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.
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
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.
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.
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
The external contacts make no connection with the control whatsoever
except to a simple 1 pole changeover relay.
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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 :-)
Martin Angove: http://www.tridwr.demon.co.uk /
Don't fight technology, live with it: http://www.livtech.co.uk /
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.
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.
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