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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?

Dmitry Grigoryev
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Jim Stevens
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1 Answers1

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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.

Janka
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    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. – Jim Stevens Jul 08 '17 at 20:03
  • I've added an explanation to my answer. – Janka Jul 08 '17 at 20:59
  • 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. – Jim Stevens Jul 08 '17 at 21:08
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    **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. – Janka Jul 08 '17 at 21:14
  • 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. – hlovdal Jul 09 '17 at 16:16
  • Thank you for the response. So I dont need to worry about current flowing in opposite directions on a common chassis ground? -6v from the cars electrical system and 12v from the fuel gauge voltage converter. – Jim Stevens Jul 09 '17 at 19:37
  • No, you don't have to worry. Currents running in opposite directions in the same conductor just cancel each other out (to a degree if they aren't the same absolute value). – Janka Jul 09 '17 at 19:40
  • So in a circuit that contains a power supply (lets say 12v) and a single light bulb and a switch to ground, does the ground wire on the back side of the switch carry any current or does it just supply a reference to ground for the circuit? Sorry I keep thinking of current flowing in a constant circle through this circuit. From the power supply through the load/bulb then the switch and back to the power supply. – Jim Stevens Jul 09 '17 at 20:26
  • Yes, sure, the current flows in a circle in that circuit. The ground wire carries a current. And because it's not a superconductor there is a very small voltage between the ground connector of the bulb and the ground connector of the switch. But this voltage is below 0.1V for the currents we are talking about, so for practical reasons, you can assume all parts of the ground are at the same potential. – Janka Jul 09 '17 at 21:03
  • I'm pretty sure you know the effects of ground failure in a car. That is when corrsion makes resistance of the ground wire at some point go up so high "ground" can't be assumed to be at the same potential everywhere. It's not a common reference any more. – Janka Jul 09 '17 at 21:06
  • I purchased an inverted voltage converter (-6v input - 12v+ ouput) and connected all of the grounds to the chassis and it worked perfectly. THank you Janka and Hlovdal. I really appreciate all of the input. – Jim Stevens Jul 13 '17 at 21:54