IRF530 mosfet act wierd

Hi all
I have this project that calls for a power mosfet, so I've bought 4 new IRF530's, and just to test one out I made this little setup, using a 12V 200mA DC adaptor.


but after about 1 sec the motor starts spinning even thou the gate is not connected?!?
It works fine if I replace the 530 with a NPN transistor, or even with a IRF830 I've salvaged a while ago.
Can anyone tell me why the mosfet behave this way?

Regards.

You need a resistor from the gate to ground so the gate voltage doesn't rise when no input is connected. As for why, mosfets don't transfer current through their gate to ground. It's very high impedance, so any leakage current will slowly turn it on unless you make sure the bigger leak is to ground by adding a resistor.

and you dont even need the 1k resistor in series with gate. Also is this a big motor?, maybe the 1n4001 is a little small if the motor is big.

Yes sir!
putting a 1M resistor from gate to ground worked, thanks #12!

the motor I tried is a 12V NB fan, not very big.

If you are driving this mosfet from a 5 volt source such as a microprocessor, an IR-F-530 is a bad choice. It requires 10 volts at the gate to fully turn it on and pass its maximum current. At 5 volts it will only pass about 4 amps. IR-L-530's are logic level devices that work well at 5 volts.

A 1N400x diode will pass many amps for a short period of time.