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I made a simple circuit following this schematic. The load is several motors. At the gate is 3.3V while the source is GND. The pull-down resistor at the gate goes to the same GND as the source.

When I put 5V at the gate, the drain outputs GND. However, when I remove the 5V, the drain starts outputting 5V.

What could have cause this and how do I fix it?

I've tried:

  1. Raising the voltage at the gate to 10V
  2. Switching the MOSFET out
  3. Also changing and resoldering on a brand new board

Both of them did not work. I have read that I could try lowering the output impedance but I do not know how to do that.

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    \$\begingroup\$ Post a schematic that makes better sense. \$\endgroup\$
    – Andy aka
    Jul 17, 2020 at 11:15
  • \$\begingroup\$ Post a schematic that shows all of the circuit. Before doing that you may want to consider how a good schematic is drawn. The small evidence in the snippet that you show gives an indication that schematic is not currently well drawn. \$\endgroup\$ Jul 17, 2020 at 11:31
  • \$\begingroup\$ What is the supply voltage? If it's 5V, you have not described a problem. \$\endgroup\$
    – user16324
    Jul 17, 2020 at 11:41
  • \$\begingroup\$ sorry for that it's my first time posting, here's a slightly simplified version of the full circuit \$\endgroup\$ Jul 17, 2020 at 12:00
  • \$\begingroup\$ You have shown +24 V on the schematic but your post suggests that you are using a +5 V supply. Which is it? What are R2 and R3 supposed to do? \$\endgroup\$
    – Transistor
    Jul 17, 2020 at 12:20

1 Answer 1

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If I read you schematic correctly, it should be like this:

schematic

simulate this circuit – Schematic created using CircuitLab

When you close the switch, R1 and R2 becomes parallel. This makes \$V_g\$ less than 1V which means the MOSFET can never turn on.

When I put 5V at the gate, the drain outputs GND. However, when I remove the 5V, the drain starts outputting 5V.

According to the schematic, it shouldn't be. Either your schematic does not match the circuit or there's something wrong with the circuit.

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  • \$\begingroup\$ nah it would seem that my circuit and the schematic matches up. I redid it a few times with the same results and i checked the connections of everything repeatedly. I have heard of people receiving "factory fakes" when they order MOSFETs so I have bought new parts (this time from a legit source) and will try again when they arrive \$\endgroup\$ Jul 17, 2020 at 14:24

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