In the following circuit, for the transistor it can be calculated that: Vce=4.5V and Ic=170mA. I also implemented this on breadboard and measured almost the same quantities.
The opamp is an LM324 and the NPN transistor datasheet is here.
Here is the photo of the transistor:
The transistor power in the circuit above must be P=VcexIc. This becomes 4.5V x 0.18A which is less than 1W power.
In the transistor datasheet the thermal resistance is given as 10 degrees per Watt. So in my case I expect not more than 10 C increase. The room temperature is 25 C and I expect 35 C for the tansistor surface.
But when I touch with my finger it feels like it is very hot like 100 C. Is my interpretation of the temperature wrong or is my expectation is wrong? Why does it get hot that much?