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neg earth stereo in positive earth car ?

Harald Kapp

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"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.
 
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