This is driving me insane. - I've tried a TIP31 (lights dimly) and an IRF730Basically, I have a 12v rail, which goes to the collector (12+) 0 to ground, the circuit is fine, if i feed 12v directly from the rail, both transistors light up the 12v bulb... but, connect the 5v to the transistor, the tip31 glows (a little) and the IRF, does not even light up.Surely, I can control a 12v source from 5v? without the need of a SSR/Relay ? (I want a 12v car bulb to switch on from 5v without using a relay, this seems futile) Thanks (yes, the IRF has a pull down resistor to stop it from constantly being on, it works if the base gets 12v, if it gets 5v, nada!) So i'm left wondering... Transistors ? in order to get the power flowing, you need the same voltage in and out? (eg 12v on base, 12v on collector vs 5v base, 12v collector)
Your description is unclear. Do you have the load connected:(a) between the drain of the mosfet/collector of the TIP31 and +12v; or(b) between the source of the mosfet/emitter of the TIP31 and 0v?Method (b) will not work because only 4V or less will get passed to the load using the TIP31, and even less with the IRF730.Method (a) will probably work with either the TIP31 or the mosfet. However, if you use the TIP31 then it may get hot because of the high saturation voltage. If you use the mosfet then it may also get hot because the IRF730 is designed to be switched from 10V not 5V, so it will not be turned on fully. The best solution is to use a logic-level mosfet instead.