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I have an N Channel Mosfets I'm trying to use as a switch for a motion sensor and an LED strip light (12V <0.8A). It seems like no matter how I hook up the mosfet it is always set to 12V or stuck at 6.5V. It's my first time using one, so I'm sure I'm missing something simple.

Mosfet: RFP30N06LE Motion Sensor: SR501 (Signal is ~3.3V)

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  • \$\begingroup\$ What voltage do you apply at the gate? What’s the recommended/specified Vgs at the load you intend to use in the transistor? EDIT: looked up the transistor. Seems well suited for 3.3 V operation. Can you please measure it just to be sure? \$\endgroup\$
    – winny
    Jul 27, 2019 at 16:28
  • \$\begingroup\$ @winny Oops sorry about that. It's a 12V source and I measure 3.2-3.4V at the gate. \$\endgroup\$
    – Hughes
    Jul 27, 2019 at 17:45
  • \$\begingroup\$ Does your 12 V source drop under load by any chance? Can you please draw a schematic (built in tool here, press edit and click on the schematic symbol) instead of a wiring diagram and post a photo of your setup. Something is fishy here \$\endgroup\$
    – winny
    Jul 27, 2019 at 21:25
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    \$\begingroup\$ Plenty of them! You can search by Vgsth on Digi-Key. Also, you can consider a NPN instead if you have some mA worth of gate drive current to spare. You will need to add a base resistor though but it you can operate down to 0.7 V. \$\endgroup\$
    – winny
    Jul 28, 2019 at 17:40
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    \$\begingroup\$ Almost. It has only 12 V Vds breakdown, so not enough for your application. I would not dare to use less than a 20 V rated one. \$\endgroup\$
    – winny
    Jul 31, 2019 at 7:36

1 Answer 1

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The gate and source form a capacitor, whose capacitance must be overcome in order for the drain/source to begin to conduct. In order to this, you can place a resistor (Something between 400-1000 Ohms should suffice) between the gate and the source. Be careful to not allow the voltage to exceed Vgs in the datasheet.

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    \$\begingroup\$ What are you on about? Why would a resistor across gate-source change anything? How would 3.3 V go over the 10 V specified in the datasheet? \$\endgroup\$
    – winny
    Jul 27, 2019 at 21:23

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