clearing a fault, using a schmitt trigger a mosfet and a mechanical switch

Discussion in 'The Projects Forum' started by newbieateverything, Mar 18, 2013.

  1. newbieateverything

    Thread Starter Member

    Feb 25, 2013
    95
    0
    hi could someone please explain the attached circuit to me in detail.

    From what i understand:

    the open circuit caused by the opened switch in series with the resistance causes a fault current in the system because the resistance has changed.

    This higher than normal current across the 200 ohm resistor causes a high voltage to flow through the mosfet and into the inverting input.
    -
    is this statement correct???

    the input is then compared to a reference( using a schmitt trigger config Threshold values set to 1 an 5V. )

    when input increases past the high threshold value set the output is low, when input is below high threshold output is high.

    The low output will turn the mosfet off and the high output will turn it on.

    When the mosfet turns on, a voltage is sent to the switch closing the switch.

    This switch should not close until the fault current is cleared and the switch doesnt re-open unless reset.

    based on what i explained is my design correct.

    Think of what i am doing as a way to clear a fault current using a schmitt trigger configuration+ a mosfet+ a mechnical switch

    please help PSPICE has a problem with the design
     
    • ph1.png
      ph1.png
      File size:
      12.2 KB
      Views:
      107
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,018
    1,537
    Is this still for your previous project? The aircraft circuit breaker.
     
  3. newbieateverything

    Thread Starter Member

    Feb 25, 2013
    95
    0
    no it is not that one has been done already or at least as far as i could take it.

    I am having difficulties understanding how a schmitt trigger works and how to pick resistors values as well as the lower and upper thresholds. Ive read on it for hours but still dont get it.

    please help.

    from what i understand when input is above upper threshold the output would be at a low, and when input is below lower threshold the output would be at a high.

    :confused::confused: please help.
     
  4. newbieateverything

    Thread Starter Member

    Feb 25, 2013
    95
    0
    hi, my input voltage say is to oscillate between 6 and 2 volts. how do i set up my resistors if i want my lower and upper thresholds to be 5 and 1.

    also are these the best choice of threshold values based on my input.

    the supply is 15 V.

    please help
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,018
    1,537
    Post your schematics directly to the forum, instead of as files. A lot of us won't open unknown file from unknown people.
     
    newbieateverything likes this.
  6. newbieateverything

    Thread Starter Member

    Feb 25, 2013
    95
    0
    On the first file i thumbnailed could you please explain how that works.
    All i know is that the fault current is caused when the switch closes causing a higher than normal current.

    How a schmitt trigger works confuses me abit, on what basis do you set threshold values?
     
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,018
    1,537
    Don't think a schmitt trigger is what you want, that is for turning on and off a circuit. A comparator is what you are looking for, like a 339 or 393.

    And your NOT loading "thumbnails". Your up loading files that people won't open.
     
  8. newbieateverything

    Thread Starter Member

    Feb 25, 2013
    95
    0
    Sorry, thought the first posted had a thumbnail that could be opened its right beneath my post.
    I want to turn my circuit off then on and back off. 1 cycle

    how can i get a reference of 0.2V going into my non inverting input??

    i dont know how to solve.

    take R5 as 10K.

    please help
     
    • 123.png
      123.png
      File size:
      11.9 KB
      Views:
      47
  9. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,018
    1,537
  10. newbieateverything

    Thread Starter Member

    Feb 25, 2013
    95
    0
    how odd thats the page i was just reading a few minutes ago.
    It it possible to do it with only 1 comparator. I was able to pull that off.

    However in this new design below i cannot get my hysteresis levels right.

    how do i choose my resistor values to get an upper trigger point of 3 and a lower trigger point of 1
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,018
    1,537
    Your circuit makes no sense to me. When I said you need two comparators, that is what it takes to make a "window", either active or inactive depending on your needs. you then use the window signal for your control signal.
     
    newbieateverything likes this.
  12. newbieateverything

    Thread Starter Member

    Feb 25, 2013
    95
    0
    would it make more sense if i said i was going to connect my trigger to a mosfet to turn it on and off?

    would study the link you gave me.
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,018
    1,537
    But thats not what your schematic shows.
     
  14. newbieateverything

    Thread Starter Member

    Feb 25, 2013
    95
    0
    your right thats not what my circuit shows because i haven't yet replaced the voltage pulse at the mosfet gate with a mechnical switch and i haven't connected the schmitt to the mosfet gate.

    sorry about that, take a look at this and please let me know if it makes more sense now

    however i am not able to turn the mosfet off completely due to a lack of hysteresis. How do i choose better resistor values?
    what formula do i use?

    my voltage at the gate shows that my schmitt output voltage swings from a high 14 to a low of 3.6V but i need it to swing all the down to a low -14V to switch it completely off.
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    What are you really trying to do?
    Where in the circuit does the fault occur?
    What do you want to happen when the fault occurs?
    How is the switch involved?
    Why did you add an LED and a 2.2k resistor in the drain circuit of your latest schematic?
    Is there a relay in the circuit? If so, you need to show it.
     
  16. newbieateverything

    Thread Starter Member

    Feb 25, 2013
    95
    0
    i dont know what to use to represent the relay on pspice. When i build it there would be some sort of relay. A push reset button.
    the fault occurs when the switch closes increasing the current flowing through the 200 ohm and 5 ohm ( 4.87 ohm resistance) from 0.1 to 4.1

    i want the mosfet to turn off, thereby forcing the mechanical switch to open.

    the LED and 2.2k resistor is to use the LED as a fault current indicator
    the LED starts off on and will only turn off when the fault is cleared.

    The fault gets cleared when the mosfet turns off forcing the mech switch open.
     
  17. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Either you are a newbieateverything including explaining yourself, or I am brain dead.:D
    What causes the switch in your circuit to close? Does the switch simulate an accidental short circuit, or is it a physical switch?
    Is the MOSFET intended to be your circuit breaker?
     
  18. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,018
    1,537
    @Ron H., he says not, but I think this is related to his first thread that was closed. A circuit breaker.

    @ newbieateverything, maybe this is what your after, In the formula on the bottom left corner, "I" will be the amperage you want to limit at. And V is .1 to .2 volts. Rs is the ohms of the resistor

    While not shown,to keep things simple, you should use agate driver IC. Between the and gate output and the gate resistor. So the mosfet will turn on fast. R gate should be between 4R7 and 47R ohms, not in the k ohm range you have been using in the schematics.
     
    Last edited: Mar 23, 2013
  19. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    You are describing a standard circuit breaker with status LEDs.

    Will one of those work?
     
  20. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Aircraft circuits are not allowed here. While not specifically spelled out in the ToS it is a matter of common sense.

    If you are qualified to work on this stuff you do not need to come to a learning electronics website to ask questions, if you are learning then it should not be on aircraft circuits.
     
Loading...