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I am working on an antique car that has a 6 volt positive ground electrical system with the exception of the fuel gauge which is a 12 volt negative ground. Currently the car has a voltage converter that takes a 5-11v input and converts it to a steady 12 volt output. However, the -6v supplied by the cars electrical system results in an output of only 5.65 volts from the converter. If I supply +6v (external adjustable DC power supply) the converter will output the steady 12v. I have found a few voltage converters online that state they convert a 6 volt positive ground input into a 12v negative ground output. The first has only 3 wires, the -6v input from the cars electrical system, the +12v ouput and a ground to chassis. The sending unit for the fuel gauge (variable resistor attached to a float) has 2 connections, one back to the gauge and the other to ground.

My questions: Can the sending unit be grounded to the car's chassis which also has a 6 volt positive ground electrical system connected to it? Can the two dissimilar voltages and polarities exist with a common chassis ground?

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As you found out yourself, you need a inverting voltage converter for that.

-6V → +12V. They share the same ground level.

schematic

simulate this circuit – Schematic created using CircuitLab

And the answer to your questions is: yes, sure.

EDIT, as asked for in the comments:

There is nothing special about ground other than we define it to have a potential of 0V. It's just a reference potential. And voltage isn't more than the difference between the potential of two points in the circuit.

So, this -6V is a property of the in wire and this +12V is a property of the out wire. Ground does not see anything what's going on on other wires.

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    \$\begingroup\$ If you dont mind to explain; how can two dissimilar voltages and polarities operate on the same ground? There isnt in crosstalking of voltages? I appreciate your reply. \$\endgroup\$ – Jim Stevens Jul 8 '17 at 20:03
  • \$\begingroup\$ I've added an explanation to my answer. \$\endgroup\$ – Janka Jul 8 '17 at 20:59
  • \$\begingroup\$ Thank you for the reply. I always thought of voltage as running in a closed loop and thought the shared chassis ground would bridge these two closed systems. \$\endgroup\$ – Jim Stevens Jul 8 '17 at 21:08
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    \$\begingroup\$ Current runs in a closed loop. Voltage does not run at all. But you don't have to care about currents in your circuit, that is what the converter takes care of. \$\endgroup\$ – Janka Jul 8 '17 at 21:14
  • \$\begingroup\$ To expand on 0V is just an "arbitrary" defined value: say you two 9V batteries connected in series. Then you can have 0V & 9V & 18V or you can have -9V & 0V & 9V. Both of those are 100% identical from an electrical current perspective. You could even have -18V & -9V & 0V. So ground/0V is more or less "just" a defined reference point, although there most certainly are some reference points that make more sense than others, so it is not completely arbitrary. \$\endgroup\$ – hlovdal Jul 9 '17 at 16:16

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