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Switching a P-channel Mosfet — Parallax Forums

Switching a P-channel Mosfet

SuperRASuperRA Posts: 4
edited 2009-12-21 20:25 in BASIC Stamp
Hello, I'm quite new here and am hoping to find some help with a circuit I plan to build.

I have a P-type Mosfet (IRF4905) that I would like to use to switch 12v going to a small pump. I've confirmed that using the mosfet works when I use a 1M ohm pull up resistor to 12v and ground the gate. The pump turns on and runs fine.

My question is will it be ok to hook up a pin of a BS1 to the gate of the mosfet? I will leave the 1Mohm pull up resistor on 12v. Can the stamp handle grounding the 12v pull up resistor? Will the stamp's pins be ok with being pulled high with 12v?

If it won't be ok, would someone suggest a way that I could make it work?

Thanks

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,560
    edited 2009-12-19 02:22
    SuperRA,

    I would lower the 1Meg to something like 10K, and use a reverse biased diode across the S-D MOSFET terminals.

    Additionally, to prevent the 12V from interfering with the 5V I/O I would use a small NPN transistor configured as an Emitter follower.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
    PMOS High Side Driver.JPG
    617 x 615 - 53K
  • SuperRASuperRA Posts: 4
    edited 2009-12-19 03:25
    Hi Beau, thank you for the response. I will try that out.
  • SuperRASuperRA Posts: 4
    edited 2009-12-21 18:45
    Hi Beau, I tried out the circuit without a stamp and if I'm not sure if I'm reading the diagram correctly. It appears that the Drain is connected to +12v and the Source is connected to the load? When hooked up that way the load is constantly powered regardless of the I/O pin state (high or low, +5v or grd to emitter). So I tried switching the Drain and Source with each other and it seems to work as expected. When grounding the emitter, the load is powered, remove the ground and the load stops. Can you confirm if this is how it should be?

    Thanks
  • Beau SchwabeBeau Schwabe Posts: 6,560
    edited 2009-12-21 20:16
    Looks to be drawn correctly... the arrow tied back to a leg is always the 'Source'. For P-mos the arrow goes out .. for N-mos the arrow goes in. The relationship that I use is a play on words between the 'N' in N-Mos and the direction of the arrow. ... 'N' <-> 'in'

    www.datasheetcatalog.org/datasheet/irf/irf4905.pdf

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • SuperRASuperRA Posts: 4
    edited 2009-12-21 20:25
    Oh yea, you're correct. I read the circuit/datasheet incorrectly. Everything seems to work so now I'll hook it to the stamp.

    Thank you!
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