Converting a Fridge into a Curing Chamber - Part 3 - The Electrics

Having assessed the problems in Converting a Fridge into a Curing Chamber - Part 2 - Controlling Humidity, and acquired the necessary thermostat, hygrostat(s) and relays, there now comes the dreaded time when it all has to be wired together!

My own control box has a thermostat and two hygrostats. There are double plugs controlled by each of these, plus one for normal usage:

Original Wiring for curing fridge / chamber

Whilst I am happy to supply details of the wiring, it's like an explosion in a spaghetti factory, so it's maybe best to look at each element separately!

Obviously, these diagrams are specific to the products I've used. Details of these are in Part 1 and 2 of this tutorial. However, they give a good idea of what's involved and may assist in working out the detail for your own choice of controller. Often the instructions and wiring diagrams supplied by the manufacturer are confusing, and sometimes, not even in English!

The normal caveats apply: you use this information at your own risk. If in any doubt, please consult a person who has the professional qualifications to do this type of work.

The Digital Thermostat

The details below show the wiring for my STC-1000 digital thermostat when using a Finder DPNO relay from RS-online (Stock No. 245-2431). The supply for both is 240V. I believe that some STC and similar thermostats run off a 12V supply and will cover the wiring for these later.

The thermostat 'switches' the live supply. However, it requires both a live and a neutral supply to run the thermostat.

Thermostat for curing fridge wiring

The wires are the standard colours: yellow/green for earth, brown for live and blue for neutral. The wire coloured red is also a live wire; it carries the power to the relay when the thermostat operates. I've coloured it red for the sake of clarity.

The top two contacts on the relay are for the two wires that when joined, will carry power to the plug socket. The contacts on the right of the relay are for the live wire, those on the left for the neutral. The connection between the top two contacts is made by applying a current to the bottom one. This operates a switch that connects the top two contacts, allowing power to pass to the plug socket. However, the power to the bottom contact remains independent of, and isolated from, the top two.

So let's look at it in detail: the earth wire goes straight to the earth contact of the plug socket. The neutral wire goes into the middle contact on one side of the relay and then from the top contact to the plug socket. There is a short link from the middle contact to the bottom one, which operates a switch connecting the top two allowing power to pass to the plug socket. There is also a neutral cable going to contact 2 on the thermostat. This, along with a live supply to contact 1, supplies the power that enables the thermostat to work; they don't supply the power that the thermostat switches, just the power to make the electrics and display of the thermostat work.

The actual bits of the thermostat that switch the fridge motor on and off are contacts 8 and 7. When the fridge is cold enough, the thermostat switch is open (off) and no power flows around the system to the plug socket. When the fridge gets too hot, the switch between contact 8 and 7 on the thermostat closes and power goes from contact 7 to the bottom contact of the relay along the wire (marked in red). This operates the switch that allows power to go between the top two relay contacts and onward to the plug socket.

By using an appropriate relay, it's also possible to have a 12v thermostat switching a 240v supply. The wiring is similar to above, except that all wires, with the exception of the earth and the wires to the top two contacts of the relay on both sides, should be from a 12v supply.

When the thermostat is all wired up and mounted, it's just a case of putting the probe into the fridge. The probe's thin wire should pass easily around the fridge's door seal. You'll also need to set the parameters on the thermostat. On most types you'll be able to set the temperature set value - the temperature at which you want it to switch the fridge off, a difference value - the amount to allow the temperature to go up before turning the fridge on again, and a delay time. The delay time ensures that the fridge compressor is not switched on and off in rapid succession, which could cause damage.

If 12°C's your ideal fridge temperature, think in terms of a band between 11°C - 13°C. Adjust the 'temperature set value' to 11°C, and the 'difference value' to 2°C. In this example, the fridge will turn on when the temperature is 13°C (the temperature set value + the difference value), and switch off at 11°C (the temperature set value). This should maintain the temperature around 12°C. If we were also to use the heating circuit (see diagram above contacts 5 and 6), the thermostat would turn the heater on at 9°C (the temperature set value - the difference value).

Wiring the Hygrostat

Having wired the thermostat, the hygrostat (humidity controller) is simple as it's similar:

Hygrostat humidity controller wiring

The difference is that the power to operate the controller is contacts 7 and 8, and the hygrostat switch is at 2 and 1.

If your problem is one of too little humidity, you'd set the controller's mode setting to humidify, connect a humidifier via this controller, and that should be all you need.

If you were going to use this to reduce the humidity, you'd still use the wiring above, but set the controller's mode to dehumidify. This can then be used to turn on a small heat source in the fridge chamber as detailed in part 2 of this tutorial.

It should be noted that the unit switches power between terminal 1 and terminal 3. In theory, you could have a humidifier connected to terminal 1, and a dehumidifier to terminal 3; the controller would switch between them. Regrettably, the changeover would be instant so that either one or the other would be running; this is not really what we're after. Ideally, we want to balance the humidity so that any intervention is minimal. For this reason, if both a humidifier and a heater are to be used, I suggest that two controllers are used with their set points about 10%RH apart. This allows for either 'running over' and prevents the heater and humidifier 'see-sawing' from one to another. Alternatively, a changeover relay with a time delay could be used.

Possible problems

Using a heater in the chamber for dehumidification presents a potential problem. You'll recall that the logic behind this is that the heater increases the temperature causing the thermostat to run the fridge motor. As the fridge cools, the humidity reduces due to water condensing on the fridge's 'cooling plate'. What's not wanted is for the heater to run when the fridge motor is already running. Firstly, it shouldn't be necessary, as the relative humidity should reduce due to the motor being on; and secondly, for the motor to be running, the fridge temperature is already high. To run a heater at this point could cause the fridge to reach a detrimentally high temperature.

A suggested solution this problem is to take the 'live' supply for the heater relay from the thermostat's heating output. This would be live only when the fridge temperature is lower than the set temperature by more than the 'difference set value': something that doesn't seem to happen in my fridge.

If you wish to try it, you'll need to wire it something like this:

Wiring for hygrostat and thermostat

Again, the wires shown in red are live wires, normally coloured brown. I've just coloured them red for clarity.

My feeling is that for this to work you'll need to have a very small 'difference set value' amount on the thermostat. It's dependent on the fridge temperature being below the 'set temperature' by at least the 'difference set value', otherwise there'll be no power to the heater relay. In this case, a thermostat with just a simple switch between heating and cooling, somewhat similar to the one in my humidity controller, may be of more use.

I feel that a more effective alternative, for thermostats with separate cooling and heating circuits, may be to introduce a 'changeover relay' fed from the cooling side of the thermostat. This would supply the power to run the heater as soon as the fridge motor turned off.

I have neither of these solutions on my controller. If either become necessary, I'll let you know how I get on.


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There are 36 comments

Graham

Hi Phil, Thanks for the excellent site.
I too built a curing chamber along the same lines as you, but since I live in the tropics, the fridge compressor will run most of the time. As you say this drops the RH – in my case to near zero. To counter this I used an ultra sonic type humidifier, but it would run almost continually and get through about a gallon of water a day! The humidity levels were also very uneven. My solution was to build from plywood a box within my chiller so as to have cold dry air outside the box and cool humid air inside. It also stops the blast of dry air from the cooling fans blowing directly on the salami. True, you loose some usable volume, but against that, I rather like the idea of my salami curing surrounded by wood rather than plastic!

Keep up the good work, Graham in Borneo.

Graham, - 02-09-’13 08:36
Max

A great series of articles! I am in the process of converting a fridge after limited success with a wine fridge, these will be invaluable. Luckily I have a friend who is a chartered electrical engineer, otherwise it would be rather daunting for a novice. I did try getting a (cheap) combined thermohygrostat on eBay. I have to say it never arrived and in the end I went with an Auber

Max, (URL) - 03-09-’13 08:43
Phil

Thanks you both for the comments.

Graham – that’‘s a great way around the problem – I did something similar in a wine fridge to solve a different; I had problems then to keep the humidity from going too high. See http://www.localfoodheroes.co.uk/?e=390 I doubt that you’ll have that problem because your fridge motor will be running more often than mine was.

Max – did you source your Auber controller from a UK based supplier? If so, can you let me have the details please.

Phil, (URL) - 05-09-’13 20:24
Max

I’m afraid I couldn’t find a UK supplier. I ended up going with the Auber store on eBay: http://stores.ebay.co.uk/Auber-Instrumen... At the moment I just have a temperature controller. I’ll set the fridge up, put some meat in and then see if I need a humidity controller too

Max, (URL) - 06-09-’13 09:44
Phil

Thanks for that Max, I’m sure that it’l help other people.

Phil

Phil, (URL) - 08-09-’13 17:43
Nate

Hi Phil, great work putting all this together!

I am just about to start building a control unit like this though I’m sticking with 1 thermostat and 1 hygrostat. Since my part of the world has 60%RH most of the year I may be able to get away with this but only time will tell. Is it possible to get an internal photo of the unit itself? I’d be interested to know how you set all your wiring up on the inside?

Nate, - 25-09-’13 03:54
Phil

Hi Nate,

Sorry, I didn’t, it’s a jumble of wires that looks like spaghetti! According to my expert, you allow enough on each wire for it to be trimmed and reconnected twice, so the wires have quite a bit of slack in them.

If I get time, I’ll open the controller he made and email you a photo of how it looks when a pro does it.

Phil, (URL) - 26-09-’13 20:27
Gordon

Do you do anything to control/introduce air flow?

Gordon, (URL) - 30-01-’14 16:44
Phil

No, I rely on the changes of air from opening the fridge door periodically. My fridge has a hole in the side, now covered, where a fan was mounted. All it succeeded in doing was cause case-hardening. I have tried fans as small as 22mm but it still caused problems.

Phil, - 07-02-’14 19:09
George

Excellent series of articles. I’m just about to sstart building my own curing chamber and was wondering if you had a larger picture of the whole wiring diagram that I can use as a guide.

George

George, - 11-02-’14 21:51
Phil

Thanks George, I’ve sent you a copy by email.

Phil, - 13-02-’14 20:06
Mick

I’m planning to build my own curing chamber and your posts helped me a lot! Printed out all 3 of them and will certainly read and re-read them again and again and I also bookmarked your site. Thank you for sharing your knowledge and experience!

Mick, (URL) - 21-02-’14 14:42
Phil

Many thanks Mick. Your website http://www.fleischtheke.info/ looks interesting.

Phil, - 25-02-’14 12:17
Mick

Thanks Phil, that’s very kind! It’s all in German but maybe Google translate can help a bit.

Mick, (URL) - 10-03-’14 20:45
Julian

Thanks for taking the time to post this info. One question as I gather the bits to convert my fridge …. why do you use a separate relay when it appears that the thermostat will do the job?

Thanks
J

Julian, - 22-05-’14 18:51
Phil

Thanks J, that’s a good question. The simple answer is that I did it because my electronics expert told me to. It’s something to do with the ability of some things to peak at far higher amps than their normal running amps. It also ensures that the system meets a requirement that the gaps between contacts have to be more than 3mm (I think) i.e. He knew the specification of the independent relay but could not be sure of the ‘actual’ specification of the one in the controller.

What it does mean is that, if I want to, I can use the box for other projects where the item being controlled is greater than 10 amps.

Hope this helps.

Phil, - 22-05-’14 19:41
Julian

Hi Phil, my Finder relay has arrived with no wiring instructions at all so I don’t know which contacts are which (maybe it isn’t important!?). Would you be able to send me a photo of the wiring of yours? Many thanks in advance.
J

Julian, - 30-05-’14 21:03
Phil

Hi Julian,

Without disconnecting my fridge and unscrewing the back of the box, I can’t really send a photo. Can you send some photos of your relay (which I assume is the same one as I mention above), including the contacts. It’ll give me a ‘point of reference’ to refer to.

Phil, - 31-05-’14 14:27
Phil

Hi again Julian,

I’ve emailed detailed instructions to you.

Phil, - 02-06-’14 12:21
Julian

Many thanks for all your help Phil. I now have a fully functioning curing fridge.
J

Julian, - 22-06-’14 12:05
fraser

Hi Phil,
Very many thanks for the excellent guide on the fridge conversion.
I’ve accumulated the bits, oh and the fridge, and am just about to follow your guide and build the electrics into a project box for neatness.
My temperature and humidity controller switch 240 volts on output, and I just wonder why you decided to use switching relays.
Both units can switch 16A which should be more than enough I hope.
Any guidance would be appreciated.
The fermentation box is next!

Regards
Fraser

fraser, - 23-06-’14 21:50
Phil

Thanks Julian, Please let me know how you get on using it. I’d love to see some pictures of your finished products.

Phil, - 24-06-’14 11:30
Duncan Wearmouth

Hi Phil,

Great articles. Got all my electrical components, getting the fridge tomorrow, so looking forward to a weekend of charcuterie engineering! Like George above, could I get a copy of the full wiring diagram please.

Kind regards

Duncan

Duncan Wearmouth, - 28-08-’14 20:01
Phil

Many thanks Duncan, I’ve emailed a copy to you.

Phil, - 28-08-’14 23:10
David Steele

I’ve built my controller following your design with two humidity controllers and a the same relays and the fridge has some Salami and Chorizo in it…so far so good. I’ve set the temperature to 11° C ± 2° but I’m not sure what to set the humidity controllers to? I’ve selected 70% and 77% but they are measuring about 85% at the moment and I’m concerned the the heater will make the compressor run constantly.

Regards,
David

David Steele, - 01-09-’14 19:49
Phil

Hi David, I tend to set the hygrostats higher in the early stages and gradually reduce it as the sausage dries. It’s one of those things that tends to settle down over a few days. I’ve had them ‘off the scale’ in the first couple of days.

There’s also a setting to stop the hyrostat ‘cycling’ the appliance on and off in quick succession – you can set a minimum time gap between use of the heater/humidifier. It’s setting F3 on the DHC-100+ and can be set for for up to 99 minutes.

I hope this helps.

Phil, - 03-09-’14 10:31
Lenny

Hi – Great Article.
I wonder, is there any chance I can have a copy of your full wiring diagram for this? Just to give me some ideas, as I am just about to start wiring mine! Many Thanks…

Lenny, - 08-02-’15 10:08
Phil

It’s on the way Lenny

Phil, - 09-02-’15 23:51
Julian Peace

Phil,

Many thanks for an excellent blog…

So, I have my fridge, I have my project box, my wiring, my temperature controller and my new sockets, but…I will need to keep the fridge in an unheated garage so will need a heater (ambient temperature currently sits around 2 celsius…) to ensure that curing can go on year round. I’m struggling to find any resources to assist with this – any suggestions?

Julian Peace, - 23-02-’15 09:48
Phil

Hi Julian

I use a heater similar to this one in my fridge:
http://www.amazon.co.uk/Hylite-1ft-Eco-T..
ou could even use a larger one of these to keep your garage temperature above freezing.
Hope this helps.

Phil, - 25-02-’15 18:37
Ralph D

Hi Phil,
I’ve been reading the blog & following all the info – been fantastic!
I’m just having a little trouble with the wiring of the relay:
You say to loop the top 2 terminals – but I can’t determine which are the top 2 terminals.
My relay is the exact same as the relay you mention (finder dpno relay – from rs online stock number 245-2431).
I have my relay positioned so that there are two vertical columns of 3 terminals and there is a small plastic bar separating two of the terminals.
The terminals on mine are numbered – would that help in determining your answer?
Would you be able to post a photo so I can see what way the relay is meant to be – I may it upside down, back to front etc!
Any help would be much appreciated.
Many thanks,
Ralph,

Ralph D, - 30-06-’15 12:12
Phil

Ralph, I’m laid up in bet at the moment, but if you send me some photos of your relay I’ll do my best to advise. What you’re trying to do is make one side of the relay permanently closed. There’s an email link here:
http://www.localfoodheroes.co.uk/?p=cont..

Phil, - 30-06-’15 14:02
Baz M

Phil,

What a top chap you are, a really informative article. I’ve been dying to try making my own Salami for years but always talked myself out of it because of my concerns of how to overcome the very problems you have addressed. Thanks for the information.

Baz M, - 21-08-’15 17:18
Phil

Hi Baz

Thanks for your kind comments.

Please let me know how you get on with your project.

Phil

Phil, - 25-08-’15 11:25
Baz

Hi Phil,

I guess I’m struggling with the same humidity problems you experienced, I am blogging my journey http://makemyownsalami.blogspot.co.uk/ and would value any suggestions you have, one question I have is how big is the container you have used for salt and is there any merit in putting the salt on a metal tray (I’m guess any moisture in the air will more readily condense on a metal container rather than a plastic one)

Baz, - 03-09-’15 19:29
Phil

Nice blog. I’ve found that the heat source in the fridge is the way to go. When I used salt, I spread it out about 1 inch thick on a plastic tray and changed it for dried salt about every two days – alternating the two trays.

I don’t find a need to do this now.

I use a heater similar to one of these http://www.toolstation.com/shop/p32265??

I hope this helps.

Phil, - 07-09-’15 12:05

I'm somewhat incapacitated at present so replies may take some time. Please post urgent enquiries at the www.sausagemaking.org forum.

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