Electronic – What could be the reason that this transistor gets too hot unexpectedly


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.

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The opamp is an LM324 and the NPN transistor datasheet is here.

Here is the photo of the transistor:

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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?

Best Answer

How to know if you need a heatsink. (Apart from the burnt finger).

This is well covered in engineering textbooks and courses. And websites such as this.

In short :

You know the power dissipation, and the ambient temperature where this lives. Let's say, 1W, and 40C.

You decide what temperature you want to restrict the junction to : below 100C, lets say 90C for a safety margin.

From the Junction-Case thermal resistance (10C/W) and the power (1W), that gives a case temp of 80C (or 40C above ambient).

Which means we need 40C/watt between case and air.
(If the 62K/W figure is correct for a SOT32 package ... google it ... you need a heatsink.

So we look for a heatsink with 40C/watt or better, thermal resistance.

(This is slightly simplified to illustrate the principle, but good enough for this application. For more detailed thermal budgeting, see e.g. the website above)

Damage can take microseconds to years, depending on the degree of overheating.