DTM7000
28
Doc.: 00293978-v1
Common earth circuits: example of the rear lights fitted to a vehicle
The following diagram shows the wiring for the rear fog lamps and rear lights with the earths for the
bulbs regrouped at a point A.
Wire the circuit and operate it.
Disconnect the rear left hand light from the earth at A and turn on the lights. What happens?
The left hand rear light and fog lamp light up dimly.
Trace the circuit the current is flowing along. What can one deduce from the voltage across the
bulb terminals?
The rear right light and left hand lights are in series, dropping their supply voltage to just 4V.
Turn on the rear lights and fog lamps. What happens?
Only the right hand bulbs light up.
Why?
The two left hand lights are supplied from two positive terminals, making a zero potential difference
across their terminals.
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A
