# neg earth stereo in positive earth car ?

Discussion in 'General Electronics Discussion' started by krawczuk, Aug 11, 2014.

1. ### krawczuk

10
0
Mar 20, 2011
hi, just came across this : http://mgaguru.com/mgtech/electric/et207.htm

about putting a neg earth radio in pos earth car, using a voltage converter/doubler .

for the life of me , i cannot see how this would work.? can any one explain ?
mark k

2. ### Harald KappModeratorModerator

10,806
2,436
Nov 17, 2011
"Earth" in a car is not really earth. Since the tires are insulating, the potential of a car's chassis is floating with respect to earth. The term "earth" is used only to refer all voltages to a common potential. Another commonly used term is "ground".

The technique is fairly straightforward. The car's chassis is defined as 0V (earth, ground). As the battery's "+" pole is connected to the chassis, the other end of the battery is at -12V with respect to the chassis.
A standard car radio is designed such that the "-" input of the power supply is at chassis potential, thus making the "+" input of the power supply being at +12V with respect to the chassis.
Obviously the two techniques don't match.

The voltage inverter creates from -12V at its input an output voltage that is at +12V with respect to the chassis. You feed this to the "+" input of the radio's power supply and the radio operates exactly as in a normal car (where battery "-" is at chassis potential).

Seen from the radio's "+"input the voltage there (+12V) has a potential difference of 24V to the battery's "-" pole. This voltage differential is bridged by the inverter.