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An electrical question..

by shibbs » 21 May 2016, 21:24

So, i have a question for you guys that know the wiggly stuff that is electrickery!
I do not., therefore i will not be doing the job, i just want to know if what i want is possible..
The fridge onboard Chin Chin was removed by the previous owner along with the whole gas system so i have just managed to find a cooler that i will put in its place. Whilst i know this won't be as good as a purpose built fridge for a boat, it was MUCH cheaper and will suffice for now. It will still fit in the space the fridge should be and the door will still close to conceal it so more than happy. I plan on getting some runners to mount it to so it will slide in and out.
It is a Waeco TC32, with 12v and 240v supply.
My question is, can this be wired up in any way so that when the boat is running it runs off the 12v supply and as soon as the shore power is plugged in it runs off the 240v without the need for unplugging etc?
Can this be done automatically or will it be a case of swapping leads each time?
Any on ehave any knowledge in this field, be good to hear if so?

Cheers all... :geek:
Stu

Draco 2500 Crystal - Mercruiser 5.7, Alpha one - Chin Chin
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by annageek » 22 May 2016, 21:44

We have the Waeco (branded Mobicool) compressor portable fridge thing (i.e. this: https://www.marinesuperstore.com/refrigeration/cool-boxes/waeco-fr35-portable-fridge-freezer which is really good, by the way) and I think I'm right in saying that it automatically favouurs the mains input over the 12V input so in theory, you can leave both connected and it'll sort its self out in the best possible way - could be wrong though.

Alternatively, you need a relay - the coil of the relay needs to be connected to mains, and the 12V positive (or negative) needs to be connected to the normally closed contacts of the relay. That way, when the mains becomes present, it will disconnect the 12V. When the mains disappears, the 12V comes back in. It's obviously important to pick a suitably rated relay, and to somehow make it all safe! Important things to look at when picking the relay are coil voltage, contact current, and contact breaking current at 12V DC (which will likely be lower than the steady contact current rating of the relay. This looks like it's be a good option http://www.rapidonline.com/finder-66-82-8-230-0000-230v-relay-dpdt-dc-30a-flange-mounting-66-82-60-4387 and has fast-on type spade connections, so will be easy to connect to. Annoyingly, the datasheet doesn't specify the DC load breaking current capacity of the normally closed (NC) contacts, as this is normally lower than the normally open contacts, but I'm expecting that your cooler draws around 4A, and I'd be fairly sure the contacts will handle that for the few hundred cycles it'll most likely see throughout it's life (and many more, most likely), as it'll only be switching 12V DC (voltage is a big factor when it comes to breaking current in any form of switch.

Simply mount this all in a plastic box project box with a couple of cable gland, and bob's your mother's brother. From a safety point of view, make sure that all cables are well restrained, and if there are any metal parts penetrating the plastic box, then you need to earth them.
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by shibbs » 23 May 2016, 05:55

Thanks very much for your detailed reply Anna, really useful, thanks for taking the time to explain.
Glad it sounds like it is achievable.
Stu

Draco 2500 Crystal - Mercruiser 5.7, Alpha one - Chin Chin
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