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?

Thanks for any explanation!

 

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 ?

 

Yes, thanks. But I have already the temp sensing circuit ready, i only wanted info on the transistors :wink: but thanks for the help!

 

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.

 

How about 2N3055? It is rated for 15A Ic(max.).

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whats the difference between this just a momentary switch?

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thanks,
david

 

David - can you be a bit clearer? Not quite sure what you are asking...