Nmos Floating Pin Voltage

Discussion in 'General Electronics Chat' started by DavS13, Aug 30, 2014.

  1. DavS13

    Thread Starter New Member

    Jun 16, 2014

    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.
  2. #12


    Nov 30, 2010
    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.
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    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.

  4. DavS13

    Thread Starter New Member

    Jun 16, 2014
    Thank you guys!