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?

 

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?View attachment 84568

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.

 

I'm a little confused when you say the output should be 0V or 5V under fault conditions.
Is either one okay?

 

Normal operation:
SW1 and SW2 closed
Vout = 2.5V

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?

 

I'm a little confused when you say the output should be 0V or 5V under fault conditions.
Is either one okay?

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

 

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?

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.

 

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.

So what generates Vout's state?