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I am trying to build a simple motor controller circuit. I have a lithium ion battery (7.4 volts, 20C discharge, 800mAH capacity) that powers the motor and I am using an adjustable voltage supply on the MOSFET gate.

I have the circuit set up according to the image below:

enter image description here

The motor pulls about 3 Amps at 7.4 volts and the MOSFET is a logic level enhancement mode n channel MOSFET. The gate threshold is about 2.5v and it can handle a continuous current up to 20 amps.

MOSFET Datasheet: https://www.vishay.com/docs/91021/91021.pdf

The MOSFET should start to turn on at 2.5V but when I probe the drain of the MOSFET with 3.3v at the gate I still see 7.4V.

This exact same setup works perfectly if I just replace the motor with an LED. I know the motor is working too. The MOSFET just does not seem to pull its drain low if the motor is connected, even with a gate voltage present.

Why doesn't my circuit work with the motor?

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  • \$\begingroup\$ Have a look at figure 3 in the datasheet. Someone can correct me on this but it looks like you need ~4.25V on the gate for the drain to sink 3A? \$\endgroup\$ – tangrs Sep 3 '17 at 22:47
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That is not a logic level mosfet. Vgs(th) may be 2.5V, but that does not mean it will be fully turned on at 3.3V. Note that Rds(on) is specified ONLY at Vgs of 10V. You should find a mosfet with Rds(on) specified at 2.7V.

When using a MOSFET as a switch, always check Rds(on) and verify that it is specified at a voltage you can live with.

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  • \$\begingroup\$ The maximum Vgs(th) is 4V. Note that at Vgs(th) the drain current is only 250 uA. \$\endgroup\$ – mkeith Sep 3 '17 at 22:51
  • \$\begingroup\$ Well darn. The part is actually labelled as an NTE2987 that I bought in a store which was advertised as a logic level mosfet. It has that part number stamped on it but I looked closely and saw the actual part number embedded in it (IRF540). \$\endgroup\$ – marco Sep 3 '17 at 23:02
  • \$\begingroup\$ Can you explain why the circuit is able to drive an LED? Is it just because of the inductance of the motor? The LED needs a much smaller current but the motor has a very small resistance, so it should still conduct somewhat right? \$\endgroup\$ – marco Sep 3 '17 at 23:05
  • \$\begingroup\$ Depending on the LED, it may glow with as little as 10 or 100 uA. That is not enough current for the motor to do much. \$\endgroup\$ – mkeith Sep 3 '17 at 23:11
  • \$\begingroup\$ Also logic level is typically used to say it works below the typical 10Vgs... Some parts will drive full on as low as 2.5V, others need 5V like this one. \$\endgroup\$ – MadHatter Sep 3 '17 at 23:21

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