Better fridge performance

Thanks for the reply and information... Yes, I know about the power supply to the compressor being 24VAC..,. I learnt that on this form browsing past threads.

I searched EBay and found a 6-32 AC to 12 DC rectifyer and bought a few to do the conversion on the fridge in question and my 15 litre dedicated freezer....

Now, on wiring it (burger all instructions came with or was posted on eBay listing) I came up with a problem... (Before I go any further, I have basic understanding of electrics)...... You see, as I want the fans to cycle on of with the compressor, I thought it best to cut into the power supply going to the compressor and this is where I am searching my head. The rectifyer has 2 AC inputs and 2 DC outputs..... The AC inputs does not specify a positive and a negative/neutral position but the DC side clearly shows positive and negative..... On an Engel fridge, the compressor only has one wire/feed to the compressor, then is earthed by other wires.

So here is what I did and the result amazed me. Input port to the rectifyer was spliced into AC positive feed to the compressor. The other AC port was wired to earth.... The DC output was loose wired as per positive and negative ports. I fired up the fridge which in turn energised the rectifyer, then tested the DC output with a multimeter and got a surprise..... Yes, AC was converted to DC but not 12 volt, but rather 24volt..... The rectifyer was clearly marketed as 12v output so, thinking the rectifyer May have been faulty, I wired up a second rectifyer and had the same results....

What have I done wrong?..... and if it is what it is, a solution might be to buy off eBay 24VDC fans

Cobba, the only thing you got wrong was the purchase. That unit is simply a diode bridge (rectifier) and a filter cap, with a series resistor for the LED. It was only ever going to provide a 24ish output voltage.

To get one that supplies only 12V DC would require a voltage regulator on the DC side, and it would need to be rated for the likely fan load. Unless rated for output side short-circuits it would need a protective mechanism, and may need a heat sink to prevent it overheating. I'd be inclined to either get 24V fans, or go with RMD's approach (I'd personally add a couple of zener clamps when putting 12V fans in series, but I'm a bit like that).

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Cobba,

As Zippo said, that unit is a diode bridge. With the four diodes it will rectify the full-wave of the AC input and produce an output which is DC of about the same voltage as the input, 24 volts.

But there is a solution for you. If you remove two of the diodes ( the black cylinders) the remaining diodes will now produce only half-wave DC and that will be about 12 volt. The capacitor (the large cylinder with a silver top) will smooth the half-wave output to be suitable for the fan.

Now you need to remove the right diodes for this to work. Numbering from the top in your photo, remove number 2 and number 4. See the modified photo below.
The easiest way to remove them is to simply snip the connecting leads at each end of the diodes with side cutters.

Note that the output from the diode board will read higher than 12v without load but will drop to about 12v when the fan is connected.

Cheers for all the advice.

I have decided to forget about trying to achieve a 12 VDC output and accept the 24VDC..... That said, 24VDC computer fans are readily available on EBay so some were ordered last night.

More thought was spent into the effectiveness of fans running on a fridge in fridge mode only and I was thinking the gain in efficiency not being great. After all, for eg, in fridge mode (+4 degrees) the fridge may only cycle 6 times per hour and compressor/condenser would not get to a heat point requiring the fans for cool down.

On the other hand, the 15 litre would definitely benefit in freeze mode as the compressor would be running 90% of the time generating plenty of heat.

Just a thought ??

Well, i have just completed the same insulation mod to my 15 litre Engel (cream body/Olive lid) so decided to wire in a couple of 24 VDC computer fans.

I thought everything looked ok until i ran the fridge for a while, and after about 15 minutes running, could smell like something electrical was burning. I then felt the center area of the fans and they were very hot, and hotter than i would expect, so i shut the fridge down, and disconnected the bridge rectifier.

I am thinking maybe the rectifier is wired wrong, and after some research online am further confused.

The confusion is all about the AC input to the rectifier. Input to rectifier has 2 ports and i gather one is for positive and the other is for neutral.

Here is where i am confused. The Engel fridge only has one wire (positive) feeding to the compressor and an earth exiting the compressor. There is no Neutral......

Having said that, i have used the compressor feed as my positive into the rectifier, and the other AC port on the rectifier, have wired it to earth.

I have tested the DC output from the rectifier and am reading 25VDC, so thinking all is ok, wired 2x 24VDC fans , but then am finding the fans are over heating...

What have i done wrong........ ??

Its all starting to get too hard and am now thinking about aborting the whole idea of running fans

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Hi Cobba,

You ask ..."What have I done wrong?"
Well that's hard to say specifically, but I will try to identify some possibilities.

Firstly, the concept of deriving power for your additional fans(s) from the AC supply tenth compressor is not a good one. This supply is derived from an inverter device and will not necessarily have a nice sinusoidal waveform. It is far more likely to have a rough square-wave form and probably some spikes. The Engel "motor" is designed to be compatible with that waveform but your rectifier board and the connected fan is not. Clearly, if your additional fan is running hot then it is being supplied with too much energy. Your multimeter is calibrated to read correctly when connected to a sinusoidal waveform and will not read correctly with other waveforms, so cannot be relied upon to show the true voltage. So you may be subjecting your fan to a non-compatible supply. This situation can be corrected by using a suitably designed rectifier board but without seeing the actual waveform it is not possible to advise you how to do that.
Incidentally, although your rectifier board was described as "6-32 AC to 12 DC rectifier" it really is a rectifier which is rated to accept a voltage between 6 and 32 volts AC and produce an output DC voltage. However this output voltage will not be "12 DC" as incorrectly advertised unless the input was 12v AC. It will be whatever the input is, simply changed from AC to DC.

I believe that your Engle already has an original 24v DC fan. If you wish to add another fan why not simply connect the new 24v fan in parallel with the supply to the original fan? Be careful to connect with the correct polarity.... positive-to-positive and negative-to-negative. If the original wiring is not clearly marked, then use a voltmeter to identify which conductor is positive.
This is the method which I would have used in the first place. Connecting to the motor supply is fraught with problems and I would only do that if I had no other choice.
I attach below a typical Engel circuit diagram which may be of use to you.
From that you will see that there are two wires connecting to the compressor motor. As a pair, they are supplying the AC power to the motor. There is no "positive" supplying the motor and the wire that you describe as "earth" (probably because it is connected to the body of the motor) is the other wire of this AC pair. There is no such thing as "neutral" in this circuit but I will not attempt to describe that in detail or it may further confuse you.

So my recommendation is to forget connecting to the motor circuit. Forget rectifier boards. Just connect to the existing 24v DC fan circuit using a 24v DC rated fan. I'll also address the requirements of fan-driven cooling air in a separate reply.

Thanks for the detailed reply and electrical diagram Alan.

The Engel fridge I am working on is a 1980’s model and has not got any allowances for a fan ( never had a fan as original) so is why I cut my required power feed for fans into the compressor supply.

I am thinking now of aborting the idea and just leaving it “as is”

Thanks for your help

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OK Cobba, then there are two possibilities:

1) Preferred method. Connect a 12v DC fan to the incoming 12v from the battery and control that fan with a simple thermostat switch in contact with the condenser. Such a switch is the Jaycar ST-3831 which will close its contacts to operate the fan when the condenser temperature rises above 50 degrees C and stop the fan when the temperature falls below 40 degrees C. This thermostat is $5.75 and is simple to mount and connect. Attach closely to the condenser with clamp screws or cable tie.


Or 2) Try removing two diodes from your rectifier board as I earlier suggested. This is not so wacky as you may imagine. A half-wave rectifier is acceptable electrical practice.

Wow and thanks Alan.....

I am thinking option one..... It makes sense to govern the fan by when the evaporator heats up.

I am on to it......

Thanks for your help

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No worries Cobba, You have made a good decision.

To be sure of getting it right I have prepared a circuit diagram below.

Of course get the polarity of the fan right but the thermostat is just a switch..... it does not matter which way you connect it. It accepts push-on connectors or you can solder the wires to the tabs.

Be careful when buying the thermostat..... some are 'normally open' and some are 'normally closed'. You require the 'normally open'....i.e. it is open until the temperature rises. That is the Jaycar ST-3831.

Thanks again for the reply and wiring diagram Alan.

I have ordered the parts however, i didnt go with the Jaycar Thermostat switch you provided a link for because of its flat base and mounting options because of this would be limited.

After some investigation, have discovered heat is the greatest at the pipe that exits the compressor and the beginning of the condenser .... That pipe in question has an outside dia of about 6mm condenser...... And then thought, maybe a control switch that has a corded probe (similar to the ones we use to measure inside fridge cabinet temps.

I tested the efficiency of readings using one of my digital fridge cabinet thermometers where i tightly tapped and bound the probe hard against the side of the condenser pipe and was happy with how it performed. With the 15 litre running at full freeze mode, that pipe would heat up to low 50's in celcius all while ambient temperatures were mild mid 20's.

I found this that i think will be ok https://www.ebay.com.au/itm/263425415737

I am hoping this will work ok.....

Before i go, i will add some advice for those Engel owners that may want to take on this mod themselves.
Before proceeding, check the power supply for your fridge and make sure the power supply is not fully enclosed in a sheet metal box/case..... If it is, its near impossible to cut into the 12 volt supply feed to power your thermistat and fans.

I have 2x same model 15 liter Engels that were manufactured some years appart. during this time there must have been a design change. One of the models (later model) has the enclosed power supply that goes full length from where the power cords plug in, right up to where the AC transformer is. Impossible to access this case to cut into the 12 volt supply, unless of course the power supply is removed which would be a major task indeed..... The earlier model 15 i have here, thank god the power supply case finishes short of the plug base (where the 12 + 240 v cords plug in) so all the wires are exposed before the dissapear into the power suppliy case.......

I hope i have confused :-(

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On reflection, I really need to edit my response to this which began...."Yes Cobba, the temperature control device you referenced should work OK."
Particularly for the benefit of anyone else considering switching a cooling fan as a function of condenser temperature.

Cobba's proposal of an electronic digital thermostat to control the fan is very much an overkill, regardless of cost.

The simple bimetal thermostat switch (Jaycar ST-3831 shown above) is much more reliable and simple to install & wire than any electronic control device.

Despite having a flat base, its sensitive area is only 16mm dia and would readily sense the condenser temperature if mounted in contact to the tubing/wire grid of the condenser, preferably close to where the tubing from the compressor connects. This could be achieved with cable ties, wire, or more elegantly with a pair of 3mm screws to a small plate on the other side. It could even be strapped to the compressor housing effectively.

My whole engineering history has been to design for simplicity, not for drama.

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Exactly Peter.
All you have to do is push the tubes on the evaporator into the holes on the compressor and pour in a bit of refrigerant (available anywhere).
Bingo.... any fool can do it. lol

The compressor/condenser package actually comes pre-charged with quick connects that match those on the evaporators, so YES, you can just push the tubes together but NO, you don't need a "bit of refrigerant".
Cheers,
Peter
OKA196 motorhome

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I'm sure that Cobba will welcome that news.

Thanks for the reply Peter but the information dosent really help as a already have a cooling system that works. The whole idea behind what i am doing is to improve performance on an already working unit.

I am sure you will be rewarded for your efforts Cobba. Power consumption (and cooling capacity) is closely related to the quality of the insulation.
Cheers,
Peter
OKA196 motorhome

I'm with you Greg. Why would you bother?

I would think he is trying to cut down the run time of the engel, to save available amp/hrs from the battery, he may well accomplish that.

I have done similar with a small waeco that I run as a freezer. 25mm closed cell foam over the case and lid, it works well.

I tested the cycle time in 35c heat with and without the foam and it was a significant saving, can't remember exactly what, as I threw out my scribble notes.

Guess I did bother

Hi Allan, Your last point is exactly how the fridge box on my Kimberley Kamper was designed. It had a small extraction fan located on the side of the Fridge Box next the the front wall of the Kamper body. It was a manual switch, but you turned it on to ensure the heated air inside the box was exhausted to the outside.

Macca.

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That's good Macca. Even if the fan ran constantly, its current drain is not enough to embarrass the battery. Provided of course that the vehicle was in use and not garaged.
However, if there was concern about the battery, it would be possible to arrange a thermostatic switch as described above that sensed the hot air exiting the fridge and operated the external fan as it was needed.