Improved fridge thermostat - Project

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Six months ago I fitted a thermostat from a Maplin Electronics kit to my fridge and the results, even through the hot Mediterranean summer, have been spectacular. Most 12V fridges are controlled by a mechanical thermostat in contact with the evaporator (cold plate). This works after a fashion but but it does not control well with changing loads (i.e. putting lots of warmish cans in), changing weather or frequent opening and closing of the fridge door. Either the fridge gets too cold, with frozen areas or parts get too warm. What is needed is a cabinet thermostat.

Maplin sell a kit type RR51F £4.99 www.maplin.co.uk You will need to search 'kit, thermostat' as I cannot find a way to get a link to post here. This kit is cheaper than buying the bits (unless you are a hobbyist and have it in stock) and it works, first time. It is easy to build as long as you can solder. The temperature sensor is a small thermistor. I wrapped a fair bit of self-amalgamating tape around mine to put a small delay in the response as well as to protect it from moisture and mechanical damage. The PCB is mounted outside the fridge and the thermistor connected by very thin electrical flex. I just close the fridge lid on the wire - I have not done anything clever like drilling a hole in the cabinet. But then in my installation the distance from the fridge lid to the electrical distribution cupboard is only a centimetre so the wire is not really noticeable. The output of the kit is a simple 'voltage-free' relay contact. I have simply wired this in series with the fridge supply, by the circuit breaker. The adjustment is on the kit PCB and is a potentiometer with a long control rod. You could mount this but for the moment I am still in the evaluation phase /forums/images/graemlins/smile.gif and have tucked it quietly into a safe corner. I never need to adjust it regardless of the weather or what we are putting into the fridge so there is no need to have easy access to the control.

I decided to wait a while before recommending this and my verdict, on a chest-fridge (i.e. the cold air does not fall out when you open the lid) is that the improvement is 100%. We no longer accidentally get frozen food in the fridge when it is set cold enough to handle hot weather or heavy loads.

If anyone has any problems with this project, or tips that might help others, perhaps they could post them here.
 
Well done! Thanks! I fiddled around and couldn't see how to do that - how did you do it?
 
Trade secret!! /forums/images/graemlins/tongue.gif


Underneath the product is a band titled "Main Products" with a direct link to the thermostat kit ....
 
What a brilliant post,truly in the spirit of PBO.
The question I would ask is if it's so cheap and easy why on earth don't the manufacturers of these units incorporate it at the production stage?
Second question could this be adapted for domestic(220v)refrigeration?
 
Nice idea...

That's a nice idea. The reduced hysteresis of the electronic thermostat will probably give some savings on power consumption too.

One point I'd mention is that the relay supplied in the kit you mentioned is only rated for 3A, and most fridge compressors take much more than that. For longer-term reliability, it would be worth upgrading the relay.
 
Sounds like a very sensible improvement on the basic type of fridge thermostat. Do you have any data that it also saves amps and thus battery useage ?
 
Yes, but you'd have to use the integral relay to control another relay as you wouldn't want to put 230V on this little pcb as it is connected to the 12V supply and the thermistor that is in the frige body without proper electrical insulation.
 
My fridge only draws 2.5A and it can keep our large fridge down at -18C even in a Mediterranean summer (albeit on 24/24). Our compressor has an Isotherm label on it but there is also a Danfoss badge so I don't know who makes it. Suffice to say, this is a 1999 unit and I would be surprised if modern compressors draw more and I would hope, less. Do check before upgrading the relay as it may not be needed.
 
Well, I don't have any data and we have so much electricity available with our solar panels and generator that I don't worry too much. However, it must be true that since you can control the temperature far more accurately, if you can tolerate a higher cabinet temperature (e.g. if you don't have sensitive food or don't need to keep it for long) then you could more easily adjust the temperature to a higher level than would be possible with a conventional fridge thermostat. We set ours to +2C which is very cold indeed, ideal for cold drinks and meat keeps almost as if it was in a freezer. If power was an issue you'd set it to +8C to +10C.
 
An excellent project with all the important details. However, I have a question. In my fridge, I find, as you, that the control is very poor especially with different loads. I aim to have the bottom of the fridge at 5C and have a car in/out thermometer to tell me what it is. The present control has a hysteresis of about 4 degrees. What is the hysteresis with this control?
 
If, as an A/C and fridge technician, I may add a few comments to the above posts.

The reason manufacturers don't use this type of control is because they cost more money then the simple mechanical thermostat.

As has been mentioned the small relay on the Maplin kit will not handle the 'starting' current of the compressor for long; yes it may only run at 3.5amps but the start up current could well be 20amps.

Also the mechanical themostat provides a form of time delay between compressor starts and this is necessary to ensure proper compressor cooling. Any compressor of this sort should not be started before about six minutes has elapsed from the last start.

Everytime the motor starts there is heat generated and the returning cool gas from the evaporator plate needs time to cool the motor windings. If there are too many starts in quick succession then the motor gets hotter and hotter and eventually can burn out.

All these things needs to be taken into consideration when designing an electronic thermostat hence the price difference between a 'proper' fridge/freezer electronic controller and a 'thermostat' from someone such as Maplin. I have nothing against Maplin or any other supplier but it's "horse for courses."
 
Sounds like you've got the smaller Danfoss compressor. The thermostat kit you recommended apparently has a temperature range of +5 to +30 degC - did you change the thermistor to enable you to achieve lower temperatures?
 
When you ask 'the hysteresis' I presume that you mean the hysteresis in the fridge cum controller rather than the thermostat per se? The thermostat has an adjustable hysteresis though you have to alter fixed resistor values. The arithmetic is very easy. As built from the kit, ISTR that the hysteresis is, from theory, a few degrees, though I have never measured it. What I have monitored very carefully is the cabinet temperature, which I have monitored with an IR thermometer and then during the hottest part of the summer in the Balearics, with a wireless LCD thermometer. I have never found the temperature to be more than three degrees out. It is possible that you might want to alter the hysteresis in a different installation but I rather doubt it...in any case, try it first.
 
The Maplin relay on my kit has a 10A dc rating. I suspect that the lower rating on the spec is due to the rating of the PCB or screw terminals which look fine for 5A+ to me (I am a qualified and experienced electrical and electronics engineer, and speak from experience). Or, maybe, sometimes they fit a lesser-rated relay. I don't know but the risk is £4-99. If you are still unhappy you can always drive a bigger external relay but you'd have to find a convenient way to mount it. Try it first. I ordered two kits - you never know when you'd like another thermostat on board!

As for the time delay, the circuit is nothing more than a simple comparator with hysteresis. The thermostat as built has hysteresis inherent in the thermistor (which I increase with tape, as you will see from my original post) and the electronic hysteresis in the comparator. I have never noticed the fridge starting within at least ten minutes or more of it switching off.

So, taking all of these things into account, we can safely enjoy a perfectly good fridge thermostat for £4-99 instead of whatever the likes of Danfoss charge. It is usually the case that add-on parts for marine products are ten or twenty times as expensive as they could be.
 
Mine is as standard although I expected to have to change it. I simply set mine towards the bottom of the range and this keeps the fridge around the sensor at +2C. There will be some variation between thermistors and tolerances on the fixed resistors, and it could be that some will need to be modified with a changed fixed resistor. It is very easy to do and there is no shortage of electronics engineers here who would assist, myself included if I am around, if needed. The circuit diagram is included in the kit. The chances are that it will work straight out of the box - mine did. If anyone wants to use it as a freezer thermostat it will need a resistor change.
 
hi
very interested in this concept as my wife needs injectable medicine which needs good control 3-8degrees centigrade. Just acquired an e bay fridge for gas/12v and intend to run on mostly on gas as my main engine is an outboard with all of the limitations for charging batteries inherent in of outboard generators (mostly 6A max at max speed - only 2/3A agt cruising speeds which often only balances instrument & radio usage). I might try the mod suggested and at least the 12v control may be better but any idea how one could improve the control of a fridge when on gas operation - actually have no idea how the temperature control works on a gas fridge!
 
the standard caravan 12v/240v/gas fridges are normal kit on a Catalac. Dont bother with the 12v setting, it uses more than 8 amps and is not run from a thermostat. The 240v setting is thermostat controlled.

I use mine on gas. again it is not thermostat controlled, and temperature will depend on ambient temperature and how often you open the door, but the gas setting is very effective.
 
Talbot has filled you in with the details of the control. Clearly you could control the 230V element or the 12V element via this thermostat, with an appropriate external relay (I assume you could sort that out OK buying from Maplin?). However, having experienced an Electrolux absorption for around 20 years on a boat I would be very dubious about using it to keep a controlled environment like that.

I have (in storage at the moment) a good quality compressor cold box which has the power to pull down to freezer temperatures (cost £260 or so in 2002). We used it as the only fridge on the Centaur for several years. It does not have good temperature control but it did have the power. You could drive one of those from this thermostat though you'd need to check current draw. That way you'd have extra fridge space of good quality and you could also carry the meds to and fro the boat safely, as it plugs into a car cigar lighter socket. I think mine is an Engel - a great piece of kit.
 
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