What does this term mean and what is it's significance?
And ..... I have a Hostmann Programmable Thermostat - Why does the display go off when I take it off the wall? It is battery operated so it must be getting something from the mains to activate the display.
They are a set of contacts that are not connected (usually internally within the piece of equipment) to any supply of electricity. You can use them for whatever purpose you wish, but you will need to provide a supply to work whatever the contacts are to control.
It means you have access to the switch contacts. The alternative is usually that one of the contacts is hard wired internally to 230V, which is OK if that is what you needed anyway.
Just a guess that it detects lack of backplate and shuts down to protect the battery. Probably thinks it is back in the original packaging. Alternatively it is actually powered from the mains. My Horstmann programmer certainly is mains powerered. However, I've never seen a mains powered programmable thermostat. In any case, it is a bit odd, as it would not be compatible with standard thermostat wiring which only has a "CH On" live, and no permanent live. You would be forced not to use the overall programmer, in this case.
Christian.
A.k.a. dry contacts, it just means that they are a pair of switch contacts that make or break according to the unit design, but have nothing else connected to them so you can use them as you wish within their voltage and current rating.
This is the opposite to a wet contact that means that either it sources a supply voltage when the contact is made, or it will sink (or draw) current (usually to earth or negative) when active. The latter is most often found in low voltage semiconductor equipment as a signalling line, the former more often at mains levels to supply such as a lamp or heater.
Isn't it just a relay then? Could a relay be called volt-free?
In article , John writes
Yes, it is any set of contacts not internally connected to a live feed or not required to be connected to a live feed for correct operation. It means you could use them for low voltage switching or some other purpose, Quite a few non electronic thermostats use the presence of mains to power an accelerator (heater) which improves the accuracy of the control temperature. It's just telling you it isn't one of those.
A relay is an example of volt-free contacts - but not all volt-free contacts involve a relay.
The ones in zone valves, for example, are mechanical - and are closed by the action of motoring the valve open.
Roger
There is a plastic pin in the backplate which enables the display. My guess is that when the front is removed the control goes into a power saving mode where it can remember the settings whilst the batts are swapped?
The external contacts make no connection with the control whatsoever except to a simple 1 pole changeover relay.
Well spotted. I hadn't noticed that. Tried pushing it in and it powers up the display (after pressing a button to wake it).
In message , John writes
It means that they don't have volts on them provided from the live connection which powers the rest of the gubbins, i.e. it's just a switch without live on the common so you can use it on a low voltage system
Many programmers now turn off the display if there is no mains applied to it. With some of the older ones, the display drew too much juice, but I would think that the main reason they switch off the display is to indicate that there's no mains to the programmer and that it won't work.
In message , John writes
The switched contacts can be
I wonder what it actually is? It's not a conventional relay by the noise it makes, and besides a relay would draw too much current for a battery device. My guess is some form of stepping motor.
You can get relays that require a pulse to start and a pulse to stop. These are much more suited to battery operation.
Christian.
AKA "latching relays". Can't see any in CPC, and don't have the Maplin cat to hand, but they are certainly available. The only problem is that in the event of a power failure they "remember" their last position. Not usually a problem with a thermostat though.
IIRC, they generally have two coils, one to latch on, one to latch off. I suspect some fancy tricks with magnets internally :-)
Hwyl!
M.
I remember a post refarding either the Tower / Horstmann programmable thermostat (one and the same). When the battery expired the contacts were left 'as is', in this case with the heating powered on. I think the poster concluded they were magnetically latched.
In message , Christian McArdle writes
But in CH controls, usually just found in the lockout function
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