Electrical question

[Tim22]

I have an Adler Barbour cold machine that is connected through a Mobitronics mps50 rectifier which allows it to run off either 110 v shore power or the boats 12 volt power. The problem is that, when connected to the shore power, the rectifier puts out 24v DC but when using the battery house power it simply passes the 12 v through. The compressor seems happy with this arrangement but the cooling fan will only run on 12v. The cold machine provides for the fans to run on one voltage or the other but doesn’t switch between them. This leads me to wonder if it is possible to convert the 12 volt supply to the rectifier to 24 volts thereby allowing the rectifier to supply 24 volts regardless of whether shore power or the house battery was being used? If this is possible, is it a desirable solution.

Thanks for any suggestions

Tim

 

[Terry Cox]

Hi Tim. Unfamiliar with your arrangement as both our AB machines run off battery supplied 12VDC, whether connected to shore power or not. Unsure of the advantage of your setup. If it were my boat I would contact Dometic and talk with one of their technical people, describe to them your situation and seek their recommendations. https://www.dometic.com/en-us/us/pr...ooling-units/dometic-cu-adler-barbour-_-20734

 

[Ken Cross]

12 volts can certainly be converted to 24, but efficiency losses would likely make it impractical.
When under battery power we have to stay on an energy budget.

Ken

 

[jviss]

12 volts can certainly be converted to 24, but efficiency losses would likely make it impractical.
When under battery power we have to stay on an energy budget.

Ken

You can achieve efficiencies of over 95% with DC-DC converters. Here's one on Amazon for $18, handles 20A:
https://www.amazon.com/Nextrox-Converter-Regulator-Transformer-Waterproof/dp/B00BWKXTUU

 

[JimInPB]

I have an Adler Barbour cold machine that is connected through a Mobitronics mps50 rectifier which allows it to run off either 110 v shore power or the boats 12 volt power. The problem is that, when connected to the shore power, the rectifier puts out 24v DC but when using the battery house power it simply passes the 12 v through. ...

I was going to write about 4 paragraphs explaining how linear & switching DC power supplies work, but for now I think that I am just going to start by saying that if an AC to DC converter is not a regulated type, then the unloaded or under-loaded output can be at a much higher voltage than the nominal value.

Motor loads (like compressors) are pretty good about shrugging off the ill effects unfiltered & unregulated power. Other equipment may have a problem with it.

You might want to get somebody with an oscilloscope to take a look at what your "24v" power actually looks like. Something as simple as a regulator may turn it back into 12vdc & make your various systems play well together.

A rectifier, a DC power supply, & a DC to DC converter are all different pieces of equipment that do slightly different things.

These are general comments. I don't know your specific system.

 

[Tim22]

Thanks for the input.

The MPS 50 rectifier was recommended by Dometic for use with the Cold Machine so one would expect them to play nicely together! Anyway, I have ordered a 12v to 24v converter along with a 24v cooling fan so hopefully this will solve the problem.

Thanks
Tim

 

[jeepbluetj]

You'd use less power just regulating the 24v down to 12v for just the fan - or _always_ running the fan off of the battery power. May be simpler too.

 

[Tim22]

I would prefer to regulate down to 12v but it’s not so simple. The rectifier has two inputs and one output. One input is for 110v ac and when in use this outputs 24v. The second input accepts either 12v or 24v and when in use it simply passes through the input voltage. The rectifier automatically selects 110v if available. My current plan is to increase the battery input voltage from 12 to 24 volt so that the output will be 24v regardless of whether running on shore power or house battery.

Regulating down to 12 volt would need to be done on the output side of the rectifier and, as far as I can make out, would be ok when the compressor is running on 110v in 24v out. I’m not sure it would work on the 12v in 12v out as I presume it would try to modify the voltage. I am really an electrical neophyte so perhaps there is something that could be put on the rectifier output that would reduce the 24v to 12v when on shore power but leave the 12v at 12v when running on the house battery.

Thanks for any suggestions
Tim

 

[Ken Cross]

You'd use less power just regulating the 24v down to 12v for just the fan - or _always_ running the fan off of the battery power. May be simpler too.

Unless the voltage is reduced using a switching type regulator, regulating down to 12 volts will throw away 50% of the power you are taking out of the battery in the form of heat. Really bad on your energy budget. This is not speculation.
The down-side of the switching type regulator is the difficulty of elimination RF radio noise caused by the internal square wave. It can be filtered out, but no surprise if you got all kinds of electrical interference from it. It really makes more sense to not regulate down.

Ken

 

[jssailem]

Just wondering here... I do not have a cold plate or a compressor so the question may be out in the blue... You want to have 24 volt supply to the unit. When I needed a heavy amp supply for a windlass I put in a battery, separate from my house bank. What if you put 2 12 V batteries in series for your refrigerator? You would have a 24 volt system separate for the refrig. Then the chance of the refrig draining the house and starter battery would be eliminated. All you would need to solve is the charging of the battery.

 

[JimInPB]

I would prefer to regulate down to 12v but it’s not so simple. The rectifier has two inputs and one output. One input is for 110v ac and when in use this outputs 24v. ...

That's not a rectifier. That is a power supply. A rectifier changes ac to dc without filtering it or knocking down the voltage.

 

[jssailem]

Here is document discussing the way to connect a charger to a parallel and series battery package.

 

[Ken Cross]

Here is document discussing the way to connect a charger to a parallel and series battery package.

The chargers would need to have isolated common terminations. The automotive type units in the illustration would work as long as they are isolated from each other. Marine chargers (with multiple battery capability may not have that isolation. I'd ask the folks who market it before buying one. If they don't have that isolation, you just shorted out one of your batteries.
Ken

 

[jeepbluetj]

Unless the voltage is reduced using a switching type regulator, regulating down to 12 volts will throw away 50% of the power you are taking out of the battery in the form of heat. Really bad on your energy budget. This is not speculation.
The down-side of the switching type regulator is the difficulty of elimination RF radio noise caused by the internal square wave. It can be filtered out, but no surprise if you got all kinds of electrical interference from it. It really makes more sense to not regulate down.

Ken

1. Switching regs are dime a dozen anymore, and halfway decent filtering will take care of the RF noise. The one I've got on my autopilot project cost like $3 and has no noise at all. Was easier to just use a cheap premade reg board than design one. (ok, ok, I lie. I can see some noise on a scope, but there's a pretty good inductance with the lengthy wire to it, + some filter caps, + the fact it's not powering much but an arduino so it's 100mA or so.. no noise affects anything in the boat including VHFs and AIS. )

2. Just the fan - not the compressor. The compressor already handles 12 or 24v anyway - so the fan is the problem. Likely pretty low power at 12v - or you wouldn't use it on a boat anyway. So even a linear reg wouldn't be the end of the world - 'cause it'd be blowin power when shore power was connected, so who cares about power waste in that case.

Now that I think of it, if the compressor will run at 12 or 24, why won't the fan? I.e they sell that "rectifier" (which I'm reading as British for "Power Supply") to run this very fridge. So if the comp is dual voltage, why isn't the fan? I assume the control board is regulating power somewhere to both the comp and the fan. Hmmm... I may need to investigate it for my own curiosity. If there's a relay - always power from 12v would work fine here.

 

[Tim22]

Hi all and thanks for all the suggestions.

As an update I just received a reply from Dometic support with instructions as to how to rewire to allow the fan to run on either voltage. I will try it tomorrow.

Also, I received the 12v to 24v converter and a 24v fan today. I installed them and it resolves the problem. I would prefer the Dometic wiring change if it works but at least I now have a back up plan.

Tim