# struggling to use MOSFETs as switches: is the problem my circuit or the choice of mosfet?

We are using an arduino to selectively apply 5V to the gate of MOSFETs, with the intention that current will only be allowed to pass through our solenoid from the supply in the situation where both the MOSFETs that a particular solenoid is attached to have a gate voltage of 5V. 5V appears to be above the threshold voltage for these MOSFETs (IRF520) but we are getting much less current than expected unless we increase the gate voltages. Can anyone explain this? Is it a problem with our circuit or the choice of parts? Is using two MOSFETs back to back like this a bad idea?

We will eventually be controlling a larger grid of electromagnets to selectively turn them on for several seconds at a time, up to 5 at a time. This grid will be up to 17x17 so it is not ideal to use a MOSFET for every magnet!

• You need flyback diodes across all of your electromagnets. – tcrosley Mar 28 '14 at 3:31

This FET's threshold Vgs (the voltage that starts the linear range of this switch) is rated at a max of 4V! That means to saturate your fets, you will need a much higher Vgs than 5V.

In case you don't know what i'm talking about: Measure the voltage at the gate (Vg) and source (Vs) at each FET. Subtract the two voltages... ie (Vgs = Vg - Vs). The datasheet for your FET recommends that Vgs be around 10V.

Your M1 and M2 should be PMOS, unless you are able to somehow generate a 10V delta from the source to the gate.

As far as your M3 and M4s, for the reasons stated in the first 2 paragraphs, a Vgs of 5V isn't going to bring the FETs to saturation.

You need 5V above the source of the MOSFET. If the FET were ON, the source would be at 5V and you would ideally have 5V ABOVE the source on the gate, or 10V on the gate.

You could use P-Channel FETs and pull the gates to ground to turn them on, or you could provide a higher gate voltage for the N-Channel FETS.

The low side FETs should be switching OK.

• Good catch by Hassan below, I assumed you were using logic level FETs, didn't check the part number. If you want to use 5V gate drive on the low side you will need lower threshold FETs. Otherwise you'll want closer to 10V drive on the low side and 15V with respect to ground on the high side. – John D Mar 28 '14 at 14:31