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MOSFET/circuit troubleshotting :-\

Discussion in 'Electronic Basics' started by Danny T, Jan 9, 2005.

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  1. Danny T

    Danny T Guest

    Things just aren't going that well *sigh* :(

    I've got a simple circuit with an IC controlling a motor via a MOSFET.
    The motor has a diode for back emf protection, two caps, etc., etc.

    Anyway... When I power it up, the motor doesn't spin as expected. If I
    replace the source/drain legs of the MOSFET with a piece of wire, the
    motor runs, suggesting to me either the MOSFET is bust or the chip isn't
    outputting a high.

    If I wire the pin up to an LED instead of the MOSFET, the LED lights
    fine. This suggests the MOSFET...

    I replaced the MOSFET with a new one - same problem. I tried
    disconnecting the gate and connecting it to 4.5V via a 10M resistor -
    motor spins fine!

    I'm at a loss... The MOSFET seems to respond to being connected directly
    to something, but not to the IC's output. It definately worked at some
    stage this morning (until I started fiddling)...

  2. Dare we ask for a circuit ??
  3. Andrew Holme

    Andrew Holme Guest

    Power supply noise from the motor might be crashing the PIC. Write a simple
    test program which toggles two outputs on for 1-second, off for 1-second.
    Make sure the watchdog is disabled. Connect one output to the MOSFET gate.
    Check _both_ outputs using a logic probe. You can make a simple logic probe
    consisting of an LED and a resistor in series. Do you have a multimeter?
    Check the gate-source voltage with that. If you have enough spare outputs,
    a slow binary count on three (or more) outputs would be a better check. Do
    you have (or can you borrow) an oscilloscope? Look at the power supply
    lines on a 'scope. Are they clean? You are blind without test equipment.
  4. Andrew Holme

    Andrew Holme Guest

    A digital photo of the breadboard layout could be as, if not more,
  5. Danny T

    Danny T Guest

    + 4.5V Drop to 3.1V
    | | 1uF 100uF
    | | Ceramic Electrolytic
    | 100uF '------------------------.
    | || .-----' | |
    | .----||-----. | _-_ --- ---
    | | || | - |___| 3V Motor --- ---
    | | | ^ - | |
    | |Vdd __ Vss | | | .--------'
    .-------o--o| |o---o---. '-----' |
    | .--------o| |o- | | |
    | | -o| |o- | ||-+ S |
    '-)--------o|__|o- | G||<- N MOSFET |
    | MCLR | '---||-+ D |
    | | | | |
    '---------------------)------' | |
    | | |
    (created by AACircuit v1.28.4 beta 13/12/04
  6. Andrew Holme

    Andrew Holme Guest


    MOSFET source S and drain D are the wrong way around (in the diagram).
  7. Danny T

    Danny T Guest

    I've tried to mark as much as I can, since any angle I took the pic from
    obscured something! Under the larger cap is another diode, like the one
    you can see. These two diodes are slightly different to the one used for
    back EMF protection, since I was using up some old bits.

    As I said, replacing the MOSFET with a cable between S/D causes the
    motor to spin. Moving the cable connected to G to and LED, also works fine.

  8. Danny T

    Danny T Guest

    I thought that's what all the caps where for? :-\

    That's what this essentially does. Both outputs are high, and when I
    connect a loose ground wire to the middle input pin on the other side,
    both motors switch off. This all works correctly with LEDs in place of
    the MOSFETs, and the motors disconnected.

    This works fine. It's only replacing the led with a wire to the MOSFET
    that breaks it. If I take the end from the PIC and connect it to + with
    a 10M resistor, the motor spins :(

    1.15V from gate to source

    I could change other things for outputs, but I don't think it'll help,
    given LEDs work fine going on and off as I connect/disconnect the input.

    I haven't tried an LED on one output and the motor on the other - this
    might show if the motor is crashing the PIC... I'll give it a try and
    post back in a min...
    I don't know anyone with one :(
  9. Danny T

    Danny T Guest

    Well, before I got to change them around, I think I've just goose
    something - I heard a fizzing and a funny smell (much like when I blow
    an LED last week). I can't find which component it might've been -
    everything looks ok... :-\
  10. Danny T

    Danny T Guest

    My motor now spins... *But* it doesn't stop when the pin goes low :-\
    (if I replace it with an LED, the LED goes off)
  11. Danny T

    Danny T Guest

    Looking at my diagram, surely the motor is grounded via the diode then
    the capacitor? Wouldn't this cause it to always be on?

    When the pin goes low, the motor slows down, but it's far from stopped :(
  12. Danny T

    Danny T Guest

    And if I disconnect Gate from anything, the motor still spins! :(

    If I take the MOSFET out and put my meter on it, it seems to have a
    *very* resistance, so I'm not sure it's that :(
  13. Danny T

    Danny T Guest

    If I take everything out and have just a motor connected in series with
    the mosfet, and nothing connected to gate...

    +ve -> Motor -> MOSFET(D), MOSFET(S) -> -ve

    The motor still runs... This doesn't seem right??
  14. Andrew Holme

    Andrew Holme Guest

    BINGO. That's wrong: that's not enough to switch the FET on; it should be
    almost Vdd.
  15. Andrew Holme

    Andrew Holme Guest

    Components sometimes go open or short-circuit when you destroy them;
    however, before you throw it away: connect the gate to the source. Does
    that stop the motor?

    You should never leave the gate un-connected: it's static-sensitive. Static
    electric charge (e.g. from just touching it) can switch it on or off, and,
    in extreme cases, destroy it. Observe handling precautions for
    static-sensitive components.

    .. You can give it It can pik up enough charge to switch off or no The gate
    is static-sensitive
  16. Andrew Holme

    Andrew Holme Guest

    This suggests you've destroyed the MOSFET. I hope you bought plenty of
  17. Danny T

    Danny T Guest

    You said I want a 100nF cap from Vdd to Vss.. Just noticed I've got a
    100uF... Guess I ordered the wrong ones...

    All I currently have are 100uF, 1000uF and 1uF (ceramic). Is what I'm
    using ok, or should I use one of the others?
  18. Miles Harris

    Miles Harris Guest

    Without one we can only guess. I'm tempted to infer that maybe the
    MOSFET needs a driver, such as a 2n7000. But that's simply a shot in
    the dark based on incomplete data.
    BTW, I assume the OP has observed proper handling precautions for the
    MOSFET? There's a lot of static around at this time of year.
  19. Danny T

    Danny T Guest

    I've changed it to a 1uF, since I *think* 100nF is 0.1uF... It doesn't
    change my problem though - motor only slows down, doesn't stop :(
  20. Andrew Holme

    Andrew Holme Guest

    The S and D of the MOSFET are the wrong way around (according to the

    The wiring around the small cap looks totally wrong !
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