Hello.
I have a caravan fitted with a Dometic RML 9930 absorpion refrigerator, which can be powered with mains,12v battery or bottled gas. . The fridge is rated 240V, 170W. We generally stay on fully serviced pitches and therefore power the fridge from the AC mains. When powered from 12v the fridge will only maintain the current temperature, making 12v power unsuitable for anything other than use whilst travelling.

I have an issue resulting from the variability of the AC mains voltage on sites (generally low and variable by as much as +/-20V too) and the resulting poor performance of the fridge. I have already added fans to provide forced airflow over the heat exchanger if the temperature inside the fridge is above the required set point (currently set to +3.9 degrees centigrade), which does help. However when we experienced 25C outside temperatures and a mains voltage of 215V on site, the fridge was not able to provide enough cooling. I find the constant adjustment and monitoring of the fridge to ensure the temperature is not too high (or indeed too low) quite stressful, impacting on my enjoyment of our caravan holidays.

So, I would like to implememt an automatic system to ensure the fridge maintains an acceptable temperature without my constant attention.
The requirement is for stability of the AC voltage to the fridge and to incorporate my additional forced heat exchanger airflow. It must also weigh less than 5kg (as there is limited load weight available in the caravan). The controller I use for the forced airflow could be used to switch the fridge on and off if it is in the required temperature range.

I have some electronics experience, but mainly with programming microprocessors interfaced with relays / Mosfets and digital sensors. I have done mains installations (under the supervision of an experienced electrician) too, so not a complete newbie to electronics.

My research to date has suggested this be implemented with a toroidal transformer with multi-tapped secondaries, using a bunch of triacs and a microprocessor to manage the switching of the secondaries depending on the input voltage, in order to provide the required 240V (the fridge does work best at 240v). I have also read about solid state variacs, but have not been able to find any DIY examples.

I would appreciate any comments on the above or suggestions for any alternative approaches.

Any help and suggestions will be very much appreciated.

Thank you.

 

In a way it's a hideous idea, but I don't see why it wouldn't work: get a 12V d.c. switching power supply to run off the nominally 240V main, and also a 12V to 240V inverter. The switching supply won't be fussy about its input voltage, so you can create 12V at around 15A, and then generate an accurate 240V from it. 170W isn't all that much power, so neither of these items will cost very much, and they shouldn't exceed your permitted weight. You'd get a bonus, too--you could run the inverter while you're on the road, and not power the fridge off 12V, so you'd get more cooling while you travel. At least, you can do that if your vehicle can supply 15 or so amps to the inverter!

 

I’d recommend you buy yourself a voltage stabiliser, these can typically handle input voltage variations of +15% whilst maintaining the output voltage better than a percent or so. You need to ensure that the unit can handle your power requirements, including the fridge compressor start inrush current.

Be aware that a number of these units are available from China, but their nominal output voltage is set at 220Vac.

 

It's an absorption refrigerator, so it doesn't have a compressor.

 

John P
I guess I have been over thinking the problem. I too can't see why your suggestion wouldn't work and I already have both items needed. I had a bit of a 'doh!' moment when I read your reply.

Hymie, I wish the van did have a compressor fridge. It wouldn't be able to run on gas, but that would (for me) be a VERY small price to pay to avoid the problems inherent the absorption fridge, which doesn't even have a thermostat as standard......

Thanks both, your help is much appreciated.

 

I wish the van did have a compressor fridge.

What would be the cost to replace it with a compression fridge?
Or don't they make them that small?

 

crutschow,
Sorry for the delay in replying. A replacement compressor fridge is well over £1,000. If I had that budget, I would rather spend it on an air conditioning unit.
I did try the power supply / inverter combination suggested by John P, however my inverter gives out 230V, which is too low, ideally I would like to run it at 245 volts - this seems to give it plenty of cooling capacity and is within the permitted UK mains voltage range (max permitted is 253V I think) and therefore should be well tolerated by the fridge. Even if doing this resulted in the service life of the heater being reduced, that would be a small price to pay for a fridge which just works.
I have considered just getting a small (500VA) variac and just 'dialling it in' to give the required voltage once on site - the issue then is the variability of the site supply voltage. If it were low when the variac was 'dialled in' and then rose, it could I suppose blow the heater.

If anyone has any idea whether an over voltage (say 270V) would do significant harm to the fridge heater (rather than just shorten its life), I would be very interested to hear.

 

I have a caravan fitted with a Dometic RML 9930 absorpion refrigerator, which can be powered with mains,12v battery or bottled gas.

Run it on gas?

 

I would appreciate any comments on the above or suggestions for any alternative approaches.

Use a 12V to 240VAC inverter and always run it from the inverter.

 

I thought I would provide an update. In the end I opted for a variable transformer. When I get on site, I just set it to give 245v output. I check it a few times a day and tweak the output voltage it if necessary. I would much prefer to have a voltage regulator in line so I always have 245v for the fridge. I could then set the output voltage on the transformer such that it is always above
245v and rely on the regulator. Does anyone know of an example circuit for an SCR AC voltage regulator where the output voltage could be set to 245V?

Thanks

 

Since the load is not that great:

The fridge is rated 240V, 170W

You may just want to but a DC to AC 220 VAC out inverter with a true sine wave out. a 12 VDC to 220 VAC 500 watt unit should not be all that expensive. Another option, as mentioned, is an AC line regulator/stabilizer. The latter may cost more. I assume when on outside mains power you are maintaining your 12 VDC power?

Ron

 

When powered from 12v the fridge will only maintain the current temperature, making 12v power unsuitable for anything other than use whilst travelling.

That's probably because it gets 14V whilst you are towing.
How about a 14V regulated power supply?
Is it like mine (which just turned 40 this year and is still going strong) which only has a thermostat on the gas?
Is there a good reason not to run it on gas?
Does the door seal need replacing?

 

An old school idea - - - Feroresonant Transformers. I happen to have several laying around. They take in 110 to 120 and give a good steady 130 out (give or take a little, haven't done any measurements on the bulk of them). I'm sure they make them (or did make them) for 240 VAC as well. Variations in the input line is compensated by the resonant frequency of the transformer. Though the sine wave is not totally sinusoidal, it's real close. I would imagine it should give you the needed regulation you're looking for. That is IF your conversion from 240VAC (respectively) to 12VDC back to 240 VAC.

Think I'll grab a few of these transformers and test them. Hook them up to a 12VAC transformer and take pictures of the resulting sine wave as well.

 

Initial test of my ferroresonant transformer is 116.7VAC in, 124.6 VAC out. Will grab a small 9V transformer and read the sine wave on the scope and post a pic. It's not a perfect sine wave, and I don't know off hand how a load will change it.

 

I also had an absorbtion refrigerator in my travel-trailer (caravan) and it too failed to cool enough on days when the power was low.
But since the power input drives a heater for the ammonia cycle cooling, and given that your unit has a twelve volt input option, what should work the very best is a switching mode DC power supply that will accept mains voltages over the wider range, perhaps 200 volts to 250 volts. Then set the output to 13.6 or even 14.0 volts and it should provide adequate cooling under all conditions, with no need to adjust anything. You will still need the extra air circulation, but that problem is already solved.
My ammonia cycle refrigerator failed after 20 years and the replacement unit cost was quoted at FAR more than I was willing to pay. So I bought a much less expensive small refrigerator that fit the same spot and installed it and that has served very well for the past ten years. For travelling we use an ice chest whikle in motion and the refrigerator while parked. Less convenient but adequate.

 

The resonant transformers regulate by saturating the iron core so that the output is constant. I have used a hundred of them in industrial machines and they do regulate and keep noise out very well. BUT they are big, heavy, and inefficient, always running quite hot. So they are totally not suitable for a caravan, where size and weight matter.

 

An old school idea - - - Feroresonant Transformers. I happen to have several laying around. They take in 110 to 120 and give a good steady 130 out (give or take a little, haven't done any measurements on the bulk of them). I'm sure they make them (or did make them) for 240 VAC as well. Variations in the input line is compensated by the resonant frequency of the transformer. Though the sine wave is not totally sinusoidal, it's real close. I would imagine it should give you the needed regulation you're looking for. That is IF your conversion from 240VAC (respectively) to 12VDC back to 240 VAC.

An ammonia absorption refrigerator is perfectly quiet.
A ferroresonant transformer. . . erm. . . . . isn't.

 

This is probably the simplest circuit, and will work fine if the voltage is low, but steady, only needing adjusting for each new location.
The transformer is 220V primary, 20V secondary @ 15VA, a few primary or secondary taps would allow fine adjustment. If

 

An ammonia absorption refrigerator is perfectly quiet.
A ferroresonant transformer. . . erm. . . . . isn't.

True. But the hum is insignificant.

 

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