Nmos Floating Pin Voltage

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DavS13

Joined Jun 16, 2014
21
upload_2014-8-30_13-6-4.png


Hello everyone,

I recently built a PWM speed control for the motor you see in the picture. As a switch I used a 2n7000 NMOSFET, and I am puzzled by what happened when it was first implemented. The motor was running when there was no voltage applied to the gate. Somehow the voltage on the gate is greater than the threshold voltage when it is a floating pin. How is it that the transistor is conducting when there is nothing attached to the gate? I am assuming it has something to do with the 15V to the motor, but I am not sure.
 

#12

Joined Nov 30, 2010
18,224
Don't do that!

Any mos gate is a capacitor with a rather low breakdown voltage. It can start up from static charge, leakage currents, or capacitive coupling when you applied power to that board. Leave it floating and you are inviting it to get damaged.
 

KMoffett

Joined Dec 19, 2007
2,918
The open gate can respond to a static charge from the environment. It's not a good idea to leave a gate unconnected. Shuffling across the floor and putting your hand near, not eve touching, an open gate can turn it on or even destroy it.

Ken
 
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