12-Volt Alternators and Indicator Lights

The output of the alternator must open / cancel the normally closed switch in the regulator.

Long ago during my army training year I bought a little Renault (for the current prise of a packet of cigarettes). The generator was bad but to stop the light from burning all the time I adjusted the regulator settings. (all three relays). The end result was a total abortion.

On the modern alternator based charging systems the alternator gets its start up power through the light. The rotor coils draw power through the light until the alternator speed is high enough for the regulator to actually get its power from an internal power point in the alternator itself. At that point the voltage being produced by the internal reference power source is equal to that coming from the light and it goes off.

When an alternator goes bad it looses its internal power source along with the main charging source. The light will then stay on even when the engine is running so you know somethings wrong with the charging system.

The alternator's output (normally 13.5 to 14.5 volts) is applied to one side of the light and the battery voltage (normally 12.5 volts) is applied to the other side of the light. The voltage difference is insufficient to iluminate the light. If the alternator output goes to 0 volts the 12.5 volts from the battery will illuminate the light.

Please explain why when there is no output from the alternator, the indicator light comes on.

Simple, really, it's to let you know that the battery is not being charged.

in case of engine stop ( no output from alternator )

current flow in one direction from battery to indicator light bulb , from bulb to alternator coil and then earthing to negative terminal for battery ( indicator bulb comes on under potential 12 V )

in case of alternator running

alternator starts to generate potential volt , as alternator voltage increase as voltage different for bulb decrease ( bulb starts to damp gradually until completely off when alternator volt equals to buttery volt)

in case of faulted alternator (no generated voltage) bulbs on same as first case

Thank you tcmtech for a good answer. Even after 38yrs in the job as an ac sparky and refrigeration engineer I was asked that question and could not answer. You are never to old to learn.