Home Automation 101: Fixing a Fridge Freezer With a PLC

Ok, so your fridge is going for overkill. Rather than throw it out and buy a new one, we can fix it for not very much money.

Instead of running all the time and using loads of electric, for less than £20 we can make it as good as new, saving the planet from the refrigerant gas and nasty insulation, as well as saving the money - we all need a fridge, right?

Once your expensive fridge starts freezing everything solid, it isn't nearly as useful as it once was. Fizzy drinks tend to explode overnight, though all the cider ends up super ice cold if you time it right. Also, it runs up your power bill!

However, with a few tools and a handy low cost unit used for industrial systems, we can sort it out.

The one thing that you aren't likely to have at home is an STC-1000 Programmable Logic Computer (PLC). You can find them on eBay or Amazon or whatever, and they range from about £7 to £20. They all do the same thing though - toggle a pair of relays, one for "More heat" and the other for "More cold". Just be sure that you get one that includes the temperature probe, and that works are the voltages you get through your mains socket. (Most units work worldwide and include the temp probe, but buyer beware and all that.)

The rest of the parts are:

Wire cutters

Mains rated wire (2 or 3 core [twin, or twin+earth])

Block connector

Electrical screwdriver

Chocolate box insulation box (Or Electrical tape if you are bodging it)

Optional:

Wire stripper

Standalone temperature sensor for the freezer section

Voltmeter (or voltstick)

A fridge is really supposed to be between 2 and 5C. Since the temperature is going too cold, you can do as we did for many months, and turn the fridge off-and-on again. Of course, this is a nuisance and relies on you being there to toggle the switch! Overnight we were seeing temperatures as low as -8.7C!

The fridge does have it's own controller, obviously, but for whatever reason it isn't working. The PLC-1000 is an industrial logic controller that simply switches a relay for heat and another for cool, according to what it sees on the end of the temperature probe connected to it. In this case we will only be using the cooling relay, relying on the ambient room temperature for the "heating".

We will simply cut the power feed for the fridge, and add the new temperature controller in line, and put the temperature probe inside the fridge body. This will turn the power on when the fridge gets too warm, according to whatever we set as the target temperature.

When you pull your fridge out, you can collect your loot. Here's mine.

You can also clean it - apparently the military pay lots for spiderweb for body armour research, but this wasn't clean enough. Damned dust bunnies!

Ok, we are working with mains voltages here, so we want to be careful.

Before you start work, air gap the fridge. As in, PULL THE PLUG! That way there can't be any sudden shocks. Now open the door and check the light doesn't come on.

Ok, now go and have a cup of tea or something. Hydration is very important. This will also give the compressor, the metal lump shown in the picture, time to cool off.

The second picture shows the wiring into the compressor, but I couldn't figure out how to get the case off, there were cable strain reliefs in the way, and though it would be neat to put it in there, it gets very hot. There's no need.

We will be working about a foot away from the compressor anyway.

The STC-1000 PLC will have come with instructions, so check those. It is pretty simple though.

Use your screwdriver to remove the wire cover, and get access to the screw connectors. (The screw comes out about 10mm, then the cover will lift off)

Determine the connection for the power, the cooling and the thermocouple temperature probe.

Wire the probe in first. Polarity doesn't matter, so just pop the the bare copper wires into the two holes and tighten them up.

Now get your new mains cable. Work out where you are putting the PLC. You will want to be able to see the temperature display. I've currently simply got it sitting on the top of the fridge freezer, some 5 feet off the ground, taped down so it can't move. Now reel off enough cable to get from the PLC to the power feed cable at the base of the fridge, and add a little bit for safety - cutting too short is a right pain. Cut a second length for the relay.

Strip the ends off the cable carefully - about 5mm of copper should be showing. Label the relay line, I used pen, but you can just use a wrap of tape on the end. Now wire those in to the PLC. Make sure you get them the right way around. Remember brown is the live and blue is the neutral. (If you've got an earth, you should wire the two earth lines together, as the PLC doesn't take the earth cable. Just use a single bit of block connector.)

Once the screws are tight, check no stray strands are bridging the wires, and no copper is showing. (a millimetre or two is fine, but no more. You can undo the screw and trim the copper then re-insert and tighten it up again) You also need to make sure you haven't tightened the screws onto the insulation, but we will check that as we go.

Locate the cable cover and split the cables apart so you can screw it back into place. There should be almost no way you could touch the back of the PLC now without a screwdriver - fingers shouldn't be able to touch bare copper!

The next wire is really hard to find - it's the one between the plug socket to the back of the compressor. :-)

Check it is unplugged again!

Chop the incoming mains cable about 6 inches to a foot away from the point it enters the compressor. I cut it about 4" away and it was a bit tricky at the end putting the chocbox on.

Ok, strip all your wires so you have something that looks like the picture. Go for about 5mm bare copper, but you will need a bit more off the outer insulation for the relay wire.

Wire the inbound power into the block connector as shown. Follow the colours and locations.

Piggy back the two neutral wires together into the first connector on one side of the block (twist together and put in the same hole) Open the screw all the way, insert the wires neatly so no copper is showing, then tighten the screw back down firmly.

Put the earth into the next one. Tighten that down.

Piggy back the lives together into the next block. Tighten that down.

Turn the socket off, and plug in the fridge plug. Check nothing's touching anything that it shouldn't be, and turn on the switch.

Hopefully there wasn't a bang! (If there was, turn it off again and unplug. Either the PLC was faulty or you wired it badly. Go turn the power back on, and call someone who knows about electricity)

Looking at the front of the fridge should show you a display as in the picture. The bright red LEDs are the PLC temperature reading (ambient room temperature) and the LCD is the little cheapo temperature monitor we were using to judge when to turn the fridge off and on again.

(If the PLC hasn't come on, turn off the power, unplug and check you didn't hit the insulation in a connector block.)

You can now put the temp probe into the fridge. The reading should start dropping.

Follow the instructions and set the PLC to whatever temperature you think is about right.

I set the target temperature to 1.6 degrees, and set the compressor saver timer to 1 minute, as the fridge still does that anyway. (Make sure you hit save - on mine it wasn't clear it wasn't saving, so I had to consult the instructions to find you have to hit the power button to save!)

This means that when the temperature is below 1.6C the cooling relay will be open, and the heating relay will be closed. When above 1.6C, the first thing that happens is the heating relay opens, and a timer starts. 1 minute later (as set above) the cooling relay closes.

You may have to play with these settings to get it perfect. I found that 2.5C was a bit too high for the freezer section to work well.

Turn off the power and unplug again.

Now we will wire the rest.

It is pretty simple, and we are simply looping the power back. The live feed goes up to the relay on the blue wire, then the switched live returns on the brown wire, which we put into the next block connector. Tighten that up, and we are done on that side of the block.

On the other side, we now need to connect the fridge itself.

The fridge blue wire goes to the block with the blue wires from the mains, the earth goes to the earth, and the live feed to the fridge goes to the switched feed from the relay. Consult the picture and notes to be sure.

Check there are no stray bits of copper on your block connectors, and plug in, then turn on.

The PLC should light up again, and whether or not the fridge has power now will depend on what you have set the PLC to. You can test this by opening the door - did the light come on?

If the fridge is still below your target low temperature, simply pull the probe out and let it warm up. Once it reaches your target temperature, you will hear the heating relay close, and 1 minute later the cooling relay will close, and your fridge light should work. Pop it back in the fridge and the relay will switch again.

And that's that!

Trouble shooting

Unplug, check the wires are all correctly into the blocks and not tightened on plastic insulation.

If the fridge has power when it is too cold, but not when warm, you've wired the wrong relay up. Check that.

If it is too cold or too warm, adjust the PLC temperature settings.

Once you are happy with everything, take the choc box and put it around the block connector, add the strain relief, and job done.

Failing that, you can just use insulation tape. (Do something though, as the little screws are live mains voltage, so mice or little fingers or whatever...)

Put the fridge back.

Enjoy your chilled but not frozen beverages. :-)

Here's the finished result.

The PLC is on and in the delay time in the first shot, as neither relay indicator light is on. The second shot shows the 'cooling' light, so power to the fridge is on.
The system now works as it should - as a great fridge.

This fridge was running about 22 hours a day, & now runs for about 2.5, which is a big electric saving as well as saving this fridge freezer from landfill.

As for the freezer part? It is currently down at -10.8C and was at -27C earlier, so plenty cold. -10 is as warm as it gets, as it is about to start cooling again.