Driving TIP31C in active region for making a continuously variable load

[manishanand14]

Hello everyone!

Please tell me the meaning of power dissipation P_D in the pic from the datasheet.


What's the meaning of T_C=25 degree and T_A=25 degree. Also I guess power dissipation will depend on heat sink mounting. Please tell me what all I must consider.

I wish to make load[continuously variable] by changing the base current and connecting 10E/5W resistor in series with emitter and gnd.

Max. current draw of the load will be 2A.

This is the diagram.

 

[chuckey]

Tc = temperature of case.
Ta = ambient temperature.
Max collector current = 2A therefore base current = 2/40 = 50 mA, you are going to flatten your 9V battery quickly, also the pot feeding the base will need to carry this current, which is basicallt too much for a standard device. feed the base via another emitter follower, then the input base current will be 50/100 = .5 mA which a pot can handle. save your self the price of a battery and connect the pot to the + line.
Frank

 

[manishanand14]

Thanks
I have made this and it is working fine.



One more thing I want to ask. How would you know or rather predict the junction temperature if you know temperature of the case i.e. the heat sink.

 

[chuckey]

From :- **broken link removed** , the thermal resistance from the case on a heat sink to the junction is 3 degrees/watt. So if the power dissipated is W watts then the junction temperature = Tcase + 3 X W degrees which must be less then 150 degs.
Frank