Latching relays with push-button reset

A 3PDT relay might do it, depending on how (what) initiates its closing.

One set of contacts provides the latch and the normally closed contact is in series with its own coil. Once the relay closes it stays closed because that contact is wired to the power source directly. To cause it to open you add a normally closed push button in series with the contact that is holding it closed. Relay will open if the button is pressed and there's no other source of power telling it to close.

Or just get a two coil device and add a PB in series with the other coil. Not too many relays allow mechanical intervention (except industrial motor starters and then it is to close the contacts only)

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I'm not sure why this application would require a (self-)latching relay at all, either mechanically or magnetically latched. If you truly need a latching function, then default's post above is a great way to do it. It gives you an electrical latching function that is easily reset with a push button.

However, I have worked on a number of furnace controllers for both oil and gas burners, hot water and hot air, and never encountered one with any kind of latching function. I'm certainly no expert, but are you sure you need this?

Bill

OK, I was avoiding resetting it electrically, but you convinced me.

Thank you, and thank you, Bill.

And strangely, the moment you convinced me, I rememberer that I have a couple that I took out of 1975 or so Royal copier machine, that my job sold me for a dollar in 1983. Very big, very heavy machine.** It has about 15 Omron relays, including two that latch. MY2K, I think I saw yesterday.

Maybe I should have said why. My new home burglar alarm needs an accessory to know when the house is getting too cold, like when the furnace fails. I was thinking that the house would be pretty cold already, and it would be better if the alarm knew when the oil-burning furnace failed. That would give whichever friend is in charge of my house an extra 2 or 3 days to get here and get the heat running. (This probably won't happen, but I want to be sure. I may drain the pipes and the toilets, etc.)

When it tries to start and fails, the original design uses a latching relay that is reset with that famous red button. I had to work on that once and I think it was a single pole single throw relay. I need the other pole to be a zone for the burglar alarm. (I"m finally getting monitoring, because I may be away for 3 months.)

And because often I have no imagination, I just wanted to make it as much like it was as possible. Also so that for whoever comes over to reset the furnace, it would look like they are used to.

You're right. I'll put in a big pushbutton, like on a 1930's front doorbell, and I'll paint it red. I could even put it on the outside of the furnace so I could put the cover panels back and they wouldn't have to move them.

I'm a little reluctant to mess with a pcb that works fine, but 10 years ago the same relay wasn't working, I think. I couldn't find another that fit*** (In those days I cared if it fit) so I filed the contact points on the one that was there. I thought it would fail again in a couple years, but it's been about 10. So I messed with it all once. I should be able to do this. I guess if I were cold as I was then, I wouldn't be hesitating.

***A new control board/unit is about 250 dollars, probably more now.)

If it has a red button that I have to push, that pushes something on the top of the relay, that's a latching relay, right?

**The Royal copier was really hard for me to get home by myself, and I couldn't have if it didn't come with a table that was about the same height as my car's trunk. It had about 15 relays and lots of other parts.

I made about 300 copies with it, but when it broke the second time, by the time I fixed that problem, something new arose.

ONCE IT WAS BURNING THE PAPER ON THE WAY OUT, SO OF COURSE I UNPLUGGED IT. i COULDN'T GET IT RUNNING AGAIN. I had bought the service manual, for only 10 dollars iirc, plus postage, and I spent hours figuring out how the thing worked, and it TURNED OUT, IT WAS ONE OF THE LATCHING RELAYS, AND WHEN i UNPLUGGED IT, IT WAS NOT IN THE STATE FOR STARTTING AGAIN. tHEY HAD NO STARTUP CIRCUIT TO RESET THE TWO LATCHING RELAYS, IT SEEMS.

I am going to need a new furnace some day, and the people on alt.home.repair were nice enough to find me an all electronic control unit with an output that says when the "relay" has tripped.

Buy a basic mercury switch thermostat and mount it by a furnace outlet - there's often one in the furnace room. Set it at the level you want to alert at. When your friend gets the furnace going, it'll open the thermostat in 10 minutes - and your friend should wait that long.