Hi,
My question is can you use a common gnd/negative if using multiple 12v supplies on a vehicle? I bit of background on the use case... I have a Motorhome that has a 12v DC system and a 240v AC system (when connected on site) and I am re wiring the control system for the heating and hot water as it is 20 year old and no longer works. The van heating can be run from the onboard 12v battery DC systems and also the 240v main hook up. It uses different heat sources for each so no problem there, however there is only a single 12v air circulation/ blower fan that need to be activated whichever heat source is used. I think I could wire up a separate switch to always drive the fan from the vehicle 12v system but I can't have a situation where the 240v heater is turned on without the blower as the element could quickly overheat. So my solution is to connect a 240v-12V transformer to the same switch that provides 240v to the heating element and then use the output of that transformer to provide 12v to the blower, suppling 12v to it automatically whenever the 240 element is switched on . The 2 +ves from the vehicle and transformer would run though a relay so only one can be connected to the motor at the same time. However is it okay to just connect the transformer 12v negative to the shared vehicle ground? Hope the question make sense. Thanks.

Here is a snippet from the proposed circuit diagram :



I guess there could be a situation where the relay switches to powering the fan from the vehicle while the transformer is still on and would this matter?
Thanks

 

Is the output of the "Transformer" 12Vac or 12Vdc?

 

You can connect both DC commons but do not power at the same time if in parallel

 

Is the output of the "Transformer" 12Vac or 12Vdc?

12Vdc. Thanks

 

You can connect both DC commons but do not power at the same time if in parallel

Thanks, I can see a situation where the 240v side of the transformer is powered on while the 12v dc load on it is not connected to the blower motor (the 5 pin relay NO/NC will stop them both being connected at the same time) however the common 12v dc gnd will be connected all the time. Is that an issue please?

 

Keep in mind that blower motors can draw a lot of current, >20 amps, depending on the speed -- check the blower fuse rating to get an idea of what to expect. This will determine the ratings of your relay contacts and the AC/DC power supply.

 

My comment is about the safety aspect! If the fan's DC power source fails while the heater is powered, you have a dangerous condition within a few seconds. So I suggest an air flow switch in series with the heater element power control, AND an over-temperature sensor thermostat in series with the heater element.
If the RV has a standard "transformer" DC supply, it actually is an isolated DC supply with adequate overload protection. BUT, if what you have is in fact actually ONLY a transformer, then you will need to add a rectifier. Power supplies often get called"transformers because people have no clue as to what they really conist of.
Sharing a common side among a number of power sources is common and if the supplies are all isolated from the mains there will be no problems at all.