipate 30W safely.

But you don't say how much is the voltage across it so we cannot calculate its dissipation.

An enclosed heatsink is useless. The heatsink needs to have plenty of air around it or a fan blowing cool air across it.

A 9.6V charger needs to have a minimum input voltage of about 15V. Therefore the transistor has an average of (15V - 9.6V)= 5.4V across it and at a current of 0.4A will dissipate only 2.16W. The transistor will be warm if the heatsink is a medium size.
 
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