[HaPPy_BoY]

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.

[sourisbulle]

hi !

the mosfet grid acts as acapacitor.

if you don't discharge the grid capacitance, it stays on for a time until leakage current do the job...
the same for igbt's

at least use a bleeder resistor of 1k...

for efficient command of mosfet or egbt you must use a push pull stage to have a fast charge/discharge otherwise the transistor will stay in linear mode and generate a lot heat... in switching mode heat is minimal (full current and minimal voltage across transistor or no current and full voltage)

hope this will help

regards

sourisbulle

[vanjast]

How did you 'remove' the signal from the gate.

If you physically disconnected the comparator output to the gate then this would leave the gate 'floating' in the ON state, and any noise on the power rails can cause it to oscillate erratically. A low value bleed resistor, as mentioned, should control this. Otherwise the comparator output, if connected, should discharge the mosfet gate normally.

If you're looking at using the mosfet to drive a motor, use a mosfet/IGBT driver chip as these have guaranteed drive capabilities for high speed switching. In addition to this, is the use of a snubber circuit across the Mosfet Drain/Source as the motor inductance will produce nasty results with your control system, if not smoking your mosfet, motor, and anything else - use fuses everywhere.
:bat: