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Flip flop - Output high, power cycle to turn back on

Discussion in 'General Electronics Discussion' started by OJdoesIt, May 2, 2019.

  1. OJdoesIt

    OJdoesIt

    7
    0
    Jan 27, 2016
    I have an external resistive device that will be powered within a circuit. I plan on using a D flip flop to drive a logic-level FET that will power the device.
    The device has 3 pins: 1-3 has a 1 ohm resistor. When the resistor element heats up, it will physically touch pin 2, thus connecting all 3 pins together. Pin 2 is used as a cut-off switch, to which current should reroute from pin 1 through pin 2 (instead of pin 3).
    I want to use this cut-off switch to somehow deactivate the flip-flop and turn off the FET. Once deactivated, the flip-flop should not turn back on until the user has power cycled the circuit.
    Here's the flip-flop I was looking at: http://www.ti.com/lit/ds/symlink/sn74lvc1g175.pdf
    What suggestions do you guys have for the topology?
     
  2. Bluejets

    Bluejets

    3,453
    698
    Oct 5, 2014
    Sounds like a dead short to me.

    Diagram of before and after and what this "device" is might help.
     
  3. OJdoesIt

    OJdoesIt

    7
    0
    Jan 27, 2016
    I posted a semi-working schematic. The device will be attached to the PADS.
    I think the circuit doesn't work because of the RC input at MR. That input isn't schmitt triggered.
     

    Attached Files:

  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,230
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    Nov 17, 2011
    As Bluejet mentioned: a dead short between Vcc and the drain of your MOSFET. The MOSFET will quickly die. What you need is a connection between pad1 and pad2 only (acting as the switch). Or another resistor to provide at least some current limiting.
     
  5. OJdoesIt

    OJdoesIt

    7
    0
    Jan 27, 2016
    There is a 1 ohm mechanical, resistive element across PAD1 and PAD3. Once that element heats up, it will move and touch PAD2, thus connecting all 3 pads together. This is where I want VCC through PAD2 to turn off the FET indefinitely until the user resets the power to the whole circuit.
     
  6. Bluejets

    Bluejets

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    698
    Oct 5, 2014
    Don't see that happening off a clock signal, and the short is still there by the way it looks to me in it's present state.

    Also this..... It will start MR at LOW and bring it HIGH until power is reset.
    The instant power connects, I think you will find the MR is high until the cap "sees" it.
    Might be instantaneous but.......
    That may well create a "race" condition.

    Go to #2 and try again.
     
    Last edited: May 4, 2019
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    9,230
    1,861
    Nov 17, 2011
    Understood, but that is a dead short circuit and will blow the transistor if you don't provide any means of current limiting. A small resistor, e.g. 0.02 Ω (exact value depends on the mOSFET's threshold voltage and the max. current) in the source connection of the MOSFET could do that. In case of the short circuit in the drain connection the high current will raise the source voltage of the MOSFET and thus lower Vgs and limit the current.
    I disagree. The circuit shown is a typical primitive reset circuit. But it lacks the Schmitt-trigger for defined operation.
     
  8. WHONOES

    WHONOES

    664
    134
    May 20, 2017
    I would use a 4013 instead. You also get two in the package plus an inversion of Q. Also, dispense with R4, use a higher value for R2 and a smaller value for C1.
     
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