Plus AND Minus 12v from AC to Thermostat

[hiker108]

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.

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

[happy2rv]

Can't find any data on 3313379.000, but did find this link which I believe to be a similar Dometic single zone control kit / thermostat. These aren't your typical thermostat installations as many have already suggested. Which is why its important to have information on specifics.

The wiring diagram for this unit on the last page, assuming its similar to yours, shows the connections. Three wires to the thermostat from the control box. Red, black, and orange. The red is +12V from control box, the black is 12V ground or the negative terminal of the battery, and the orange is a data wire that carries all of the control signals. All three connect between the thermostat and control box. The black wire at control box also connects to 12V ground (negative). A separate wire on the control box connects to 12V from the converter or battery. This is the +12V supply line. The control box may or may not do something to switch the 12V its supplying to the thermostat, but it is supplying the 12V.

So, answering your original question again, when you disconnect 12V from the control box, you disconnect it from the t-stat. The red and black lines should have 12V on them and the orange won't necessarily have a readable or reliable voltage since it is a data wire.

[ChuckSteed]

Not quite correct. The 12 volts DC supply to the air conditioner electronic control box is directly wired from the power distribution panel DC bus in the RV directly to the air conditioner control module..

As long as battery is installed and cut off switch not engaged there is 12 volts DC power supplied to that control box on top regardless of whether you are plugged into shore power or not.. the converter has nothing to do with the DC supply going to that AC control module...

There is no voltage divider network doing what you are describing. The Thermostat...if it is LCZ or CCC2 gets its 12 volts DC power from on top of the AC unit via the control box.. it is routed down t9 thermostat using a 4 wire telephone cable

2 of those wires supply 12 volts DC and GROUND to thermostat unit. The other 2 wires are used as a BUS line to communicate signals from thermostat to control module on air conditioner on top of roof...

That’s the way the system works.

[hiker108]

Thanks all. I think I got it figured out. The control box in my Dometic A/C is a Dometic 3313379.000 When the 12 volts from the converter are plugged into this unit, I think there is a passive resister network that divides the 12 volts and sends it out to the thermostat as a -3.8 volts and a +8.2 volts. So the thermostat is happy with this.

[ChuckSteed]

The DC power and GROUND to your thermostat on this Dometic are supplied from 12 volts DC that is coming from the battery to the air conditioner control module.

If there is no 12 volts at the controller then the thermostat will be dead and the ECM for the air conditioner will not work.

Power is supplied in this setup from air conditioner control module to thermostat across a 4 wire phone cable .... however it is not wired at the RJ45 plug like a standard telephone cable.

The other two leads on the phone cable are for CANBUS communication back and forth from the thermostat to the controller.

Additionally most Dometic units also have the furnace being communicated with across this same telephone line and there will be a separate lead from Zone 1 air conditioner control module running down to the furnace control board.

[opnspaces]

Hiker, as happy2rv posted above we would like to help you solve whatever problem you're encountering, but it's not really clear what problem you are trying to solve.

If you merely seek a simple answer about minus 12 volt. The answer is that - 12 volt is simply ground on the RV. You can get that ground by running a wire to a ground wire of one of the 12 volt accessories. You could run a ground wire to the negative terminal of the converter. You can even run a wire straight back to the negative post of the battery.

if you need more concise help than thins it would be very helpful to us if you can give us a more concise explanation of what you are running into. Include any model numbers of components if available and even what wire colors are used at each end and the labels of the terminals they wires are connected to. Of course a few closeup and in focus pictures of the wire connections of your actual unit will work as well or better than you trying to describe everything,

[happy2rv]

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.

[Chris_Bryant]

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

[jkwilson]

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.

[time2roll]

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.

[time2roll]

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)

[Ozlander]

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

[Old-Biscuit]

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)

[hiker108]

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.

[hiker108]