I was thinking about the same thing. Because fets are voltage operated, not current and does not consume a lot of current on the gate, am I right?
Right you are.
I want this project to be a stable active load to measure power of the power supplies.
Well, I can of course measure voltage on the output of Opamp (I think it's better than positive input because negative input is not connected to the ground but to the drain of transistor as a overload protection) but will it be reliable and linear source to measure?
??
What value the power resistor should have to handle 8A ?
R = E/I = 1.2/8 = .15 ohms, but at least 10 watts.
But --- Let's say your supply is 12 volts. At 8 amps the FET will have 10.8 volts across it at 8 amps or almost 88 watts. The FET cannot handle that much power. It will need a big heat sink just to do the 3 amps.
I don't know if it will be worth to do measuring circuit because I've got most of the parts and attiny/atmega + lcd will cost 10 times more. And I won't use it often. What do You think?