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I have built a test circuit to test powering a DC motor through a 30N06 N-channel MOSFET

When running a 5V 1A power supply (bench supply) through the MOSFET, and touching the gate from GND to the 5V supply, I see that 1A of current flows through the MOSFET as expected:

enter image description here

At this time, I can see that the resistance of the drain -> source is showing as maxed on my multimeter when gnd is applied to the gate, and 0 ohms when the 5V is applied to the gate (again as expected.)

When I put the DC motor after (or before) the MOSFET in the flow of current, the amount of current that flows drops to ~0.47A:

enter image description here

Additionally, when checking the resistance of the MOSFET in this case I am seeing maxed measurement (maximum resistance from the MOSFET) while the motor is being powered, and seeing 0-20k ish ohms when gate is driven to GND.

When the motor is given the 5V 1A source directly, 1A flows through the motor and it spins far faster:

enter image description here

I am very confused as to why this might be. My expectation from both testing without a load and from educational material is that it should drop to near-zero resistance and allow the full 1A to flow through the motor.

The MOSFET is not getting warm. I've also tried with connected a 100k and 1m ohm resistor between gate and source, no difference.

Note that this circuit is not my final circuit and was just a test for making sure that I can control the motor with a MOSFET.

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    \$\begingroup\$ The gate voltage isn't 5V in any these circuits. \$\endgroup\$
    – user253751
    Feb 16, 2023 at 22:56
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    \$\begingroup\$ In your first case, where you're connecting the FET directly between the power rails -- OMG. That's how you let the smoke out. The FET won't magically limit the current -- you're just lucky that the power supply (apparently) did. That's a schematic to burn up FETs, power supplies, or both. \$\endgroup\$
    – TimWescott
    Feb 16, 2023 at 23:01
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    \$\begingroup\$ @TimWescott it's a current-limited lab power supply, apparently \$\endgroup\$
    – user253751
    Feb 16, 2023 at 23:07
  • \$\begingroup\$ None the less, don't do that. \$\endgroup\$
    – TimWescott
    Feb 16, 2023 at 23:15
  • \$\begingroup\$ Yes I’m using a current limited power supply to reduce the components for testing \$\endgroup\$ Feb 16, 2023 at 23:18

2 Answers 2

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The motor must be connected between the +5V and the drain of the MOSFET for it to be a switch. The gate-source voltage must be 10V for all of that MOSFET part number to fully turn on. You have the MOSFET as a follower with a gate-source voltage of 2.5V.

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  • \$\begingroup\$ Thanks @audioguru, now I can get full power when applying 5v to the gate, but when I ground gate it drops only to 0.46A. Seems I have the problem inverted now. I added a 1Mohm resistor between gate and gnd which did not seem to help \$\endgroup\$ Feb 16, 2023 at 23:22
  • \$\begingroup\$ Ok never mind, I switched out the mosfet for another one and the issue went away \$\endgroup\$ Feb 16, 2023 at 23:29
  • \$\begingroup\$ I will note that much lower voltages worked for my case, as I find tha at 3.6v I only see 0.07ohms resistance \$\endgroup\$ Feb 17, 2023 at 0:46
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This is a more-or-less behavioral view of how your circuit is going to act. A FET needs a certain amount of voltage between gate and source to turn on fully -- less than that and it runs in "source follower" mode, where the source (what's connected to the motor) is a more-or-less fixed voltage below the gate.

As the motor (modeled as the resistor and coil inside the dotted lines) gets more current, its voltage goes up. That voltage subtracts from the voltage across the FET's gate to source path. Eventually, the FET conducts less, and that limits the current.

schematic

simulate this circuit – Schematic created using CircuitLab

This is what you (probably) want to do next, after you've found a FET that works with a 5V \$\mathrm{V_{GS}}\$. The FET source is grounded, meaning that all of the power supply voltage is seen across the gate-source path, so it'll turn on fully. I've added a "catch diode" so that when you turn the FET off the resultant inductive kickback from the motor won't destroy the FET. I've also added a switch so you can turn it on and off, and a resistor (R2) to make sure that the FET gate is pulled low when you release the switch.

schematic

simulate this circuit

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  • \$\begingroup\$ In that second diagram you have the mosfet after the motor, will the issue mentioned in the first paragraph occur again? \$\endgroup\$ Feb 16, 2023 at 23:25
  • \$\begingroup\$ And thank you for the detail! \$\endgroup\$ Feb 16, 2023 at 23:29
  • \$\begingroup\$ "In that second diagram..." I'm not sure what issue you mean. Clarify, please? This is a standard way you make motors turn. \$\endgroup\$
    – TimWescott
    Feb 17, 2023 at 0:02
  • \$\begingroup\$ Sorry, yes. The fix that worked for me was putting the mosfet before the motor rather than after (and also getting a new mosfet...). In your first diagram you have the mosfet before the motor like I now have working. In the second diagram you have it after the motor which I believe was my original issue (although I should test again with the new mosfet). \$\endgroup\$ Feb 17, 2023 at 0:33
  • \$\begingroup\$ After testing I get the issue only when the motor is before the mosfet \$\endgroup\$ Feb 17, 2023 at 0:51

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