[SOLVED] Mosfets always on...

Hi all,

I have a project in which I’m using STP16NF06L mosfets to switch a 12v solenoid. My problem is that the mosfets are always on, regardless of whether I’m providing a voltage to the gate or not… I’ve attached an image showing my circuit. You’ll see that I’ve wired 2 mosfets two different ways but left the gate without a voltage and the LED is on.

I was expecting that the LED would be off until i provide a voltage to the gate leg.

Any ideas what I’ve done wrong?

Don't know why you hook them up differently.
Connect correctly and try a gate resistor to ground, maybe 10k.

When you leave the gate unconnected, you have what is known as a floating input. Because the gate terminal is such a high impedance, you can easily pick up noise/unwanted signals that will turn the MOSFET on. The gate terminal has some capacitance to the source/GND once this is charged up, the FET will conduct until the charge leaks away.

You should make sure that the gate is connected through a resistor to GND as bluejets suggested.

Rather than try several different ways of connecting up your circuit, why not spend a little extra time and connect it up the correct way the first time?

The structure of the MOSFET contains a parasitic/protection diode so when you connect it up the wrong way round then it is likely to conduct permanently.

Thanks for the help guys. But you say "connect it up the correct way" like it's so obvious. The examples i found didn't have the gate connected to ground, but now that i re-read some of the sources, i now realise what they meant. I see now where my understanding was wrong.

Thanks again

Can you edit your Subject title and add [SOLVED] to it please, it makes it a bit more informative that a problem has been solved.

Thanks.. Tom... :slight_smile: