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I'm using the following MOSFET:

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

I have 5V going into Gate, and source is connected to ground. My understanding from the datasheet is that, this would be enough to allow Vds.

My multimeter does detect the Vgs of 5V, but does not detect any voltage between drain and source.

Why is there no voltage between drain and source even though i've met the threshold Vgs?

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  • \$\begingroup\$ Because a MOSFET is a transconductance device and converts input voltage into output current. \$\endgroup\$
    – Andy aka
    Apr 24, 2020 at 18:08

2 Answers 2

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The FET is an N-channel, so it’s being turned on with a positive gate-source voltage. Since your source is grounded, it would be expected that the drain would also be ground when Vgs is above the turn-on threshold.

What is connected to the drain? It would help to post a schematic at this point.

If nothing is connected to the drain, use an ohmmeter to test the drain-source resistance. It should be close to zero with the FET on, and infinite (open) with the FET off. Make sure the + side is on the drain otherwise you’ll be measuring the body diode.

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  • \$\begingroup\$ Drain is not connected to anything right now. Can you only check voltage when there is a load? What i'm trying to do: 5V supply does not have enough current for my servo. I was trying to amplify the current using a transistor. \$\endgroup\$
    – Yousuf
    Apr 24, 2020 at 18:20
  • \$\begingroup\$ Added to my answer. \$\endgroup\$ Apr 24, 2020 at 18:26
  • \$\begingroup\$ I just tested. Resistance between Source and drain shows value of 1. For Source and Gate, it shows close to zero. I put the setting on 2000K. \$\endgroup\$
    – Yousuf
    Apr 24, 2020 at 18:40
  • \$\begingroup\$ Actually wait, i just unplugged the MOSFET from my breadboard and put it back in. Checked the resistance again. When i put the probes up to the drain and source, it gets zero. The weird thing is, when i remove the 5V input, it still shows the resistance as zero. \$\endgroup\$
    – Yousuf
    Apr 24, 2020 at 19:01
  • \$\begingroup\$ Maybe you have a miswire, or a damaged device. Source-gate resistance should be open circuit. \$\endgroup\$ Apr 24, 2020 at 19:42
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If your power supply does not have enough current to drive the SERVO then adding a transistor or MOSFET will not magically give you more current for the SERVO.

If what you really meant was that the controlling MCU does not have enough drive on its output pin to control a servo then it is possible to use a transistor to buffer the MCU output. The transistor can switch more current to the SERVO than a typical MCU pin.

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  • \$\begingroup\$ But don't people use MOSFETs in circuits in order to amplify current? \$\endgroup\$
    – Yousuf
    Apr 24, 2020 at 19:29
  • \$\begingroup\$ Sure but that amplified output current has to be supplied from some power supply. It will not magically bump up the ratings of your power supply. \$\endgroup\$ Apr 24, 2020 at 19:32

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