Fail Safe Latching Relay

Discussion in 'General Electronics Chat' started by eaztridah247, May 6, 2015.

  1. eaztridah247

    Thread Starter New Member

    May 6, 2015
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    I am looking to use a latching relay that also includes a fail safe state if power is loss. I know normal latching relays remain in the state they currently are in if power is lost, but I am looking for one that fails to a certain state upon loss of power.

    I have searched for a component that does this but have come up empty. Does anyone have any ideas?

    Thanks.
     
  2. Reloadron

    Active Member

    Jan 15, 2015
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    The circuit is latched, my motor is running. Bang, power fails... I have coffee... Power returns, do you want my motor to start automatically or do you want the circuit unlatched and motor not running?

    You can have either.

    Ron
     
  3. crutschow

    Expert

    Mar 14, 2008
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    You can use a standard relay in a latching configuration.
    The default for such a relay is the power off or unlatched position.
    Do you want the latching relay controlled by one alternate-action push-button or two (set, reset) push-buttons?
     
  4. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    You have just described an ordinary relay... if power is lost, it goes back to its initial state
     
  5. eaztridah247

    Thread Starter New Member

    May 6, 2015
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    In your scenario, I want the power loss to cause the latched relay to fail to motor not running state.
     
  6. eaztridah247

    Thread Starter New Member

    May 6, 2015
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    I cannot use a regular relay because of how I am making the circuit. For example, a lever hits a switch at setpoint x, it comes off of setpoint x but the latched relay remains in the latched state until the lever hits another switch at setpoint y or I lose power.
     
  7. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    In that case a diagram or graphic would be most helpful
     
  8. cmartinez

    AAC Fanatic!

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    Come to think of it. You may get the results you want using several ordinary relays arranged as logic gates
     
  9. eaztridah247

    Thread Starter New Member

    May 6, 2015
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    reason i can't use relays used as logic gates is because there is a state where neither switch is engaged that the motor is on and another when neither are engaged and the motor is off, thus why I have to use a latch relay.

    My main question, does a latch relay exist that does what I want, fail to a specified state upon loss of power.
     
  10. Reloadron

    Active Member

    Jan 15, 2015
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    Yeah, then you will need to really provide a good description of exactly, in detail, what you are looking to do. For example if you are driving a traversing motor back and forth with limit switches and reversal. Even then for any project the load currents need known that any switches or relays will handle.

    Ron
     
  11. Reloadron

    Active Member

    Jan 15, 2015
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    You can configure any DPDT relay to do that using one set of the relay's normally open contacts to latch the relay. If power is lost the relay will drop out and need to be latched again using for example a pushbutton switch.

    Ron
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I still don´t see why a standard latching configuration of a normal relay wouldn´t work. You have one set of NO contacts to turn the relay on, and a set of NC contacts that turn the relay off.

    Can you describe the condition where you think this wouldn´t work? Or even better, can you write down all the states?
     
  13. Reloadron

    Active Member

    Jan 15, 2015
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    Here is an example of a simple latching circuit:

    Relay Latch.png

    Just an example. Additional relay contacts on RT 1 can be used to power whatever you wish. The circuit can also be any voltage you wish. Pressing momentarily SW2 latches the relay using its own contacts. It remains latched till either power is removed or SW1 is momentarily pressed.

    Ron
     
  14. hajivitra

    New Member

    Apr 7, 2015
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    nice information
    thaks all
    [​IMG]
     
  15. eaztridah247

    Thread Starter New Member

    May 6, 2015
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    States:
    Switch 1 (at 50 gallons) Switch 2 (at 80 gallons) Motor State
    0 0 On
    0 1 Off (Latch till 50 gallons)
    1 0 On

    Problem being, lever hits the 80 gallon switch and you lose power. The latching relay is latched into the "motor off" state. Say the liquid depletes all the way to 20 gallons with no power. Once power is re-established the latching relay is still keeping the motor off, but I really want "motor on"

    Reason I can't have "00" state just reset the latch, is there is a time when the liquid is depleting from 80 to 50 gallons that the "00" state occurs (no switch is hit) and I want the latch to still be enabled not turning the motor on.

    Hopefully that clarifies things.
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    So in the truth table for above example logic function 0 0 represent the initial state of Switch 1 and switch 2?
    Max.
     
  17. eaztridah247

    Thread Starter New Member

    May 6, 2015
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    correct
     
  18. MaxHeadRoom

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    Jul 18, 2013
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    I think you need to describe the whole process quite a bit more, the detail is very sketchy so far in order to come up with a logic circuit.
    What initiates everything? Is fully automatic needed etc.
    Max.
     
  19. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    The truth table was helpful, but maybe a better way would be if he were to write pseudo-code for the process he's trying to control.
    I'm still certain that logic gates is the way to go.
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Or any logic controlling device such as PLC or Smart Relay?
    Max.
     
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