Novice needs help with what should be a simple P channel mosfet project

Discussion in 'General Electronics Chat' started by woodworkerjb, Dec 22, 2015.

  1. woodworkerjb

    Thread Starter New Member

    Mar 20, 2012
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    I thought this would be a simple project but it has left me scratching my head. I pulled all my hair out on the last project so scratching is all I have left. The project is as follows:

    I have a 12v winch for my trailer. I want to use a 2 channel remote control for activation. The motor uses the standard starter solenoid to energize the motor. My research indicates the solenoid will pull 4 amps. The solenoid gets it ground through the frame of the trailer. The remote is simple enough, you just apply +5v and ground once a channel button is pressed it sends 5 volts to the channel pin.

    Because the solenoid is permanently grounded I decided to go with a P channel (irf9540) mosfet so that I could switch the positive side to the solenoid vs negative. My true problem comes from the fact that I need to now pass a negative 5v to the fet to activate where the remote passes a 5 volt positive.

    I tried using an opamp to invert the +5v to -5v but failed at this idea as well.

    In sum: I have a remote control module that outputs +5volts to a channel pin. I am trying to use a p channel irf9540 mofset to switch the 12v 4amp motors solenoid on and off.

    Any ideas on my approach or a whole new approach would be appreciated.
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Use a transistor to invert your 5V control signal to drive the P MOSFET. I'll attach a schematic shortly...
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    You're confusing the fact that the gate voltage relative to the source needs to be negative to turn on the P MOSFET with needing a negative voltage WRT ground.
    upload_2015-12-22_10-7-21.png
    When 5V is applied to R2, Q2 will "invert" it and ground the gate of the MOSFET to turn it on.

    You need to add a snubber diode to protect the MOSFET.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    In case you don't know, the "snubber diode" , such as a 1N4001, goes from MOSFET drain terminal (cathode) to ground (anode).
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    For completeness...
    upload_2015-12-22_10-27-15.png
    Values for resistors not critical. If loading on 5V control is an issue, R2 can be larger. Diode can be anything you have on hand. Even a 1N4148 signal diode can take 2A for a μS. Just about any NPN transistor will do.

    EDIT: snubber must handle 4A peak current...
     
    Last edited: Dec 22, 2015
    woodworkerjb likes this.
  6. crutschow

    Expert

    Mar 14, 2008
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    The op stated the solenoid draws about 4A, so a 1A or larger diode, such as a 1N400x, should probably be used.
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
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    Thanks, I missed that...
     
  8. woodworkerjb

    Thread Starter New Member

    Mar 20, 2012
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    0
    Dennis

    Thank you so much that was a great way to solve my problem. I have tried it and it works. What would you suggest I add to the circuit to ensure the MOSFET stops conducting after the +5 volt signal is removed?
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
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    You're welcome. The 10K resistor should be sufficient. If for some reason it isn't, you can make it smaller.
     
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