fridge charging electronics

[chuzzlewit]

I'd like my boat fridge to come on automatically when the engine is running. A voltage sensitive relay would work but costs £50. There are cheaper devices for a car to power a caravan fridge only when the car engine is running.
There must be a simple circuit one could make up to switch on a relay when the alternator is working or domestic voltage reaches 12.5V. Any ideas?

 

[john_morris_uk]

Some of the new fridge controllers auomatically sense when the batteries are being charged from any source (engine, mains power etc) and switch the fridge to a 'super setting'. Why not enquire with one of the boat refridgeration manufacturers?

 

[Guest]

Its not just a matter of one voltage level otherwise the relay will chatter. You have to have a delay or two switching levels. But even that can cause trouble with solar panels etc. The panel gets the battery over 12.5V but as the fridge starts the battery drops below and the relay opens. Not good for a fridge. This used to happen in the mornings with a commercial fridge control.

As for other circuits there is a W output of an alternator that can be rectified and used to sense the engine running. You can also use a slaved oil pressure switched sensor. The Ignition feed. It has a lot to do with how the engine is wired.

Do you want it to only work when the engine is on or to change its temperature setting when the engine is on. What happens in port when on shore power? To change its setting is little bit more complicated but the easiest way is to fit 2 mechanical bulb type temperature sensors and supply power to the correct one depending on the situation.

 

[Stemar]

A cheap & cheerful solution is to use an ordinary relay switched by a feed from the field connection on my alternator (the wire that goes to the charge warning light.) It's off until the alternator starts to charge. Cost about a fiver from halfords.

What it won't do is cut out if the output is there, but inadequate, which may happen when the engine's at idle. I don't know if this would be a problem for you, but it isn't for me using it for split charging.

 

[Albert Ross]

I wired up a relay so that the fridge is turned on when the light on the engine rev counter comes on. It means that the fridge is on whenever there is battery power to the engine (whether or not you've started it) but it's really simple and saves going up and down to turnmthe fridge on every time you start the engine .. wiring diagram in PBO last autumn.

 

[Ships_Cat]

Hi Chuzzelwit

Our freezer control panel has a two position switch one position of which only permits the freezer to run when the engine is running. It senses from the engine control panel key switch. This is quite a common arrangement and would seem easy to do something similar for any other charge source.

I have forgotten exactly how it is wired up now and realise this is not exactly what you are looking for, but if is of any use send me a PM and I'll trace it next time I am at the boat in a couple of days.

John