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555 Mosfet driver problems

Discussion in 'General Electronics Discussion' started by Scook, Nov 16, 2012.

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  1. Scook

    Scook

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    Oct 13, 2012
    My objective is driving the gate of an N channel Mosfet with the 555 timer in monostable multivibrator.
    I've completed the 555 circuit and it gives me my desired pulse just fine, but when the 555 output is connected to the gate of the mosfet, the mosfet turns on without ever triggering the 555 to pulse...
    The mosfet configuration is
    S: Ground
    D: Light
    G: 555
    and here is my circuit.
    555.jpg

    Additional data:
    During all of my test's i am using my DMM with Neg lead to drain and Pos lead to Pos bus.
    The mosfet is in working order, it switches fine in any test configuration.
    And yes, I will put a diode across the light, once I've corrected this problem.



    thanks in advance to anyone who can help
    -Steve
     
    Last edited: Nov 16, 2012
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Have you checked the output of the 555 (aka gate of the MOSFET) for any residual voltage?
     
  3. Scook

    Scook

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    Oct 13, 2012
    Do you imply that there could be a small continuous voltage on the 555 output causing the fet to open? Even if there was a small voltage coming out, i doubt it could open the fet's channel, or could it? Because this isn't a logic level fet,

    Before connecting the 555 output to the fet's gate, I check and see no voltage coming out, but the very moment I attach it to the fet's gate, boom...opens the channel.
    All of this without even touching my momentary trigger. I've even opened (removed) the momentary push button trigger, read no voltage on the 555 out, reconnect the lead to the fet's gate and still it opens the channel.

    I know the 555 is not unintentionally firing because the mosfet stays on forever until I remove the lead from the gate.

    ps. Danke fur Ihre Zeit herr Kapp!
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Connect the drain to the source (after disconnecting it from the 555's output) and see if you have a turned on, or turned off mosfet.
     
  5. duke37

    duke37

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    Jan 9, 2011
    Could be incorrect ground problems because of the two power supplies.

    Put a led with a seies resistor directly across the source and gate to check the switching voltage.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Does this happen only in the simulation or in the real circuit?
    If in simulation: try an additional 10kOhm resistor from gate to source.

    Harald

    P.S.: thanks for your P.S. in German, but generally please use English as a courtesy to all other forum members.
     
  7. Scook

    Scook

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    0
    Oct 13, 2012
    This happens with the real circuit, the simulation runs perfect.


    Okay so no voltage coming from the 555 output (untill triggered), it is attached to gate and turns on my mosfet (my original problem)
    With DMM pos lead to + bus and neg lead to mosfet drain ,when pulsed from the 555, the mosfet channel then closes and then when the pulse ends, it returns to open channel again.

    keep in mind that when i take the 555 out of the equation and test the mosfet, it works fine when providing the gate with a pos voltage from the bus or even a 9v batt.
    This IS an always off fet unless triggered.

    and yes Harald, I have tried the 10K R and it did not make a difference, so it's not floating on.
     
    Last edited: Nov 17, 2012
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Could you explain taht a bit more? I'm currently not so shire what this means.
    What is the + bus?
    And this description sounds like the 555 does indeed turn the MOSFET on and off, contrary to your other claim.

    Surely I'm missing something here.

    Harald
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Is the OP possibly having a problem where the 555 self-triggers at the end of the timing period of the monostable?

    Perhaps there is noise on the power supply rail.

    A capacitor across the power supply rails, near the 555, may help.
     
  10. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    You need to to pull down voltage in pin4 or reset first everytime you power on your circuit.

    You can do this by placing a RC circuit. Capacitor and resistor in series. Connect another end of resistor to +VCC and negative pin of capacitor to ground. Then connect pin4 to connection of resistor and capacitor in series. Value of resistor could be 1ufd and resistor 4.7k. This will temporary reset your NE555 and not trigger its output everytime you power of circuit.
     
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    True but his time constant is only about 500mS, yet (he said) the lamp is always on.

    Chris
     
  12. Scook

    Scook

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    Oct 13, 2012
    Alright, I feel I have not been very concise with my circuit, and I apologize for any confusion.

    This is the EXACT circuit I'm having problems with, the two thick lines at the top are the Negative and Positive bus lines that supply a common voltage and ground source for everything in the circuit.

    EP.jpg

    Data:
    1. I read a normal Positive going pulse from the 555 with my multimeter every time I push the trigger.
    2. Reading no voltage from the 555 output, I attach the 555 output to the Mosfet's gate (never touching the trigger) which immediately turns the Mosfet on, completing the circuit and illuminating the light.
    3. When I trigger the 555, the light and mosfet turn off for 400ms (my 555 pulse time).

    (mosfet test)
    I have removed the 555 output from the mosfets gate and instead connected a wire from the 5V Bus to the mosfets gate , which turns the mosfet on proving that it is operating normally.

    Anyone?....
    Thanks again.
    -Steve
     
    Last edited: Nov 18, 2012
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Usually we like to see a ~10R resistor in series with the gate but it's to suppress ringing. I don't think it's going to cure this issue and I don't think it's related.

    Chris
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Measuring between which points? Not accidentally between +5V and the 555's output?
    What is the voltage from gate to source after you have completed the connection between gate and 555?

    Me neither.

    Are you sure your physical setup is exactly as your schematic?
     
  15. Rleo6965

    Rleo6965

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    Jan 22, 2012
    Measure Mosfet dc voltage between G and S. Reading of +5V will turn on the Mosfet and lamp. OV should turn off Mosfet and lamp. Place your test probe exactly on Mosfet terminals. Negative probe on S and positive probe to G. This will accurately test voltage on Mosfet and isolate problem.
     
    Last edited: Nov 18, 2012
  16. Scook

    Scook

    12
    0
    Oct 13, 2012
    Yes haha, and very aggravating , Considering how simple this circuit is!

    DMM leads are at correct polarity Harald, I had hoped it was a simple mistake like this, but no =[.

    (*steve*) got me wondering about noise on the rail tho, because I am using an ATX computer PSU I converted for benchtop hobby.
    I havent hooked it up to an O-scope yet but I'll bet there's alottt of noise.

    I Specifically remember using this PSU for a non-inverting op amp and watching the analog multimeter dance when connected to the amps output.

    I'll check VGS tomorrow as well, thank you all for your time.
     
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    You have a scope but you've been futzing around with a DMM? :confused: Also, since you suspected noise, substituting another supply or battery pack would have been logical troubleshooting progression

    Chris
     
  18. Scook

    Scook

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    0
    Oct 13, 2012
    Oh no I wish I had a scope (my school does tho), I'm not great with money. I have to salvage 40% of my components, but I have a closet full of expensive and some rare components, all because of my wasteful neighbors who throw out any electronics that are over a year old, or if something new comes out....unbelievably selfish some people.
     
  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    651
    May 8, 2012
    That's not selfish, it's wasteful. Maybe you live in Boca? :p Actually, I retract that statement. They are selfish because they could have given the stuff to charity.

    BTW, I see you have a face warmer started. Good thing because it's getting chilly. :D

    Chris
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I think this is a hint.

    What's the bet it's a P channel mosfet?
     
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