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.

  • \$\begingroup\$ How many volts across the MOSFET (drain-source) when it's ON? \$\endgroup\$ Aug 27 '15 at 10:41

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?


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