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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\$ Nov 30, 2017 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, 2017 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\$ Nov 30, 2017 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\$ Nov 30, 2017 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, 2017 at 18:58

2 Answers 2

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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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I think you've got a defective or counterfeit part. The RFP30N06LE is a logic level MOSFET and should be on fully on at 3V.

With 4A on 4.04V, your motor should have a resistance of about 1 ohm. Adding a fully on Rds of 0.047 ohms, you should get 4/1.047=3.8A and read a voltage across the device of V_DS=3.8/0.047=0.17V.

If you get 1.5V across the 1 ohm motor with V_gs=4V, then A = 1.5/1 and the V_ds= 4.04-1.5=2.5 and the Rds=2.5/1=2.5 ohms, or 2.5/0.047=53 times higher than spec for a "30A, 60V, ESD Rated, 0.047 Ohm, Logic Level N-Channel Power MOSFET" that is designed to be directly driven by "transistors can be operated directly from integrated circuits."

If the part is to spec, it can deliver 20A with V_DS=4V, V_GS=3V:

RFP30N06LE should handle 20A with Threshold voltage chart of RFP30N06LE showing ~2V

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