240/12V input selector

[Guest]

My fridge unit is powered by 12volts only, what I would like to do is put a black box that takes both a 240v and 12v feed (outputting 12v to the fridge) but selects the 240v supply (saving the batteries) if it is available. Does such a unit exist ?

Regards

Mike

 

[Guest]

Hi Mike

No

Regards
Brian

PS. You need to know the MAXIMUM start-up current of the compressor as any unit that is designed for you will require that MAX current to be it's constant running capacity. Electronically it is not a problem to design this unit - given that the designer knows the parameters that he has to work to .. easy peasy

regards again

Brian Moffat

Brian

 

[LadyInBed]

Re: 240 to 12V Device

Yes, its called a Battery Charger!

 

[VMALLOWS]

As Brian indicates achieving what you ask for is trivial for an Electrical Engineer...but probably expensive for you (it's the liability insurance, etc....:,).

I would go for NIGELB's suggestion. Assuming your charger can supply at least the normal running ampacity of the fridge (probably 3-5 amps) just leave it as a float charge on the battery at about 14v (13.8 if long term). Your battery will be kept fully charged at the same time. The start up surge will not be a problem because its taken by the battery and very quickly replaced.

 

[byron]

You my friend are in luck, I just happen to have a spare one, they cost almost £200 each., you can have it for £35. For others peoples info. these items are a fairly standard fit on larger Princess yachts that run a cocktail fridge and a large food fridge.
email me byrona@royal.net

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<P ID="edit"><FONT SIZE=-1>Edited by byron on Mon Mar 11 23:10:42 2002 (server time).</FONT></P>

 

[philip_stevens]

Well done Byron. Some people "just say no" without really knowing.

I also have a similar unit for constant power to a 12v user. It has 12 and 240 input, and if the 240 fails, the 12 is still available. Mine is rated at 6 amps and was made by Communication Aerials of Salisbury. Whether they are still in business is another matter.

Something like a UPS for computer stuff.

regards,
Philip

 

[ccscott49]

I'm afraid Brian always answers like that, see his answer on battery chargers, he thinks we are all congenital idiots! Still he does make some valid points now and again!

 

[ChrisP]

Pop in to the local electrical suppliers and buy a motor contactor with suitably rated 4 pole change over contacts and 240V coil and industrial mounting box.
Wire the 12V supply to the normaly closed contacts and the 240 to the normaly open contacts and the coil to the 240 volt supply.
When there is no 240V available the fridge will be fed through the N/C contacts. As soon as the shore supply is connected the coil will pull in the other set of contacts and the 240v supply will be connected to the fridge and the 12V supply will be disconected. QED auto change over. This will also work as an auto change over for a generator / shore supply. Thus protecting the genny from the nasty shore side volts.

Good luck and if you need more info send me a private message and I'll explain more.

ChrisP

 

[vyv_cox]

My Mobitronic battery charger is intended to operate 12 volt equipment "straight through". It can easily cope with my 12 volt fridge.

 

[byron]

Chris, the 'box' I have supplies 12 volts to the fridge(s) permenantly. If you are plugged into 240 volts it by-passes the batteries and supplies the 12 volts. The moment you take away the 240 volts it auto-switches and supplies the power from the batteries. It's a really cool bit of kit.

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[robp]

Surely this will poke 230V AC up a 12V DC fridge?

 

[ChrisP]

Thats why you use a 4 pole contactor. 2 for the 12V and 2 for the 240V. Not rocket science.

 

[ccscott49]

Byron, If this guy doesn't want it, I'll have it! Colin.

 

[byron]

First come, first served, if you want it Colin its yours. Problem is it is a weighty bit of kit and you will need come and collect it. Also while you are here plug it in and test it to satisfy yourself it is working (it is)
email me to make arrangements byrona@royal.net

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[andyball]

I agree with ChrisP.......I've used one of these as an "auto-changeover" switch as a battery back-up for an aquarium - sounds very different, but the requirements were the same : to switch back/forth between 240V AC and a 12V battery(+ inverter in this case) set-up.

It worked fine, and was very cheap.......a look in Maplin/RS for 240V relays/bases may also be useful.

 

[robp]

No I know it's not. I started life as an electronics engineer 30 years ago. Just the way it reads Chris.