Fridge Temp Control Question

[BFL13]

I have a Dometic S630 absorption fridge from 1991. AFAIK newer ones work the same way. Apparently the freezer cools down and then eventually the fridge cools down to its proper temp range just above freezing.

This works ok when on propane. My 120v heater is irreparable.

When I use my jury-rigged AC method powering the 12v heater with 12v (15.2v actually to get the watts needed) from a converter on its 120v input, I get 200w instead of the 300w that the 120v heater would do.

With colder ambient outside, this 200w is enough to get the freezer cold enough, and the fridge part too, but now in summer, the fridge part only gets down to about 9C, which is too high for the job. Have to go to propane to get the fridge part down to its proper range.

Is there any way to increase the amount of cooling from the freezer part down to the fridge part? ( I have the eyebrow setting on "coldest" already) If there is, I would likely need to put it back the other way for when on propane so the fridge does not freeze, but one step at a time!

Thanks.

[Chris_Bryant]

One thing to keep in mind is that is basically a Norcold- Dometic built cooling units for Norcold for a while and in the deal they marketed the Unit with the server name.

[wnjj]

BFL13 wrote:
I think I got that this time! Have you a link to such a relay with illustrations for the terminals you describe--"common", N.O. pins, etc? Thanks!

I have ordered a 120v element but it is 325w instead of 300w in hopes I can make it work from the 120v output from the board. The specs for the 300 and 325 in amps and ohms are not much different.

The 120V option will be your best bet. If you still want to try the relay hack, Google for lighting relay diagrams since they’re basically doing something similar.

On automotive style relays, pin 85 and 86 are the coil , 30 is common and 87 is normally open. https://www.the12volt.com/relays/spdt-and-spst-automotive-relays.asp

[BFL13]

pianotuna wrote:
BFL13,

325 watts is what mine draws. I hope you will be pleased with the results.

The Dometic in my ride is a 6 cubic foot.

I could not find the 300w rated part for mine on Amazon, but figure this 325w rated part will be close enough. The rated 325w must vary in real life anyway depending on what the "120v" is at.

Doug, you are right, but it was worth it for me to do the 120v powered 12v trick. It worked ok in winter ambients where the fridge part was already almost in the proper temp range, but not now in summer. Can't get it down past 9C/48F and moving the thermistor did nothing as was predicted. the freezer part is cold enough though. I need to drill a hole in the freezer floor to let some cold down into the fridge! (Not!)

[pianotuna]

BFL13,

325 watts is what mine draws. I hope you will be pleased with the results.

The Dometic in my ride is a 6 cubic foot.

[dougrainer]

pianotuna wrote:
34 amps @ nominally 12 volts. Mine operated just fine on a 35 amp PD converter with wizard. It was not a "planned" test, but rather happened by accident.

The alternator doesn't seem to power the fridge well at all in 12 volt mode.

dougrainer wrote:
2. Even if you have a 3 way refer, operating on 12 volts is ONLY for when the unit is in Transit and your Chassis engine Alternator is supplying the needed voltage/amps to power the 12 volt element. It is NOT designed to operate on a Power Converter, as it takes a lot of power to run that 12 volt element.

I should have clarified. 4 Cubic Foot and smaller RV refers will usually operate fine on 12 volts. 6CF and larger are NOT designed to run on 12 volts unless the Truck Alternator is engaged. 6 CF and larger, the 12 volt element is a MAINTAINER, not a cooling mode Heat element. Using a Inverter/Charger or a Power Converter to run a 6CF and larger on 12 volts is foolish and a detriment to a Power Converter. Why put the strain on those chargers when you can operate on 120 or LP? Doug

[BFL13]

wnjj wrote:

BFL13 wrote:

wnjj wrote:

BFL13 wrote:
I don't know if the terminals for the 12v element on the board, when seeing the 12v would "back feed" the board, same as if the board were getting power from its regular 12v inputs.

Yes, it would likely back feed into your whole RV 12V system. If you do want to go this route, buy a 12V relay and use the wires heading to the element to power the coil and then use whatever voltage the 12V element can take through the switches terminals to the element.

Still not clear on this. You have the relay being operated by the temp control switching the 12v element terminals? So you still need regular 12v into the board at its 12v input terminals.

Now the 16 or whatever voltage supplied externally to the element is being switched by the relay.

How do you get say 13v to the relay from the board's 12v element supply terminals, while the relay is switching say 16v? Two different voltages. I don't understand how this relay works so it can do that.

Basically, the relay coil “replaces” the heating element as far as the control board is concerned. Instead of switching the element on, it activates the relay coil. Then the control board can continue to operate at 12V. The heating element is connected from a separate supply (e.g. 15V) through the relay common and N.O. pins to the element. Connect the supply to the “common” terminal on the relay and the element to the NO terminal, with the other side of the element grounded. If you figure out which is already ground, you can just leave the element grounded at the control board connection and add the coil ground to it.

I think I got that this time! Have you a link to such a relay with illustrations for the terminals you describe--"common", N.O. pins, etc? Thanks!

I have ordered a 120v element but it is 325w instead of 300w in hopes I can make it work from the 120v output from the board. The specs for the 300 and 325 in amps and ohms are not much different.

[wnjj]

BFL13 wrote:

wnjj wrote:

BFL13 wrote:
I don't know if the terminals for the 12v element on the board, when seeing the 12v would "back feed" the board, same as if the board were getting power from its regular 12v inputs.

Yes, it would likely back feed into your whole RV 12V system. If you do want to go this route, buy a 12V relay and use the wires heading to the element to power the coil and then use whatever voltage the 12V element can take through the switches terminals to the element.

Still not clear on this. You have the relay being operated by the temp control switching the 12v element terminals? So you still need regular 12v into the board at its 12v input terminals.

Now the 16 or whatever voltage supplied externally to the element is being switched by the relay.

How do you get say 13v to the relay from the board's 12v element supply terminals, while the relay is switching say 16v? Two different voltages. I don't understand how this relay works so it can do that.

Basically, the relay coil “replaces” the heating element as far as the control board is concerned. Instead of switching the element on, it activates the relay coil. Then the control board can continue to operate at 12V. The heating element is connected from a separate supply (e.g. 15V) through the relay common and N.O. pins to the element. Connect the supply to the “common” terminal on the relay and the element to the NO terminal, with the other side of the element grounded. If you figure out which is already ground, you can just leave the element grounded at the control board connection and add the coil ground to it.

[BFL13]

Chris Bryant wrote:
Boy, that does look smaller diameter...

EDIT --It might just look thinner by being behind the other one, hard to tell. 300w vs 225w--would the 300 be fatter?

This is the 12v one:

https://www.ebay.ca/itm/Refrigerator-Heating-Element-Dometic-2943467-007-Direct-2-U-SKU-2910/1637226...

Says it is 225w and 18.75 volts. Howinheck can it get 18.75 volts? EDIT--the table linked below shows that the 18.75 is amps, not volts as shown on the eBay link.

The 120v element is part # 2943469.003 at 300w. Listed here, but can't find one right off using that number. They do change the numbers. Lots of those 325w ones, but don't know if I can use one. It would be clamped onto the pipe, not sitting in its sleeve.

http://techsupport.pdxrvwholesale.com/wp-content/uploads/2019/01/Dometic-Refrigerator-Technical-Data...

[Chris_Bryant]

Boy, that does look smaller diameter...

[grizzzman]

You could use a boost converter to feed the 16 volt side. Or a buck converter for to the board power.

[BFL13]

wnjj wrote:

BFL13 wrote:
I don't know if the terminals for the 12v element on the board, when seeing the 12v would "back feed" the board, same as if the board were getting power from its regular 12v inputs.

Yes, it would likely back feed into your whole RV 12V system. If you do want to go this route, buy a 12V relay and use the wires heading to the element to power the coil and then use whatever voltage the 12V element can take through the switches terminals to the element.

Still not clear on this. You have the relay being operated by the temp control switching the 12v element terminals? So you still need regular 12v into the board at its 12v input terminals.

Now the 16 or whatever voltage supplied externally to the element is being switched by the relay.

How do you get say 13v to the relay from the board's 12v element supply terminals, while the relay is switching say 16v? Two different voltages. I don't understand how this relay works so it can do that.

[BFL13]

It is all about the watts for the 12v element. At 100w it does not cool down the fridge, but at 200w it does. The 120v element was pulling 300w, so it works great.

EDIT--to be more accurate, those 100w and 200w measurements were not at the element but were what the converter was pulling according to the Kill-A-Watt. Jack up the converter's adjustable voltage and get more watts.

Getting enough voltage out from the 12v element's board terminals is the problem, with all the voltage drop in the board, from chassis connection block to the board, and from the battery to the chassis block.

I have 14.7v out to the element, but the thin pos wire out from the board to the fuse and on to the element is very warm/hot. Some wasted power there, but needs to connect to the board terminal so can't go big on the wire really.

[pianotuna]

34 amps @ nominally 12 volts. Mine operated just fine on a 35 amp PD converter with wizard. It was not a "planned" test, but rather happened by accident.

The alternator doesn't seem to power the fridge well at all in 12 volt mode.

dougrainer wrote:
2. Even if you have a 3 way refer, operating on 12 volts is ONLY for when the unit is in Transit and your Chassis engine Alternator is supplying the needed voltage/amps to power the 12 volt element. It is NOT designed to operate on a Power Converter, as it takes a lot of power to run that 12 volt element.

[BFL13]

Here is the photo from the old thread on this. Tinypic is closing, so fingers crossed--



Since then, I have broken off most of the 120v element so there would be room for clamping a new one on as suggested above. Not sure I can get the 12v one out, but also it seems to be thinner, so not sure the fatter 120v would fit in the 12v sleeve. I will check that all out when I get the new 120v element.

I don't mind using the shore power to run the converter to supply the 12v element for a day or two before a trip. It is all propane once we head out and go off-grid.