2N7000 MOSFET Switch Does Not Turn On When Gate Voltage is Applied

Other Hardware General Electronics
1

I would like to use a 2N7000 MOSFET as a switch. Here is a link to the datasheet:

I watched a tutorial on how to use a 2N7000 to control a 6V buzzer from this website:

I am using Electronics Workbench v5.12 to test the concept before building the circuit. I built two circuits. The first circuit uses the MOSFET as a switch, whereas the second circuit uses a SPDT switch. I used the same MOSFET switch circuit that was used in the tutorial. The switching element (MOSFET in one circuit, SPDT in the other) is connected in series between a 6V supply and a 100ohm resistor. I expect to see 60mA through the resistor when the switch is closed. I connected a 3V supply to control the gate of the MOSFET. I only get 0.006uA through the MOSFET when I apply 3V to the gate. I get the same current when I apply no voltage to the gate. I get 60mA when I close the SPDT and 0A when I open the SPDT. The attached an image with circuits to this post.

My SPDT works as expected, and since a MOSFET acts like a switch, I am pretty sure that my circuit topology is correct. Furthermore, to rule out if insufficient gate voltage was an issue, I stepped the gate voltage in steps of 10V from 10V-100V and I got 0.006uA through the resistor in each step. I also tried inserting a 1k resistor in series between the gate supply and the gate (as I have seen in some tutorials on the web) and got 0.006uA. I also tried using a 10k pulldown then a 1k pulldown resistor (since I plan to control the MOSFET from my Arduino Uno) and still got 0.006uA.

Thank you for reading this post. Any help would be greatly appreciated.

switch_closed.bmp (3.18 MB)

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3

I did not tie the grounds together to a common ground. However, I assume that the simulator treats the circuit as if the grounds are tied together because everything is referenced from the same ground.

4

Well clearly they are not common, you see leakage current only. In a real device with
a floating gate the behaviour would be less predictable.

5

Thanks for your feedback, guys. I'm out the house right now but I'll tie them all together and run the sim again when I get home. Is there anything else wrong with it?

6

I still get leakage current (0.006uA) when I tie the grounds together (see attachment). Any more suggestions?

switch_closed2.bmp (3.02 MB)

7

I swapped out the 2N7000 N-Channel Enhancement MOSFET for a 2N2222 NPN BJT and a 3k resistor in series between the base and the 3V supply. The BJT switch behaves as expected. The collector-emitter current (Ice) is ~60mA when I feed 1mA into the base. This matches the current that I expect, as depicted in the circuit with the SPDT switch in place of the transistor (see attachment).

Although I can continue using the BJT in my project, I would like to learn how to use the MOSFET because I understand that, as a voltage controlled device, the MOSFET puts less drain on a battery than a BJT, which is a current-controlled device.

I appreciate all of the help that you all have provided me. I would still like to know how I can get the MOSFET to work in my simulation. Is there anything else that I should try?

bjt_switch.bmp (3.18 MB)

8

Hi, if the transistor model works, then the model of the MOSFET must have a parameter not set properly.

I'm not familiar with that version of the workbench, last one i use was 15 years ago.

Tom...... :slight_smile: