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I have the following circuit to power DC motors at 5 V using a RPi 4. The MOSFET is a RFP30N06LE (https://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf), 1n4001 diode, 10k resistor.

enter image description here

When applying 3.3 V at the gate, I am seeing extremely little current flowing. I would expect 3.3 V to easily allow lots of current to flow based on:

enter image description here

When using a http://www.unisonic.com.tw/datasheet/30N06.pdf that has a higher Vgs(th) of 2-4 V, I see much more current flow, and the motor spins up (somewhat slowly) by itself.

I am confused as to why I am getting such little current flowing through the motor, I have to use my hand to help it start turning over. As well as why the FET with the higher threshold would allow more current to flow. I also tried with 3 units and observed the same results.

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    \$\begingroup\$ All source voltages must be zero because the source is connected to 0 volts. Are you sure you are measuring it correctly and using a voltage range and not a current range? \$\endgroup\$
    – Andy aka
    Mar 6, 2023 at 21:04
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    \$\begingroup\$ Where did you get the RFP30N06LE from? Can you show us a photo of it? What do you mean by 'voltage at the source'? \$\endgroup\$ Mar 6, 2023 at 21:21
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    \$\begingroup\$ > Where did you get the RFP30N06LE from? Can you show us a photo of it? What do you mean by 'voltage at the source'? --- Amazon, not in an anti-static package. I've just ordered a batch from sparkfun to check that it's not a counterfeit (although they were many 4&5 star reviews). \$\endgroup\$ Mar 6, 2023 at 21:44
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    \$\begingroup\$ I would recommend using 5V for Vgs. For an application like this, you really want to drive the FET into saturation. \$\endgroup\$
    – Troutdog
    Mar 6, 2023 at 21:45
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    \$\begingroup\$ > Just to be sure: The grounds of both supplies are tied together? --- yep! \$\endgroup\$ Mar 6, 2023 at 21:46

2 Answers 2

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It is doing what I would expect. You are at the threshold voltage of that MOSFET, it starts to enhance but will not be fully enhanced until the Vgs reaches at least 5 volts (the graph is very granular). All measurements for Vgs should be taken at the gate and source leads, any wire etc in between such as additional wire in the source can have a voltage drop giving you an invalid reading.

There are other MOSFETs that will work. It is possible you have a substitute MOSFET that has slightly different characteristics labeled as the original part.

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This image shows that the 3.3V VGS may not be clearing the miller region, so the drain voltage may not be zero. As other have said, you need more gate voltage.

enter image description here

Also the silicon diode should be replaced with a Schottky diode. The reverse recovery current of the 1N4007 goes straight to the gate through the miller capacitance, slowing turn-off thus increasing heat.

Edit: The above diagram is a bit misleading. At least it shows the gate charge with the miller plateau. The VDS graph shows VDS decreasing as soon as VGSth occurs. In fact when driving an inductive load with a flyback diode, VDS does not start decreasing until the current through the diode is zero at the start of the miller plateau. ID is maximum. There is significant power dissipation in this region. During the miller plateau. VDS drops to zero. Notice that VGS does not change significantly. This is due to all the input current is being used to charge the miller capacitor CGD. For a fast turn- on, there must be significant gate current. The RPi will not be able to support the required current an it’s output voltage will be less than 3.3 during switching.

Most often a gate drive circuit is required between MCU and MOSFET when using PWM.

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  • \$\begingroup\$ This makes sense, also will look into the other diode! \$\endgroup\$ Mar 7, 2023 at 12:26

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