Plus AND Minus 12v from AC to Thermostat

hiker108 wrote:
I have a Dometic A/C and a Dometic single zone thermostat. The thermostat requires plus AND minus 12 volts. The schematic of the A/C control box indicates that the plus AND minus 12v come from the power converter and are passed on, thru connectors, to both the A/C control box and also to the thermostat.
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Where does the minus 12v come out of the converter ?

If I unplug the plus and minus 12v connector to the A/C control box, the voltage at the thermostat drops out. Why?

HELP!

Thanks

I’m not sure if you are confused or just using the wrong terms. There is no minus 12V in an RV system. Minus 12V would be 12V below zero or 24V below your 12V output. This is different from the 12V negative (from battery negative) and 12 positive (from battery positive) leads that make up the RV 12V system.

The pic I have posted shows the external plus and minus 12v into this unit and same outgoing to thermstat. I suspect that schematic may be incorrect. It may be that this control unit generates the minus 12 volts for the thermostat. I will try to post pic of thermostat wiring.

The MINUS 12VDC is a 'Negative' from the NEGATIVE Terminal supplied by battery...either at converter Battery Negative or Battery Negative post.
NOT a 'Chassis Gound connection.

Wire bundle coming out of control box......
MINUS 12VDC is the BLACK wire
POS 12VDC is the RED Wire
Furnace are the TWO BLUE Wires
Load Shedding are the TWO YELLOW Wires (Used with a EMS---Energy Management System Control)

That's saying the + side of 12 vdc and the - side of 12 vdc.

hiker108 wrote:
Where does the minus 12v come out of the converter ?

The converter has a 12v+ and 12v- terminal same as any battery charger. The 12v+ connection goes through a main fuse to the 12v stack of branch fuses and to the battery. The 12v- goes to the frame and to the 12v- bus. Battery 12v- goes to the frame.

(generally)

hiker108 wrote:
If I unplug the plus and minus 12v connector to the A/C control box, the voltage at the thermostat drops out. Why?

Because the control box supplies the 12v (+ & -) to the thermostat.

hiker108 wrote:
The pic I have posted shows the external plus and minus 12v into this unit and same outgoing to thermstat. I suspect that schematic may be incorrect. It may be that this control unit generates the minus 12 volts for the thermostat. I will try to post pic of thermostat wiring.

12V - is not minus 12V.

That picture is not for a single zone box, it is for the CCC.

More confused than ever? It would help if you identified the models of the components involved and the problem you are trying to solve. Without knowing the components involved, any answers will be speculation and generalities.

I think its probably safe to say, what you are referring to as minus 12V is indeed 12V negative or 12V ground as others have suggested. Most things installed in an RV aren't fed directly from the converter, but +12V is fed from the load center. 12V ground is usually fed from a bus bar located in or near the load center.

As to your second question "why?", I believe you posted your own answer.

hiker08 wrote:
The schematic of the A/C control box indicates that the plus AND minus 12v come from the power converter and are passed on, thru connectors, to both the A/C control box and also to the thermostat.

When you disconnect the power feeding the A/C control box, you disconnect the power feeding the thermostat.