P channel MOSFET Wiring not working

Discussion in 'General Electronics Chat' started by teeveeloon, May 29, 2016.

  1. teeveeloon

    Thread Starter New Member

    May 29, 2016
    3
    0
    Hi all, MOSFET Problem.jpg

    Essentially I've got a P channel MOSFET that I'm trying to connect.

    The voltage of the battery is 4.2ishV (18650)

    I have attached my sketch of the circuit

    I want the voltage at "D" to be zero when the switch is off and full voltage when it is on.

    At the moment, it reads 4.2 with the switch on and anything between 0-4.2V when it is off. How do i get it to be zero the whole time when the switch is off??

    Many many thanks, I've tried so hard to figure this out.
     
  2. MrAl

    Well-Known Member

    Jun 17, 2014
    2,438
    492
    Hi,

    Connect 10k resistor from source to gate.
    You can really use anything between about 1k and about 100k, but 100k might make it turn off too slow so 10k is good.

    A mosfet has an internal capacitance that stays charged when the gate is open, so a resistor is required to drain that capacitance to let the mosfet turn off.

    Also, a small series resistor in the gate might be a good idea too, like 50 ohms. That is to limit gate current when turning on. Min about 10 ohms, max about 100 ohms.
     
  3. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    TeeVeeloon,

    I had a little trouble figuring out your circuit diagram at first, so I re-drew it in a more standard schematic form. One problem you have is that you've connected your positive supply (4.2V) to the drain of the PFET. Are you aware that FETs have an intrinsic diode? This is an internal diode that is part of the physical construction of the FET, and in the case of the PFET it points from drain to source which is not what you want for your circuit. You need to turn the PFET around. See the attached drawing of your original circuit, with the intrinsic diode shown, and my suggested changes. Also, MrAl is right, you need a resistor from the gate to source. Always use a gate-source resistor on your FETs to ensure they stay off. Good luck with your circuit!




    upload_2016-5-28_22-48-40.png upload_2016-5-28_22-48-40.png
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
    3,249
    625
    Check the diagram again. The device is connected connected correctly; it's just missing the resistor between gate and source.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
    4,855
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  6. dl324

    Distinguished Member

    Mar 30, 2015
    3,249
    625
    Welcome to AAC!
    As mentioned by the earlier posters, you need a resistor between gate and source.

    It would be helpful if you included part number(s) and drew schematics in a more conventional way.

    upload_2016-5-29_6-46-56.png

    Note that the dashed line for an enhancement mode device is often drawn as a solid line when hand drawing. That can be confused with a depletion mode MOSFET which is on with no gate bias, but they're seldom used.
     
    Last edited: May 29, 2016
  7. teeveeloon

    Thread Starter New Member

    May 29, 2016
    3
    0
    Thanks so so much guys, put a 10K on and it works great!

    Just to iron out the other problems now

    Cheers
     
  8. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    As other said, you might want to try to draw the schematics in a more conventional way. I still can't figure out what you drew :)

    I thought the rectangle was a resistor tied to the source, and the half circle with a D in it was the battery tied to the drain!
     
  9. teeveeloon

    Thread Starter New Member

    May 29, 2016
    3
    0
    What software do you guys use to create the schematics?

    Thanks
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
    3,249
    625
    I used the free version of Eagle.
     
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