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UK Caravanning (uk.rec.caravanning) A forum for the discussion of caravanning undertaken by residents of the United Kingdom, whether in the UK or abroad. It encourages the interchange of views on the merits of models of caravan, makes of tow car, accessories, caravan sites, caravan clubs, and other related topics. The term caravan is to include trailer vans, motor caravans and trailer tents.

Fridge fan



 
 
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  #1 (permalink)  
Old July 12th 03, 09:08 AM posted to uk.rec.caravanning
Mike
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Posts: 29
Default Fridge fan

Some time last year I installed a computer fan on the cooling fins of my
fridge this was switched on when the temperature rose to a predetermine
temperature of 20 C. The thermostat I used was the size of a thumb nail a
bimetal that clicked on and off , this bimetal strip about 1/2 long provide
its own spring tension so making it a positive on and off action. I sent to
interested partys copies of photogrphs of my instalation. Unfortunately this
thermostat I used is no longer made.
I suggest you use a fish tank thermostat but failed to realise that these
only proved power on when the temperature falls. to use one of these
thermostats one needs a relay which will be perminatly on but will switch of
when the temprature rises about a predetermind temprature say 70 F, this
will switch off the relay, this relay will have a set of contacts that
connect power when the relay is switch off.
What we really need is a thermostat that connects on temperature rise. If
you use a central heating thermostat, get one that has two sets of contacts
(power on and power off ) these are used in boilers that have an pump over
run so that when the boiler is satisfied the heating is switch of but the
pump continues to run until the temperature of the boiler is reduced so the
boiler doesn't continue to heat water in the heat exchanger and boil,
useualy used in boiler with cast iron heat exchangers. It also occurs to me
that these thermostats may not work at a low enought setting to be any good.

If anyone knows where to get the thermostat I used, also if you know of a
better DIY arrange please let me know.
TIA Mike


  #2 (permalink)  
Old July 12th 03, 10:15 AM posted to uk.rec.caravanning
Nigel Crompton
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Posts: 68
Default Fridge fan

On Sat, 12 Jul 2003 08:08:30 GMT, "Mike"
wrote:

Some time last year I installed a computer fan on the cooling fins of my

Me too. Couldn't source a suitable stat so hard wired it to a switch.
The fan's inaudible during the day, but noticeable at bedtime so it
gets turned off. Remembering to turn it back on in the morning is the
trick!
  #3 (permalink)  
Old July 12th 03, 04:45 PM posted to uk.rec.caravanning
Mike Williams
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Posts: 56
Default Fridge fan

I like the simplicity of a simple mechanical thermal switch for such
purposes, but unfortunately they aren't that easy to find. There are plenty
available in car accessory shops of course, but they are designed to switch
at high temperatures and so are no use to you. You can get adjustable
versions (with a little adjusting screw in the side) and also microswitch /
snap disk versions (with the snap disks themselves available at all sorts of
temperature values), but I can only find them at specialist dealers or
manufacturers and it wouldn't be economical to buy them other than in bulk
quantity. You could (as has already been suggested) use a room thermostat,
but they don't seem to be particularly cheap, at least those that I have
looked at, and some types are not suitable anyway. If you can "drive" a
soldering iron then you could easily knock up a thermostat switching unit
yourself for a couple of pounds (just a simple op amp, a thermistor, a
little pot (not the "smoking" kind!) and a few other bits. In fact, Maplins
sell a kit containing all of the components you will need (including the
board and all the components and an output relay) for less than a fiver. If
you want to go that way then have a look at Maplin's Thermostat Kit (Order
Code RR51F) which they sell for 4.99.

Mike

"Mike" wrote in message
...
Some time last year I installed a computer fan on the cooling fins of my
fridge this was switched on when the temperature rose to a predetermine
temperature of 20 C. The thermostat I used was the size of a thumb nail a
bimetal that clicked on and off , this bimetal strip about 1/2 long

provide
its own spring tension so making it a positive on and off action. I sent

to
interested partys copies of photogrphs of my instalation. Unfortunately

this
thermostat I used is no longer made.
snip






  #4 (permalink)  
Old July 12th 03, 06:04 PM posted to uk.rec.caravanning
Jim Crawford
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Posts: 13
Default Fridge fan


"Nigel Crompton" wrote in message
...
On Sat, 12 Jul 2003 08:08:30 GMT, "Mike"
wrote:

Some time last year I installed a computer fan on the cooling fins of my


Me too. Couldn't source a suitable stat so hard wired it to a switch.
The fan's inaudible during the day, but noticeable at bedtime so it
gets turned off. Remembering to turn it back on in the morning is the
trick!


Same here, we go to France next week and I plan to use it then. It shifts a
fair amount of air for it's size, I was quite amazed.

Jim


  #6 (permalink)  
Old July 15th 03, 05:39 AM posted to uk.rec.caravanning
bowtiejim
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Posts: 138
Default Fridge fan


"Peter Sheppard" wrote in message
...

"Mike" wrote in message
...
Some time last year I installed a computer fan on the cooling fins of my
fridge this was switched on when the temperature rose to a predetermine
temperature of 20 C. The thermostat I used was the size of a thumb nail

snip

Radiospares has something close (although the contacts switch the wrong

way
:-( , have a look at:

http://www.rswww.com/

and search for item number 228-2664 (2.20 + postage + VAT).

You could use a simple 12v relay with changeover contacts to make it

switch
the way that you want.

Good luck

P


Using a room stat may cause a problem as normally they have a max of 25
degrees which will be exceeded most of the time! A cylinder stat would be
best. It could be wired to a 12v relay with open and closed contacts (5
bladed version). The fan(s) are then wired to the contacts which are closed
when the preset temp is exceeded, i.e. no current passing through the stat.
Both Maplin and I believe Halfords sell the five bladed versions. (Maplin's
version is about 1.50)

The stat is connected to pin 86, and pin 85 to the return to the battery.
Pin 30 is connrect to the positive side of the battery and pin 87a to the
fan(s). When the temp is below the preset, pin 87 is live, but when the
preset is exceeded, no current flows through the stat and pin 87a becomes
live. For a diagram see the five pin ISO relay on:
http://www.mgcars.org.uk/electrical/body_relays.html

It is important to insulate pin 87 so that no stray wiring of metal comes in
contact with it which might cause a short and subsequent fire. Naturally
the feed to the stat and that to pin 30 should be fused.



  #7 (permalink)  
Old July 15th 03, 02:01 PM posted to uk.rec.caravanning
Mike Williams
external usenet poster
 
Posts: 56
Default Fridge fan

"Roger Mills" wrote in
message ...

What you *actually* want is a stat whose contacts *close* as the

temperature
rises - since this can directly switch the fan without requiring a relay.

A
relay causes extra complication *and* consumes current when the fan is not
on.


If you can drive a soldering iron then my advice would be to build your own
little circuit to do the job. That way you can set both the switching
temperature and the hysteresis to whatever you want.

Mike

If a room stat has change-over (as opposed to simple on/off) contacts,
that will do fine. The fact that the actual temperature may be higher than
the max *switchable* temperature isn't a problem as long as you want to
switch within the switchable range. [ISTR that the OP wanted to switch at

20
degC - which is fine]. If the actual temperature is (say) 30 degC, the fan
will still run - which is what you want.

I'm not sure that a cyl stat would work. On the plus side, cyl stats

usually
have change-over contacts, so you can use the pair which closes when the
temperature rises. However, on the minus side, they are designed to switch
at higher temperatures - usually around 60 degC (but may still be able to
switch at lower temperatures). More importantly, though, they are designed
to detect the *surface* temperature of the cylinder rather than the
surrounding *air* temperature. I would have thought that air temperature
switching is what is required for the caravan fridge application.

Roger



  #8 (permalink)  
Old July 15th 03, 03:49 PM posted to uk.rec.caravanning
[email protected]
external usenet poster
 
Posts: 2
Default Fridge fan

On Sat, 12 Jul 2003 08:08:30 GMT, "Mike" wrote:

Some time last year I installed a computer fan on the cooling fins of my
fridge this was switched on when the temperature rose to a predetermine
temperature of 20 C. The thermostat I used was the size of a thumb nail a
bimetal that clicked on and off , this bimetal strip about 1/2 long provide
its own spring tension so making it a positive on and off action. I sent to
interested partys copies of photogrphs of my instalation. Unfortunately this
thermostat I used is no longer made.

Use the fridge stat driving a relay with a 240v ac coil, then contacts to
drive the fan with 12vdc

Ie when the fridge calls for cooling the switch will then drive the rly
closed and off again when cooling is not required.
  #9 (permalink)  
Old July 15th 03, 03:57 PM posted to uk.rec.caravanning
Roger Mills
external usenet poster
 
Posts: 11
Default Fridge fan


wrote in message
news
On Sat, 12 Jul 2003 08:08:30 GMT, "Mike" wrote:

Some time last year I installed a computer fan on the cooling fins of my
fridge this was switched on when the temperature rose to a predetermine
temperature of 20 C. The thermostat I used was the size of a thumb nail

a
bimetal that clicked on and off , this bimetal strip about 1/2 long

provide
its own spring tension so making it a positive on and off action. I sent

to
interested partys copies of photogrphs of my instalation. Unfortunately

this
thermostat I used is no longer made.

Use the fridge stat driving a relay with a 240v ac coil, then contacts to
drive the fan with 12vdc

Ie when the fridge calls for cooling the switch will then drive the rly
closed and off again when cooling is not required.


I had always assumed that a solution was required which would work
regardless of which fuel source was being used for the fridge. Your solution
*might* work when it is on mains electricity - but I don't see how it would
work for gas. I'm not *too* sure about the mains bit even - I'm not sure
whether an absorption fridge (as opposed to a commpressor-based fridge) has
a conventional thermostat.

Roger


  #10 (permalink)  
Old July 15th 03, 05:14 PM posted to uk.rec.caravanning
Mike Williams
external usenet poster
 
Posts: 56
Default Fridge fan

wrote in message
news
Use the fridge stat driving a relay with a 240v ac coil, then
contacts to drive the fan with 12vdc


Unfortunately that will only work when connected to the mains, whereas the
other solutions will work irrespective of the source of energy (mains, gas
or battery).

It is possible, of course, to use the fridge stat contacts to control the
fan switching (as I briefly mentioned and then disregarded in my earlier
post in this thread), but it would definitely be more work than it was worth
(considering that other methods are very much easier), especially if you
wanted it to work irrespective of whether there was a mains supply or not
(which most people would very definitely require). But you would have to be
very careful about the design of the circuit. One way would be to provide an
isolated supply via a high resistance source to the "mains input" side of
the contact and to rectify and measure the voltage at that point with
respect to the other contact. In that way a voltage would be "seen" across
the contacts when they were open whether the mains was connected or not, and
the voltage would not be present when they were closed. Your circuit could
then repond to that voltage (present or not) to control some sort of switch
(a relay, perhaps) in the separate fan circuit. There are other ways of
using the existing fridge stat contacts, but all that I can think of would
require more work than it was worth, and you would have to be very, very
careful about isolation of the mains from the DC supply.

Mike



 



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