I was experimenting with an IRF840 and connected a small load transformer inline from battery 12V+ to the Drain pin and the Source pin to 12V- leaving the Gate pin completely unconnected. To my surprise the MOSFET started to get warm.

Could someone please explain what is happening here because when I connected the Gate pin to 12V-, it stopped conducting.

Many thanks

 

hi,
An Open circuit MOSFET Gate can assume a 'high' state, so the MOSFET will be partially On.
As the MOSFET is not fully switched On, its dissipation will be high.
E

 

Many thanks Eric, I understand a little more now.

Cheers
Bob

 

hi D,
This simple LTSpice shows the effect, note between Vg = 2v and 3v,[partially On] the FET heating.

E

 

Hello,

In some schematics you will find a resistor of 10K to 100K from the gate to the source mounted.
This will avoid the mostfet to be conducting partially.
The gate can be seen as a capacitor and the resistor will discharge the capacitor.

Bertus