Why does transistor get so hot

Thread Starter


Joined Jan 30, 2011
If it is rated for 200ma continuous current, and my constant charging circuit below is set to only supply 80ma to my nimh battery? Makes no sense. I could understand if I was drawing near or above the maximum rated collector current. 80ma of power flows through the transistors collector and out the emitter. The LED's below have been replaced with my battery to be charged. 4k7 connects to + power rail at the top. I used 2n3904(collector current rating of 200ma). BTW>> I already know about the lm317t constant current setup. I need to know why this setup is running hot.




Joined Apr 30, 2011
There are only two possibilities: too much base current or too much collector current. Since you say your collector current is well within specification, that leaves… Once you've driven the transistor fully into saturation, additional base current just makes heat. You only need 8mA of base current to achieve 80mA of collector current. According to your formula, the emitter resistor should be 8.75Ω.

Last edited:


Joined Mar 6, 2009
You may be within the collector current rating but you need to consider the device power dissipation.

The supply is marked as ranging from 6-15V.

If the battery under charge is 6V and the supply is set to 15V then the collector-to-emitter volatge would be around 8.3V. At 80mA for Ic this gives a collector dissipation of 664mW. So the transistor may well start to feel hot.


Joined Feb 12, 2009
80mA or current flows through the transistor - not power!

The LEDs together only drop 3V or so (depends on the type of LEDs). The emitter resistor drops 0.7V. The rest of the voltage is dropped across the transistor. Not surprising it's getting hot.

BTW. My definition of "hot" is if you can't hold your finger on it. This is approx. 60deg C or more.

Measure the voltage across the transistor, then P = V x A.

Reduce the supply voltage or use a bigger transistor with a heatsink.