mosfet power-gate turns on off on its own will

Discussion in 'General Electronics Chat' started by Toebs, Mar 11, 2015.

  1. Toebs

    Thread Starter Member

    Oct 9, 2014
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    I made a "power-gate" using a PNP and an NPN mosfet to cut power from a sesnor when I am not reading from it.
    My problem is that the circuit seems very noise sensitive - if i touch (with finger) the wire (Pin2) from my uC to gate on the first mosfet (Q10) the circuits goes "on" and the sensor starts drawing current.

    Right now it is built on breadboard, so that might be the cause?

    I have attached a schematic of the small circuit, on this the sensor has been replaced with a diode (D1) and resistor (R2).

    the sensor I intend to use is a VH400 from vegetronix.

    How can I alleviate this problem?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You need a pull down resistor across the gate, source on the nfet Q10, like R1.
     
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  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    You seem to have Q2 installed backward:

    upload_2015-3-11_6-30-18.png
    Its source should be connected to the +5V. I agree on the need for a pull-down resistor on Q10 to dissipate static from the gate.

    John
     
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  4. Toebs

    Thread Starter Member

    Oct 9, 2014
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    About Q2 you are right ofcourse, i drew it wrong in schematic but it is connected thed right way on the breadboard :).
    I have added a pull down on Q10, would 100k be suitable?
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    10K should work.
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    I think 100K should work. You are just draining a relatively small capacitance. I tend to use something smaller, like 1K to 10K. It does make a voltage divider with your gate driver, so if that drive is relatively high impedance, then use a higher impedance turn-off resistor.

    John
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @Toebs

    I've experienced something snimilar and played forever with the pull-down resistor. The root cause ended up being the Microcontroller. In my case, the PIC16f628A. I had MCLR reset turned on without a pull-up resistor on the Vpp/MCLR pin. Without the pull up resistor, the micro keeps resetting.

    EDIT: The pull-down resistor on a gate is always a good idea (especially in an H-bridge) so you don't energize the P and N on the same side of the bridge during the initialization phase of the Microcontroller startup. In any case, once started up, and assuming you are not tri-stating (setting as input) the pin that controls the mosfet gate, there is really no reason for the pull down resistor and the presence/absence is not going to change anything.
     
    Last edited: Mar 11, 2015
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  8. Roderick Young

    Member

    Feb 22, 2015
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    I think GopherT may be onto something. While I agree with him and the others about adding a pull-down resistor, I suspect that the microcontroller pin is floating, either by design (output turned off or in input mode), or accident. Even with no pull-down, I don't see how the gate of Q10 could be so sensitive to a touch, if it was being actively driven to 1 or 0 by the microcontroller.
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I agree I was editing my post while you made this one. Essentially saying the same (I think).
     
  10. Toebs

    Thread Starter Member

    Oct 9, 2014
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    Thanks I will look into it at try it out. Will report back when I have something. :)
     
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