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I have a rfp30n06le (N channel MOSFET) that I'm trying to power a brushed motor with.

  • The source is connected to ground of a 1 cell lipo battery.
  • The drain is connected to the negative side of the motor.
  • The gate is connected to a 3.3v pin of a microcontroller.
  • The battery is reading 4.04v
  • The +bat to drain is reading 4.04v also

When I hook the motor up to the positive end of the battery, the bat+ to drain reads 0v, and the motor doesn't do anything.

When I change the gate voltage to the 4.04v from the battery, the motor starts turning slowly, and the bat+ to drain reads 1.5v.

It seems as if the MOSFET isn't allowing much current through, but increasing the voltage allowed more current to flow. This makes me think that the gate to source voltage needs to be higher for the MOSFET to allow enough current to go through.

Am I reading the data sheet wrong? shouldn't I be able to use this MOSFET with a gate-source voltage of 3.3v?

rfp30n06le datasheet https://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf

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    \$\begingroup\$ Measure the actual voltage between the gate and the source of the mosfet when you are trying to turn it on and post the result. \$\endgroup\$ – Dean Franks Nov 30 '17 at 18:41
  • \$\begingroup\$ @DeanFranks gate-source voltage from the 3.3v source is 3.15v. gate-source voltage from the battery is 3.99v \$\endgroup\$ – Jcweaver Nov 30 '17 at 18:47
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    \$\begingroup\$ I'm guessing you are having RdsOn issues. At Vgs = 3.15V, the device is not close to fully on (saturated) and you are going to get a non-trivial voltage drop across the source and drain. You might want to switch to a lower voltage mosfet with a logic level gate (low Vth threshold voltage). \$\endgroup\$ – Dean Franks Nov 30 '17 at 18:53
  • \$\begingroup\$ You could also use a gate driver with a charge pump which allows your 3.3V microcontroller output to drive the gate at 9 or 10V. Note that some gate drivers with charge pump have a maximum duty cycle on the gate signal (they assume PWM) and a minimum PWM frequency. What are the current requirements of your load? \$\endgroup\$ – Dean Franks Nov 30 '17 at 18:55
  • \$\begingroup\$ @DeanFranks I need about 4A on the load at max torque. I notice the datasheet says that Vgs = +10, -8 and the Vgs(threshold) is min 1 and max 2. Should I actually be trying to reach Vgs rather than just try to pass the threshold? \$\endgroup\$ – Jcweaver Nov 30 '17 at 18:58
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  • Credit goes to @DeanFranks

I was using a gikfun brand one which apparently needs 5v gate to source voltage

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