Thanks Graham.

What I am thinking is using a double pole double throw switch (15amp) switch.

If I run the output from the transformer into one side of the switch and the output from the cigarette lighter socket into the other side of the switch, I would the be able to select either output to supply the fridge.

Does this sense, or is it overly simplistic??

Thanks, KB

Do you have a battery charger ? I leave mine on 12v ( house batteries) then when I hook up 240v. The battery charger takes care of business .. I extended the 12v lead using heavier wire so fridge can be moved .. Much safer than playing with 240.. 

greyhoundtom wrote:

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Sorry to stick my nose in here and as readers of my posts would have gathered from my naive questions I have very little expertise in these maters.

However the first idea that popped into my head when I read the question asked, was to simply leave the fridge plugged into the tug battery. To then both ensure that the tug battery is always fully charged regardless of the circumstances as well as keeping the fridge running, to permanently wire up up a battery charger to the tug battery under the bonnet and then plug that into any available 240 power supply overnight.

Yes I know stupid idea but sometimes I just can't help myself particularly when I've had a nice drop of port after dinner. Lol

Cheers, Tom

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 HI Tom

In no way a stupid idea

In fact a very safe way to do it 

NO PLAYING around with 240 V[ILLEGALLY] & mixing voltages on the same switch

No 240V leads going through doors or windowswhere they are very likely to get damaged & liven up the WHOLE vehicle !!

PeterQ

See http://www.australiandirect.com.au/assets/files/FMguide.pdf

AFAICT, the "transformer" the OP is referring to is in fact an AC-to-DC adapter, so the usual cautions/lectures are misplaced.

In fact the idea of a DPDT (15amp) switch is not too bad, although I expect that the switch may not last too long. Instead I would use a DPDT relay.

         _
        | |
        | o
        | (  1A fuse
        |  )
        | o
        | |
        | |___
        | |   )||
 1N4001 | V   )|| Relay
  diode | -   )|| coil
        | |___)||
        | |
  DC+ o-)-+--o
        |    __--o-----o  V+
 CIG+ o-)----o
        |                     to refrigerator
  DC- o-+----o
             __--o-----o  V-
 CIG- o------o

            DPDT
            relay
           contacts
        (relay not energised)

Connect the cigarette lighter output (CIG+/CIG-) to the normally closed terminals (ie those that are on when the relay coil is not energised).

Connect the adapter output (DC+/DC-) to the normally open terminals, and power the relay coil from the same source. This will ensure that the relay automatically switches over to the adapter whenever 240V is present. You would also need a 1A diode for back-EMF protection. An additional fuse would provide reverse polarity protection in case the wrong adapter were used.

Ideally the cigarette lighter output should only be active when the engine is running (not cranking), so the refrigerator would not be draining the battery when the alternator is not charging it.

The above setup will ensure that you never need to disconnect anything, and the AC/DC switchover will be automatic.

Edit: The OP could add a 1K resistor and LED across the relay coil to indicate when the refrigerator is running off 240V.

grahamg wrote:

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thanks for your concern oldtrack123, I have no concern for the engel fridge is it designed this way.

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HI Graham

 I have NO concerns with your set up  perhaps you misunderstood my post 
Yes, Engels do have that as an automatic energy source selection[INBUILT}

Have done for something like 30plus years

PeterQ

The fuse shown in dorians diagram is in the relay coil pos wire, and the diode is reverse polarity across the coil, to protect the coil supply circuit from voltage spikes caused by the back EMF of the relay coil when voltage is removed from the coil.

You can get relays with suppression diodes (or resistors) internally fitted.

Baz421 wrote:

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Mobitronics have a AC/DC rectifier available. I have 2 (both used for years 1 for 12 years and one for 8 years) for use in van for 12v fridge and for Waeco 18l portable fridge.

Plug and play. IF plugged into 240v you use that supply and if it fails or you change manually you use 12V DC.

So simple.

---

I notice that the MPS-50A has a 6A continuous rating. Is that enough? Plus it costs a bit more than a switch ($170).

https://www.getawaeco.com.au/files/Manual_Coolpower_MPS_50A.pdf

dorian wrote:

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Baz421 wrote:

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Mobitronics have a AC/DC rectifier available. I have 2 (both used for years 1 for 12 years and one for 8 years) for use in van for 12v fridge and for Waeco 18l portable fridge.

Plug and play. IF plugged into 240v you use that supply and if it fails or you change manually you use 12V DC.

So simple.

---

I notice that the MPS-50A has a 6A continuous rating. Is that enough? Plus it costs a bit more than a switch ($170).

https://www.getawaeco.com.au/files/Manual_Coolpower_MPS_50A.pdf

---

Never had a problem with current ie overload on either.

Costs ye,s but if hardwired (options are cig lighter output OR  hardwired and we use one of each) nothing to do but turn on 240V switch which I think was the OP question ie keep it simple.