"martin" wrote in message
...
On Sun, 14 Mar 2004 20:29:03 -0000, "Ophelia"
wrote:

please explain what is a 'digital plug in stat'?

a separate plug in thermostat with a digital display.

Thank you

O

On Sun, 14 Mar 2004 20:54:16 GMT, "kenty ;-\)"
wrote:

look on ebay.co.uk ,auctions, type in digital thermostat .I got one for £25
+£3P+P.The one I got has a digital display with a cable & probe.You can
connect electric tube or fan heaters through them,the temperature is
displayed,it looks like a car stereo player with a cable and probe.They are
meant to within 1deg accurate,there is someone on there selling the same
thermostat all the time,you can bid for one ,which I lost.Or you can buy one
now for £25 if you cant wait,just look through the list of items and find
one with a buy it now logo.

I've bought one of those, should get it tomorrow. Read the
specification supplied by the vendor before deciding if it will do
your job. Here it is below as pasted from his email.


'Main technical features Range: -50÷150°C
Resolution: 1°
Accuracy: ±0.7° (-30÷110°C)
Sensor type: PTC1000
Outputs:
Out 8(3)A 240V~
Power supply: 230V~ ±10% 2W
115V~ ±10% 2W
Operating temperatu -10÷50°C
Front protection: IP54
Dimensions: 77x35x77 mm'

I bought mine to control a 1.5KW heater in my workshop. The guy says
he has customers using them with 3KW fan heaters. Personally, seeing
those contact ratings I would be dubious. I would use the thermostat
to operate a suitably rated mains relay - good quality mains relays
are usually fairly cheap and easy to find on the surplus market.

Rod

Weed my email address to reply
http://website.lineone.net/~rodcraddock/index.html

In article , Rod
writes

I would use the thermostat
to operate a suitably rated mains relay - good quality mains relays
are usually fairly cheap and easy to find on the surplus market.


Um........... What exactly is that then Rod? I thought mains relays were
some sort of collection of wire. Does it help electrical equipment in
the greenhouse?

janet

--
Janet Tweedy
Dalmatian Telegraph
http://www.lancedal.demon.co.uk

In article , Rod
writes

I would use the thermostat
to operate a suitably rated mains relay - good quality mains relays
are usually fairly cheap and easy to find on the surplus market.


Um........... What exactly is that then Rod? I thought mains relays were
some sort of collection of wire. Does it help electrical equipment in
the greenhouse?

janet

--
Janet Tweedy
Dalmatian Telegraph
http://www.lancedal.demon.co.uk

On Sat, 20 Mar 2004 20:20:39 +0000, Janet Tweedy
wrote:

In article , Rod
writes

I would use the thermostat
to operate a suitably rated mains relay - good quality mains relays
are usually fairly cheap and easy to find on the surplus market.


Um........... What exactly is that then Rod? I thought mains relays were
some sort of collection of wire. Does it help electrical equipment in
the greenhouse?

janet

The relay (electrically operated switch) built into the thermostat in
question is only rated for switching 8Amps for the 'normally open
contact' and 3Amps for the 'normally closed' contact. In a change over
switch such as those normally present in thermostats you have a
contact where power goes into the device which flips between two
contacts where the switched power comes out. In it's 'settled
quiescent state' the moving contact sits on the 'normally closed'
contact. When the thermostat activates the switch the moving contact
flips over to the NO contact and turns on the heater. This is all fine
and dandy but 8Amps at 220-240V is only about 1800Watts and we want to
switch 3000W + any inrush surge so those 8A contacts are doomed to a
fairly short unreliable life (they get burnt or even sometimes weld
together). Most heating control thermostats other than those sold by
the likes of Two Wests are only expected to switch heating control
circuits, that doesn't involve a lot of power, say a few Watts or tens
of Watts compared with the thousands of Watts for space heating.
The way round this is to use the thermostat in the way it was intended
to activate the heating control system - in this case a big
electrically operated switch (Relay). So now all the switch inside the
thermostat has to do is switch the few milliwatts needed to activate
the relay - the big silver coated main contacts in the relay are doing
the real switching job. The relay is just an electromagnet activated
by a small control current through a coil. The magnet pulls the
contacts over as required. Another big plus in such a system is that
the most unreliable part of this combo of digital thermostat and
external relay is the relay and they only cost a few pounds to buy new
- even the 30Amp part I'm using.
That's quite an essay to cover something that is really quite simple
and very easy to demonstrate. Apologies for that if I haven't made it
clear.



Rod

Weed my email address to reply
http://website.lineone.net/~rodcraddock/index.html