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.

 

Do you have a schematic of the circuit you assembled (attach as a .PNG file)

Can you provide a photo of the circuit you assembled (attach as a .JPG file)

 

Power mosfet gates have substantial capacitance and need to be driven low to turn off. Just removing a signal will not discharge the gate.

 

The circuit is attached (tran21.gif & 05122010398.jpg) .. I hooked the output of the oscillator to non-inverting input and ground the inverting input of the comparator lm358N. The output of the comparator is to the gate of the mosfet.

Also, i wonder if mosfet is suitable to be used in dc-dc luo converter. I'm building one voltage regulator to drive servo controller with 10 servo motors attached.

 

I like to know more about the gate voltage of the mosfet irf540. I read from net that it is required that the gate voltage to be referenced to the drain voltage in order to fully turn on the mosfet. Is this true for irf540 also ? Let's say the next time i supply 12V to the drain then would I need Vgs=12+4 V ?

 

I read from net that it is required that the gate voltage to be referenced to the drain voltage in order to fully turn on the mosfet.

That is not correct. The gate voltage is relative to the source voltage. Full conduction comes when the gate is at least 10 volts positive with respect to the source.

 

In the case of a p-channel enhancement type mosfet then the gate voltage will need to be taken negative with respect to the source.

hgmjr

 

...when i removed the signal from the gate, the led kept turned on.

With the circuit you posted (which should work)? What is the value of the resistor connecting the gate to ground? As noted, the gate is a capacitor and the charge must be drained off for the state to change.

FWIW, I have a handful of that exact same MOSFET and it's what I use whenever I need one. No particular reason, I just had to pick one and that was it.

 

In the case of a p-channel enhancement type mosfet then the gate voltage will need to be taken negative with respect to the source.

Quite true, but the IRF540 is N channel. The gate would have to go negative if it were an IRF9540.

 

Quite true, but the IRF540 is N channel. The gate would have to go negative if it were an IRF9540.

Yeah. I was aware that the target device was an n-channel. I was just elaborating on your comment about the n-channel for the sake of completeness.

hgmjr

 

Sure. Hopefully the OP got something useful from that.