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Old 08-08-2020, 01:33 PM   #1
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Inexpensive Refrigerator Fire Prevention

My twenty year old Dometic RM1282 continues to work flawlessly on both gas and AC power. In spite of this, reports of RV refrigerator fires in recent years have become a concern. These fires occur because of damage to the refrigerator’s boiler tube due to overheating. Commercial devices are available that prevent this overheating but I’d never got around to pulling the trigger to order one. A few months back, a fellow iRV2’er posted a description of an inexpensive homemade device that could prevent the overheating problem. Here is a description of my installation of this device.

The device itself is described as a High Temperature Thermostat and it costs less than $20. This circuit board, with digital temperature readout, comes with a temperature probe and has an onboard relay that operates as an on/off switch for an external device (your refrigerator). This thermostat operates off 12 vdc and has a range of 30–999° Celsius. Operational parameters are easy to set.

After receiving the device, I familiarized myself with its setup using a cup of hot water and a cup of ice water. With my (cheap/free) Harbor Freight meter, I watched the relay switch open and closed as the temperature probe was moved from cup to cup. Convinced that I understood how everything worked, I mounted the thermostat in a small plastic box (recycled from a box of screws from Lowes).

Before the actual installation, I modified the supplied temperature probe for better results. The probe was made for a screw-in application so I removed the threaded part, with a cheap Harbor Freight file set, to end up with a small cylindrical probe. This process took about 5-10 minutes and provided exactly what I was looking for. I then fabricated a clip, from a hose clamp, to hold the probe against the refrigerator’s boiler tube.







For installation, I followed the same instructions found in videos for similar commercially made protection devices. Simply gain access to the boiler tube and mount the probe as high as possible. Best results are obtained if the probe is mounted above the indentations in the tube.





My refrigerator has a control board that operates on 12 vdc. If this 12 volts is removed, the refrigerator turns off. Return the 12 volts and the refrigerator turns back on. Actual testing proved that this happens if the refrigerator is in AC mode or Gas mode. If the supply line for this 12 volts is connected to the relay on this thermostat board, it will control the power to the refrigerator, turning it off and on.

After everything is in place, it was time to set the thermostat to control the refrigerator. To determine the normal operating temperature of the boiler tube, the thermostat was set for 300° Celsius. With the refrigerator operating on gas, the readout on the thermostat was observed for several hours. The next day, after a total cool down period, I tested again with the refrigerator operating on AC. In either case, the temperature never got above 182° Celsius. With this figure in mind, I reset the thermostat to cut off power if the temperature reached 185° Celsius and turn back on at 182° Celsius. I have monitored this on a couple outings and it seems that 182°C is my maximum operating temperature.

If you install this board, you must perform your own temperature test because every installation is going to produce different results. Contributing factors for this are the age of the refrigerator, its usage and the mounting position of the temperature probe. Another point I should make is that your off/on temperature settings will be maintained by the circuit board even if it is disconnected from its power source (such as turning off your salesman switch).

Because the thermostat board was so cheap, I bought a spare just to have an extra. Should the board fail, you can easily take it out of line by removing the wires from the relay and twist the ends together. This will return your refrigerator wiring back to its original state.

Does this inexpensive device provide the same safety as the commercial units? I don’t know. Am I happy with my investment, installation and feel more secure? I certainly do.

Feel free to PM me if you have questions but I prefer to reply via email. I usually read this forum with an Android tablet app which makes typing a reply slow.

Special thanks to iRV2 member nehog for his mention of this device and answering some of my questions.

Device name: Mini High Temperature Thermostat
Model number: 34687-MP
https://www.mpja.com/High-Temperatur...info/34687+MP/
Company: Marlin P. Jones & Assoc., inc
Web address: www.mpja.com
Price with shipping: < $20.00



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Old 08-08-2020, 02:12 PM   #2
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Thank you for posting... It's very well documented!

The 29 in the display is 29 C or 84 F.

Marlin P. Jones & Assoc., inc. / www.mpja.com has been around for years... I first purchased some ham radio parts from them in the late 1970s. It's a GOOD COMPANY.

For what it's worth... should you need metric conversions...
A very useful FREE conversion program for any Windows computer from Win95 to Win10 is here:
https: // joshmadison.com/convert-for-windows/

it handles all popular units of distance, temperature, volume, time, speed, mass, power, density, pressure, energy, and many others.

Mike
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Old 08-08-2020, 02:56 PM   #3
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Thanks for the complement. That picture was taken right after installation, it was 84 in the shop that day. I didn't think to take another picture after it reached operating temperature. MPJA does have a lot of neat stuff.
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Old 08-08-2020, 04:02 PM   #4
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Nice find! Ordered a couple. Nice write up.
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Old 08-08-2020, 06:16 PM   #5
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I replaced my Norcold with a 12 volt DC compressor fridge. Keeps the temperature more consistent than the Norcold good. Have plenty of solar and battery to run it. I only used my Norcold 2 years before I tired of it.
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Old 08-25-2020, 04:09 AM   #6
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W4KMA de K7SSC

What box did you use for the circuit board enclosure??
tnx
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Old 08-25-2020, 07:55 AM   #7
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Quote:
Originally Posted by BrigitteM View Post
W4KMA de K7SSC

What box did you use for the circuit board enclosure??
tnx
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Anything will do of course. Mine was from a box of Kreg "pocket screws", from Lowes. You could probably find a small food container at a dollar store that would work.
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Old 08-25-2020, 08:10 PM   #8
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Does a twenty year old control board have 2 AC relays? If it is a 3 way frig is there one more for DC?
AC relay points are known to arc. The surface can become like velcro. They can also weld shut. The AC heater can run away, even when 12v is removed.

To prevent that, 2 AC relays are used in series. One takes all of the abuse. The second is for safety. Removing the 12v will turn it off. I'm not sure when this safety feature was introduced. I bet Dinosaur control board replacements have it.
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Old 08-25-2020, 10:07 PM   #9
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Nice work! I run the Fridge Defend ARP unit, which does essentially the same thing with a few other features thrown in. I am very happy with it, but your solution is about one-tenth of the cost!
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Old 08-26-2020, 08:09 AM   #10
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very creative, thanks for posting!
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Old 09-12-2020, 03:17 PM   #11
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I added this unit to my 21 year old Dometic fridge. This is what my perk tube looks like .





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Old 09-12-2020, 06:09 PM   #12
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Quote:
Originally Posted by Pappion View Post
Does a twenty year old control board have 2 AC relays? If it is a 3 way frig is there one more for DC?
AC relay points are known to arc. The surface can become like velcro. They can also weld shut. The AC heater can run away, even when 12v is removed.

To prevent that, 2 AC relays are used in series. One takes all of the abuse. The second is for safety. Removing the 12v will turn it off. I'm not sure when this safety feature was introduced. I bet Dinosaur control board replacements have it.
YES 2 AC Relays (K1 & K2)
Yes if 3-Way
YES Dino Boards have 2 also
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Old 09-13-2020, 05:48 PM   #13
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WOW! Thanks for the tip! I used to buy from that company waaay back when I had an electronics business! I saved this for future needs!
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