guestion about a mosfet

[Lucifre]

i have a mosfet with source grounded, +5 v and led with resistor connected to drain. when i apply 5v to the gate transistor turns on... the problem is when i want to turn it off... when i remove the wire from the gate led still glows when i ground the gate it turns off, but it turns on again as soon as i disconnect the wire from ground. I knoe the transistor is ok because i bought two of them and both of them act the same way. Is that the way mosfet should work? i thought that if you disconnect the wire from the gate it would turn off.

 

[IanP]

Because of very high input impedance of the Gate, if left open, it can pick up some charge ..
Connect 100kΩ (or more) resistor between the Gate and the Source ..
This resistor should fix your problem ..

Regards,
IanP

 

[manojshukla]

Use proper biasing.
Without biasing, if you will connect the +5V to gate and later remove it, it is possible that gate drain capacitance may get charge upto +5V, and may lead to diode in on condition. As soon as you ground the gate , the diode will be off, as the gate-drain capacitance get discharged.

 

[on1aag]

Hi Lucifre,

It's the gate/source capacitance that is still charged, you should connect
a high value resistor across this capacitance to discharge it.

on1aag.

 

[mkhafaji]

This is high-impedance or Z condition when you don't connect anything to such nodes. The digital circuits always take it az hi because of very high input impedance of the gate and its capacitor. To solve the problem your biasing shouldn't have free nodes.

 

[Lucifre]

Thanks every one for replying