Thermostat N/O and N/C is N/C ever used?

[fixitflav]

This discussion is, of course, all about the markings and (per the OP) how they can/could be confusing.

I don't think we have yet seen a case of any non-electronic stat, or any associated wiring diagram, using "NO"/"NC" terminology - they seem to invariably use "C"/"1"/"2" or "1"/"2"/"3". Are you aware of any (non-electronic) stats or diagram that use "NO"/"NC"?

As Eric has demonstrated in the other current thread, and as may be what the OP in this thread has discovered, at least some electronic stats seem to be different, in that they do use "NO"/"NC" descriptors - but, as Eric has suggested, this probably relates to the state of the contacts when the stat is not powered - leaving one to have to work out how "NO"/"NC" correspond to the usual "C"/"1"/"2"/"3". Indeed, as has been said, Eric's one has a jumper which enables one to change that relationship!

Kind Regards, John
Edit: too slow at typing again, probably due to greater verbosity!

No, I'm not aware of any using N/O and N/C, but I don't get into it much these days. I agree it could be ambiguous, depends what you consider normal, maybe electrical guys have a clear convention.
If in doubt you can always set it to say 50deg C on the bench, so it will definitely be calling, and check what it's doing with a multimeter.

 

[EFLImpudence]

Or just a mistake?

Here they use OFF and CALL.

 

[fixitflav]

View attachment 138986

I see that only the Danfoss stat uses NC,NO. I wonder if there is something different about them.

I think it's just what Danfoss choose to call them. All 3 contacts are used with all the stats.

 

[JohnW2]

No, I'm not aware of any using N/O and N/C ...

See below.

.... but I don't get into it much these days. I agree it could be ambiguous, depends what you consider normal, maybe electrical guys have a clear convention.

As has been said, there is a clear convention in relation to any contacts which are electrically actuated (as in a relay or contactor) - in which case "normal" means when no power is applied to the device. However, as I said in my first response in this thread, in the context of contacts which are not electrically actuated (e.g. a traditional stat), what is 'normal' depends upon the context and application - hence potentially confusing/ambiguous.

In any event, I'd forgotten this from early in the thread - an example of Honeywell using NC/NO in relation to a "SPDT Controller", which would usually/often be a stat ....

If in doubt you can always set it to say 50deg C on the bench, so it will definitely be calling, and check what it's doing with a multimeter.

Indeed - that's what one ends up having to do if the markings are not crystal clear.

Kind Regards, John

 

[EFLImpudence]

In that diagram, what does switch 1 do?

 

[JohnW2]

In that diagram, what does switch 1 do?

When, with the stat in the "NO" position (calling for heat), the motor has fully opened the valve, that 'switch' closes, thereby connecting the motor to the "NC" contact of the stat. That means that when the stat no longer requires heat, and returns to the "NC" position, the motor is activated to close the valve (switch 1 remaining closed until the valve is closed).

As I've said, that's more 'sophisticated' than a 'standard' motorised valve, which is simply closed ('passively') by a spring when the voltage (switched by a SPST stat) is removed.

Kind Regards, John

 

[EFLImpudence]

The switches are not rigidly connected as it looks in the diagram, then?

They have a separate cam each.

 

[JohnW2]

The switches are not rigidly connected as it looks in the diagram, then? They have a separate cam each.

No, they have a single cam and switch together.

When the motor has fully opened the valve, the cam operates both switches. SW2 then switches off the supply to the motor and, as per previous post, SW21 connects the motor to the "NC" contact on the stat., waiting for that contact to become live when the thermostat is 'satisfied' and therefore the motor has to run again to close the valve.

Kind Regards, John
Edit: typo corrected

 

[EFLImpudence]

Ah, like the door lock motors in Fords.

I was confused by the dotted line which looks like it is pulling both switches at once, like a diagram of a relay.

 

[JohnW2]

... I was confused by the dotted line which looks like it is pulling both switches at once, like a diagram of a relay.

It IS 'pulling both switches at once' - when the motor has fully opened the valve, the cam 'pulls' both switches simultaneously, opening SW2 and closing SW1 (as per the 'non-pulled' switch positions shown in the diagram).

You missed (or, at least, didn't comment on) my previous typo, now corrected!

Kind Regards, John

 

[EFLImpudence]

It IS 'pulling both switches at once' - when the motor has fully opened the valve, the cam 'pulls' both switches simultaneously, opening SW2 and closing SW1 (as per the 'non-pulled' switch positions shown in the diagram).

Are you sure?
Surely, the switches must be opened by oposite sides of the cam.

You missed (or, at least, didn't comment on) my previous typo, now corrected!

I knew what you meant.

 

[JohnW2]

Are you sure?

Pretty sure, yes - otherwise it wouldn't work!

Surely, the switches must be opened by oposite sides of the cam.

Not necessarily - it just depends on where the fixed contact is relative the the movable one which the cam 'pulls' (more likley 'pushes', I would have thought!).

Anyway, sticking with 'pulling', if the cam pulled both movable contacts towards it when 'activated', it could (if the fixed contacts were in the appropriate places) pull the movable contact of SW12 away from its fixed contact ('opening' it) whilst pulling the movable contact of SW21 towards its fixed contact ('closing' it).

Kind Regards, John
Edit: typos corrected

 

[JohnW2]

Not necessarily - it just depends on where the fixed contact is relative the the movable one which the cam 'pulls' (more likley 'pushes', I would have thought!). ... Anyway, sticking with 'pulling', if the cam pulled both movable contacts towards it when 'activated', it could (if the fixed contacts were in the appropriate places) pull the movable contact of SW12 away from its fixed contact ('opening' it) whilst pulling the movable contact of SW21 towards its fixed contact ('closing' it).

Kind Regards, John

 

[fixitflav]

See below.
As has been said, there is a clear convention in relation to any contacts which are electrically actuated (as in a relay or contactor) - in which case "normal" means when no power is applied to the device. However, as I said in my first response in this thread, in the context of contacts which are not electrically actuated (e.g. a traditional stat), what is 'normal' depends upon the context and application - hence potentially confusing/ambiguous.

In any event, I'd forgotten this from early in the thread - an example of Honeywell using NC/NO in relation to a "SPDT Controller", which would usually/often be a stat ....

View attachment 138989

Indeed - that's what one ends up having to do if the markings are not crystal clear.

Kind Regards, John

I assume that's a 2-port valve, so am I right thinking it has an additional limit switch, not shown, to start the pump and boiler only when the valve is fully open? To avoid risk of them starting when dead-headed if the valve should fail to open. Of course that's not needed with a 3-port valve (mid-position or not) as there's no way both ports can be closed at the same time.

 

[EFLImpudence]

John, your diagram.
It can't be like that or switch 2 would open (as well) as soon as the cam moved.

I think something like this as clearly the diagram does not allow reversal of the motor: