TIP122 Heating Very Much, Is this normal?

[losturcos]

Hello,
I have a 24VDC regulation circuit which converts 60VDC input to 24 VDC by using TIP122 and 7824.
The voltage on the TIP122 transistor is about 25 Volts and the current passing over the transistor
is about 70-80 miliampertes generally.
Actually this is a part of a circuit. My problem is TIP122 is heating very much without heatsink.
Even with heatsink(not a very big one), it is heating, but it can be touched by hand. Is this normal?
Thank you.

 

[FoxyRick]

With the TIP122 as an external pass transistor, it is dropping 60-24 = 36 volts Vce.

At 80mA that gives a power dissipation of 2.88W, plus a little bit for the base current and Vbe.

Yes, it will get quite warm dissipating that amount, that's normal. Choose a heatsink to keep it comfortably touchable, as it sounds like you have.

 

[losturcos]

Thank you very much. Btw the circuit is as below:
View attachment 24Vreg.bmp

 

[FoxyRick]

Ahhh... the TIP122 is used as a pre-regulator to drop the voltage to the 7824. In that case...

With the base at 37V (24V+13V zeners), the emitter will be at about 35.8V, so the TIP122 is dropping 24.2V (now the 25 volts you mentioned makes sense to me).

So, it will be dissipating about 2W - still enough to make it quite warm, as you have experienced.