How to install a heat sink in a dc circuit

Question

i have a heat sink board and i want to know how to install it to keep the transistoers cooler becaucose every time i close tthe circuit the transistor heat up and burn

Answer 1

you didn't said what kind of transistor. because the transistor which driving load will heat. that is normal. these type of transistors have metal side at back. this may collector or emitter. while you placing heat sink the heat sink must not contact metal part of transistor. it must be separated by mica or rubber sheet with plastic bushed bolt and nut. confirm that the heat sink fixed tightly.  

Answer 2

you have it backwards. You install (mount) power transistors to a heat sink.

Determine which transistor is over loaded. Calculate from the schematic the maximum power dissipated in that transistor. From that and from the data sheet for the transistor, decide on a heat sink and attachment method that will allow the transistor to operate without exceeding it's maximum junction temperature.

To do this you add up the thermal resistance of the various elements in the heat sink, that is, the thermal resistance of the junction of the transistor to it's case, the thermal resistance from the case to the insulator, the thermal resistance of the insulator, and the thermal resistance of the heat sink. The numbers should be in terms of degrees C per watt.

Now you multiply that number by the number of watts the transistor is dissipating, for example, 100 watts x 2ºC/watt = 200 degrees.

Add that to the ambient temperature, say 35C to get 235C, which would be the junction temperature, which has to be less than (hopefully a lot less then) the max junction temp, typically 400C

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