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While Designing battery charger circuit using LM317, how is the transistor used for CC selected.Also other than heat sink can any other method be used to control heat.
Welcome to the forums!
I am assuming that you are referring to Figure 22. If so, really a general purpose transistor is fine. The LM317-N datasheet suggests a 2N2222
When the transistor turns on due to overcurrent and pulls the ADJ pin to ground, the transistor just needs to be able to handle the current. And also the transistor needs to be able to handle the different voltages.
There is also an app note here: www.ti.com/.../snva581.pdf
It doesn't really talk about the transistor, but more about how these kinds of circuits operate.
As for heat, controlling the input to output voltage is pretty critical. Reducing your voltage drop across the regulator goes a long way, but of course it also depends on output current.
What are you looking at for input voltage, output voltage, output current?
You can use a resistor at the input of the regulator to dissipate some heat, but it depends on your setup.
