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Puzzled by MOSFET switch

  1. Nov 30, 2011 #1
    I have the following circuit wired up on a protoboard.


    The datasheet for the BSS138 MOSFET is here. I'm puzzled regarding what I'm seeing happen with this circuit - when I apply 3.3V at the gate resistor, the MOSFET turns on fully and I see 3mV at the output. This, of course, is expected.

    However, if I remove 3.3V from the gate resistor, the pull-down resistor turns the gate off. I expected to see approximately 3.3V at the output, yet I only see 2.7V. If I replace the 3.3V on R1 with 5V, the output shows 4V. In other words, a volt is being dropped on R1 when the MOSFET is off. Is this expected? Somehow, I expected the MOSFET to have an immensely high resistance when off and hence expected approximately 5V being dropped on it when it's off.

    Are my expectations wrong for this MOSFET?
  2. jcsd
  3. Nov 30, 2011 #2
    What is the Output tied to . . some kind of load?
  4. Nov 30, 2011 #3


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    Science Advisor

    One way this could happen is if you used an analog voltmeter to do the measurements.

    If it drew about 1 mA while taking the measurements, you would get voltage readings like this..

    It could also just be a faulty FET.

    One easy test would be to put a milliammeter in series with the supply and see if you can cut off the current when the gate voltage is dropped to zero.
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