I'm controlling a coil running at 32V from a microcontroller running at 5V via a BUZ71 N-Channel enhancement mode MOSFET. The coil has a resistance of about 30 ohms and it draws about 0.6A at 32V.
I supply +5V (or about 4.9V according to my multimeter) to the gate of the BUZ71 via a 1K resistor, and I've also grounded the gate with a 100K resistor to ensure it stays off when the microcontroller isn't supplying 5V.
I've got +32V going into the coil, and the coil connects to the BUZ71's drain. The BUZ71's source connects to ground without a resistor.
The whole apparatus seems to "work", and the coil is energising as expected, however when I measure the voltage going to the coil, it appears to initially spike at about 22V and then drop down to about 10-15V. It's hard to see exactly what's going on and when because my digital multimeter only updates about twice a second.
I'm totally new to MOSFETs, and I haven't even really use transistors a great deal. I've looked at the data sheet for the BUZ71 and I think +5V on the gate should fully activate it, but I'm not sure.
Does the setup I've described here reflect how I should be using the MOSFET? Is +5V enough to fully activate the MOSFET? Is the BUZ71 appropriate in this case, and if not then what other MOSFET do you recommend?
Big thanks.
Best Answer
While your setup sounds OK, I suspect you're doing something odd. To begin with, a 30 ohm coil should draw about 1 amp (32 volts/30 ohms), so your current draw of 0.6 amps suggests that you've got a ground problem in your coil circuit.
I assume you're using your 5-volt ground as your reference. Try activating your FET, then measure all 3 pins. Is the source reading 0 volts? What is the drain reading? Are you sure you've got your 32 volt ground properly tied to your 5-volt ground?