In this highly inefficient circuit:
(I forgot to insert a 1K resistor before the pot in this diagram)
I have 2 components that I use as the TRANSISTOR of this circuit. A STP55NE06 (TO-220) (which for some reason, is shortened a P55NE on the marking and fades off quite after a while), and a large TIP35C (TO-218).
The circuit works as expected, however they get really hot, where the STP55NE06 would reach about 90 degrees celcius and the TIP35C about 80 degrees celcius, due to the obvious wasteful power dissapation.
Recently, I noticed that in the TIP35C, the bottom back of the plastic looks a bit burnt. I looked in the datasheet and saw that it could handle the current and voltage of the bulb and the temperature as I measured wasn't over. Do I need a heatsink? Also I think I made an error and had the base voltage over 5V (12V in fact). Would that be the reason? The transistor is working fine as I can see from this circuit but I'm a bit worried about discolouring of the plastic.
In addition, the STP55NE06 looks not so good as well. THe bottom back of the metal looks a bit burnt (or is that corrosion) but the plastic isn't as burnt as the TIP35C.
Have I done anything wrong? Is this normal?