The old Refrigerator not operating on AC Trick

Hampton

Hampton

Air Defense Dept
TECHNICAL Contributor
Nov 26, 2006
7,628
Panama City, Fl
Boat Info
2008 44 Sedan Bridge
Engines
Cummins QSC-500's
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As I took the boat apart today, I noticed that the refer wasn't working on AC in the salon. I plugged another accessory into the outlet and it worked fine. I studied the electrical guts, and this is how it works: AC and DC both into a black box. Only DC out.

My refer is a German made Danfoss. The black box accepts 100 - 240V in (AC), 12/24V in (DC), and pukes out a max of 25V.

Some numbers: Danfoss BD 35 Compressor
101N0220 low EMI Electronic Unit

http://www.1stflash.com/files/Danfoss-Cooling.htm#Danfoss-Electronic-Controls

Looking for the black box with AC and DC in, DC out.
 
On some units the switchover happens with a simple relay with a 110 volt ac coil. When dockside power is connected the relay is energized closing a normally open set of contacts and allows 110 volts ac to operate the compressor. Disconnect shorepower and you de-energize the relay allowing 12 volts dc to flow through the normally closed contacts.
Whenever the refrigerator refuses to operate in the 110v mode the culprit is almost always this relay.
Do you have a copy of the wiring schematic for your unit?
 
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I'll have to get it from the boat later today. I have 12 volt power wired to a round tube-shaped object on the wall of the boat. That power leads into a black box marked 12/24V input. Into the same box is a standard 110V AC power cord plugged into a receptacle also on the wall (inside cabinets). It is labeled 100 - 240V input. On the other side of the box, there is 12/24V out which goes to the rest of the frig's electrics.
 
It's like Republicans in Congress - they don't work when they're in a black box either. :grin:
 
Labbur said:
replace the small in line ac fuse. mine did ths several times. Twist out and a small 3 amp fuse
Click to expand...

I didn't see an inline fuse. The black box had a regular AC cord and plug (like any applicance) coming out of it. Other than that, it was DC in and DC out. Maybe on mine there either isn't one or it's inside the black box. By the way, the box is velcroed on so well I didn't pry it loose for fear of breaking it. Was yours like that?
 
Need help finding a power supply unit for Dometic RPD-110. This unit takes 110AC and 12V DC and puts out 24V DC to the fridge. Any ideas?
 
Talked to a couple of Dometic guys. Once I got the number, it was easy to find all over the internet. MPS-35.

How long could this have been going on? I don't ever recall verifying AC power to the fridges. I guess it just means that the battery charger took on the load when AC was available. Not the end of the world unless it kept the batteries from being properly charged and/or cycled, or unless it did damage to the charger. It's not much power, though.
 
Hampton said:
Talked to a couple of Dometic guys. Once I got the number, it was easy to find all over the internet. MPS-35.

How long could this have been going on? I don't ever recall verifying AC power to the fridges. I guess it just means that the battery charger took on the load when AC was available. Not the end of the world unless it kept the batteries from being properly charged and/or cycled, or unless it did damage to the charger. It's not much power, though.
Click to expand...

Unknown as to how long it has been going on. FWIW, the fridge on Saint Max has an indicator light to tell you it is on. When it is on. I can tell a difference in the brightness of the light when going from 12VDC to 120VAC. The compressor sounds a bit louder as well.

That may be a simple test for you for the future.
 
dpmulvey said:
Unknown as to how long it has been going on. FWIW, the fridge on Saint Max has an indicator light to tell you it is on. When it is on. I can tell a difference in the brightness of the light when going from 12VDC to 120VAC. The compressor sounds a bit louder as well.

That may be a simple test for you for the future.
Click to expand...

That sounds just like mine. I've always kind of known that, but can't remember the last time I saw the light on bright, if ever.

Is this the same with LCD screens? LCD screens run on 12V DC, so when I bought my DC TV, I expected it to be as bright as any, but it isn't. So, does a 12V DC LCD screen run brighter when actually connected to 120V AC and then converted to 12V DC? Like the fridge light gets brighter and the compressor runs lounder, does the LCD screen shine brighter?
 
About $65 delivered. 100 - 240 V AC in, 12 - 24 V DC in, 24V DC out. Should be plug 'n play.

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Old power unit - AC not working, so replaced with new power unit.

Should have been plug 'n play, but wasn't. Almost called a refrig tech for a house call, but finally got it working. Here's how/why:

Pulled fridge, disconnected old power unit.
Connected new power unit - required many new fasteners - even ends with connectors didn't match.
Tested fridge - worked well in both AC and DC. Turned power back off.
Removed old power unit - held in with HEAVY velcro glued on both unit and fridge.
Glued velcro onto new unit and put in place.
Disconnected wiring connectors to run smartly behind accessories on fridge back.
Reconnected wiring and started to put fridge back in slot.
One more test - DC on - Lights go dim, tv loses power, then all comes back on - EXCEPT FRIDGE! AC test - nothing on fridge. Rest of boat fine.
Take it back out. Remove new power unit. Re-wire old power unit (DC had been working on it). Nothing. I thought I had fried the fridge's (expensive) control unit.
Broke out voltmeter. DC Power good to capacitor (where fridge gets DC). Capacitor held a good charge. Used a light bulb to drain capacitance. AC good too (used hairdryer to test outlet).
DC out from power unit good at 12.4V.
Wired new power unit back on. 12.4V out on DC and 28V out on AC.
Started trouble shooting. Hot wired power unit to fridge controller - it worked!
I HAD A BROKEN WIRE CONNECTOR BETWEEN THE POWER UNIT AND FRIDGE CONTROLLER! : (
Replaced connector. Re-mounted new power unit - re-glued sticky, messy velcro. Zip-tied excess wiring. Tested good!!!!!!!!!!!!!!!!
Re-installed fridge. Cleaned up. (This is where all of the spilled coffee goes.)
Time from voltmeter discovery to finished - 1/2 hour.
Time spent freaking out - 1 hour +.

Lessons learned - When capacitor is drained, expect a large draw on your DC when you first apply power. I thought I had fried the fridge, but I guess it was just re-charging the capacitor. The broken connector was a coincidence - not associated with the power draw.

Also, break out the voltmeter earlier in the process - it would have saved me changing out the new power unit for the old one and back again.

Now, for $65 and a few hours labor, the little light comes on bright in the fridge when AC is available (oh, and the compressor runs in high gear for better, quicker recovery).
 
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Hey John,

Two questions:
1. How dirty were your coils when you pulled out the fridge? Wondering if I need to pull mine to clean coils...
2. What are the steps to pull it out? Any traps? Or straight forward?
 
I was truly surprised how clean it all was. There was a very light sprinkling of dust on surfaces. The coils were clean. The boat walls had a lot of coffee on them.
 
I've got the exact same behaviour in my fridge. 12V good, 120V no good.

I'll r/r the power supply in May.

Yes, I said in May.

Damn endless winter!

Thanks for the info, John.

Did I mention May???
 
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