Hi guys. I'm doing some project which will use mosfet as a switch. So, i tried to play with some mosfet first; the irf540. I tried out the simple circuit like turning led on/off found in the internet. I have a crystal oscillator connected to a comparator to provide signal to the gate and i use supply voltage of 12V from sla batt to the drain. When i powered up the led turned on, but when i removed the signal from the gate, the led kept turned on. Then when i removed the supply voltage and reconnect it back the led did not turned on without signal from gate. So what is the problem ? Is the mosfet spoilt ? I tried to find any statement on this on the net but i'm very confused as I'm new to building practical circuit other than simulation.