Hi, I'm plaing to make a thermal switch, and i want to do it with a transistor (relays click anoyingly) but I'm not sure of what transistor I should use. The voltage throught the transistor will be about 12 volts and the current about 5 amps. Now I'm wondering if anyone could help me, will a normal biopolar (BD243) do the job or would it be better with a fet, or even a mosfet?
You could use an Lm311 op amp as a comparator thermistor as your temp detecting device, with something like a 10k resistor as a pull up on the output(its an open collector op amp) and use a BUZ11 mosfet to switch with.....Does that fit the bill ?
A BD243 will do the job, though it's only rated at 6A, so you will be getting quite close to its limit. You may want to consider something a little higher rated (eg, MJE3055 - 10A).
Using a mosfet has advantages and disadvantages. They require very little drive current (several pA into the gate), but tend to have a higher "on" resistance than bipolar transistors. However, if you are prepared to pay more, you can get MOSFETs with a low on resistance.