DPDT relay as a latch?

Discussion in 'General Electronics Chat' started by umphrey, Apr 24, 2015.

  1. umphrey

    Thread Starter Member

    Dec 1, 2012
    39
    1
    Hello,

    I have a circuit with 2 switches that are normally closed but when they get hot they open. I also have a DPDT relay and a circuit that monitors voltage. Is there any way I can connect this so that the relay will latch if SW1 or SW2 opens?

    Normal operation:
    SW1 and SW2 closed
    Vout = 2.5V

    Fault condition:
    SW1 or SW2 opens
    Vout = 0 or 5V

    After I get a fault condition I want Vout to stay 0V or 5V until the power is cycled. Can I do that with a single DPDT? If not how can I do that with 1 latching relay? 2015-04-24_15-02-56.png
     
    Last edited: Apr 24, 2015
  2. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    A latching relay is pretty easy if you had normally open switches that closed when hot - you'd just put one of the relay normally open contacts in parallel with the sensor switches.

    Without scratching my head too much - I think you'll need another relay to act as an inverter, the auxilliary relay releases when the sense switch opens and a normally closed contact closes to energise the one wired for latching.

    The other ways of doing it pretty much involve power resistors dissipating heat.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,973
    3,219
    I'm a little confused when you say the output should be 0V or 5V under fault conditions. :confused:
    Is either one okay?
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,669
    804
    Do you mean WHEN SW1 and SW2 are closed, or AFTER SW1 and SW2 get closed? I.e. is the system supposed to have some kind of memory?
    I guess it should, but your conditions don´t make it clear enough. I would imagine that when SW1 and SW2 are closed and you push a reset button the system will start. Then whenever SW1 or SW2 goes open, the system stops and waits for someone to push the button and reset it. Am I right?
     
  5. umphrey

    Thread Starter Member

    Dec 1, 2012
    39
    1
    They get read into a DSP and if Vout is less than 1 volt or greater than 4 volts then the DSP reads it as a fault
     
  6. umphrey

    Thread Starter Member

    Dec 1, 2012
    39
    1
    The act of closing SW1 and SW2 shouldn't affect the state of Vout. Once either is open then Vout should stay in its state until power is removed.
     
    Last edited: Apr 24, 2015
  7. crutschow

    Expert

    Mar 14, 2008
    12,973
    3,219
    So what generates Vout's state?
     
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