Need the wiring diagram for the aux circuit feeding caravan fridge power while the engine is running.

Understanding Caravan and Tow CarUnderstanding Caravan and Tow CarIn the beginning.... The first and oldest electrical system on caravans, that was installed way back when most caravan's were no more than small garden sheds on wheels were the rear lights that replicated the rear lights of the towing vehicle, usually no more than two side lights and two brake lights.
Illumination for inside the caravan was provided by gas lights fed from a small gas cylinder usually clamped to the "A" frame of the caravan. It wasn't long before the lighting system needed to be upgraded, as direction indicators became mandatory on cars, additional wiring was needed to replicate indicators on the rear of the caravan. Skip forward a few years and a rear fog light became mandatory, so a further upgrade was made to the lighting electrical system to allow the rear fog light to be installed. The wiring for these lights was connected through a 7 pin plug to the car that is commonly known as a "12N" connection.Sometime in between the indicators and rear fog light changes, people decided they wanted to replace the gas lights inside the caravan with 12 volt lights that could be run from the car battery. People often used old car spotlights or other 12 volt lights that were found in cars of the time as interior lights and these soon drained the car battery, so an "upgrade" was to install a second battery in the caravan that could be used to provide power for lighting. These were usually old car batteries. It would be another few years before the dedicated "leisure battery" came along. Originally, you would have to take this battery out of the caravan and charge it at home using a standard car battery charger. Again, in a continual developing process, someone came up with the idea of being able to charge the second battery in the caravan from the tow car while driving along the road, or when on site, being able to run the engine of the car and using a long lead be able to plug the caravan into the car to charge the battery for the next nights use of the lights, So a second "supplemental" electrical system was born and the "12S" (S for supplemental as to differentiate it from the 12N or Normal) connection came into being.he 12 Volt systems
The 12 volt systems are split into two. The first, and as we have seen, the oldest is the caravan road lights, that is to say the lights that are required by law to have on all trailers. These lights consist of "tail" or "rear" lights - rear side lights (red), brake lights (red), direction indicator lights (orange), reversing lights (white) and fog lights (red). Due to the size of a caravan front marker lights (white) are required and from 2013 side marker lights (orange) on longer caravans.
12N & 12S Vehicle Wiring
These lights are connected to the car via a 7 pin "12N" type plug or by a more modern "continental" style 13 pin plug. The road lights are usually a complete system with all the supply and earth connections being separate from any other electrical system on the caravan. One of the most common faults with road lights is problems associated with the earth lead, but more of that later.
The only road lights that are not connected via the 12N connection are the caravan's reversing lights. The original 7 pin socket when it was developed was thought to have enough connections for everything anyone could ever want. Unfortunately as the years progressed, the number of "spare" connection on this plug diminished, to the point where there was no spare connection for the reversing lights. However, with the advent of the 12S, this was taken care of.The second 12 volt system is the supplemental system. This is designed so that when towing, you can charge the caravan's leisure battery, power the fridge and in some cases power an electrical brake system. The design of the supplemental system has to take into account a number of things:-
• It must be able to charge the caravan's leisure battery only when the car battery is fully charged and the engine is running.
• It must be able to power the caravan's fridge - but not allow the fridge to flatten the car battery if the engine is not running.
• It must be able to power the internal 12 volt electrical system of the caravan, but turn everything off when the engine is running.
• It must not interfere with the correct operation of the road lights of the caravan or towing vehicle.
OK, so lets look at the first one -
"It must be able to charge the caravan's leisure battery only when the car battery is fully charged and the engine is running"roblems with this type of connection. If the car battery is flat and you try to start the car, a heavy current will be drawn from the caravan's leisure battery and would cause the caravan wiring to over heat and possibly cause a fire. It would also damage the plug and socket connecting the caravan's wiring to the car, as they are not designed to take the high currents involved in starting a car engine. So how can we do this safely?