The pump draws 0.5A @ 12v while connected at +12v directly. I havent measured it in my circuit.I have tried to use IRF510/530 as they have same pinout as 3055's but it didnt work at all.What transistor would you suggest ?
To drive a power BJT transistor fully into saturation, you should use a base drive equal to one tenth the collector current.
Do NOT use a darlington configuration, as the saturation voltage will be about 1.4V instead of 0.1V or thereabouts.
How much current does the pump draw (need)?That MJE3055 has very little gain (typically around 50 or so, but can be as low as 20) and is probably not turning on all the way.For instance if the pump needs 2 amps, you would need at least 100ma into the base to ensure the transistor is fully on. I think you might want to add a 2N2222 or 2N3904 as a Darlington front end to the MJE3055.