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Help with MOSFET as a switch

Discussion in 'General Electronics Discussion' started by jgauthier, Apr 26, 2013.

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  1. jgauthier


    Mar 22, 2013
    Hey Everyone,

    So, I built a circuit and everything was working very well. I was using a MOSFET as a switch. However, I was not happy with my design, because I was "swtching" ground. Effectively, I had two ground busses. One on the drain, and one on the source completely isolated. I didn't think this was really what I wanted to do, or was what anyone did.

    So, I thought it, and rewired it so that I had two separate VCC busses instead. This seemed to make a lot more sense. Ground is ground everywhere on the circuit, and when the MOSFET is switched, it will connect to the two VCC busses.

    The problem I ran into with this change is that now the MOSFET, even though the gate is low (I just added a resistor to pull it to ground), both the source and the drain are have ~5V.

    I included a before and after.

    Attached Files:

  2. duke37


    Jan 9, 2011
    You will need a resistor from gate to source to turn off the fet when the switch is open.

    To switch the Vcc together, you will need a P channel fet and ground the base to turn it on.
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