Why MOSFET makes Motor ON directly?

Question

In my project I have to operate DC motor. For this I am using Arduino as controller and MOSFET for switching. I have connected MOSFET GATE pin to Arduino's pin 12.

I have used this schematic....

Main problem is as I give +12v supply to one end of Motor and other end of Motor to Drain of MOSFET and other connection same as Image. But without connecting Arduino output pin to Gate of MOSFET motor staying on continuously.

In my case I am controlling GATE pin using pin 12 of Arduino,which will send pulse to MOSFET to control motor(ON/OFF).

What wrong am I doing?

Answer 1

The circuit you show should work. You probably don't have something connected correctly. For example, if the drain and source of the FET were flipped, you'd get exactly the symptom you see due to the body diode of the FET conducting.

Check the FET datasheet and your connections carefully.

Measure the gate voltage with a voltmeter, and verify that it really is 0 V.

Answer 2

Two possibilities:

1. Incorrect connection of the MOSFET - especially swapping Source and Drain will do this. Careful measurements with a multimeter should show the Source-Drain connection as a diode - if it's wrong way round, the diode conducts, running the motor.
2. Electrostatic damage can destroy the MOSFET. If it appears to be a short circuit both ways round - OR if excessive leakage to the gate means there's voltage across R2 - that's what has happened. Be more careful with the next MOSFET and follow proper electrostatic handling procedures.

Answer 3

Additionally, but highly unlikely you might be dealing with a depletion mode MOSFET which is a "normally on" device as oppose to the common enhancement mode MOSFET which are "normally off" devices. Depletion mode MOSFET are not as widely available as enhancement mode devices but there as still a few available and since you do not posted a actual part number for the device you are utilizing I believe this could be highly unlikely but plausible scenario

Answer 4

What kind of current does the motor draw, & which MOSFET are you using? If you draw too much current, the MOSFET will be ruined, and, they often go to low resistance on the Source to Drain, I've seen enough blown up power supplies to know this. The Gate pin should read low resistance to the Source pin, but, high resistance to the Drain pin.

There are MOSFETs with zener diodes built in, to reduce ESD damage potential. Also, many MOSFETs in the TO-220 package have the 3 leads Gate-Drain-Source, viewed left to right, BUT, not all!

Answer 5

MOSFET has to be open if You want to drive a motor through transistor. You are using MOSFET with N channel, that means gate voltage has to be positive (like +5V from Arduino), in other cases it won't run.