On 09/03/2020 17:46, Chris Green wrote:
wrote:
Hi All,

Back in the dark ages (60s,70s, 80s) cars had little bulbs that would come
on to tell you your battery was no longer charging.

There have been a few people on the Peugeot007 self
Help FB group Im on whose alternators have died, but their cars not charging
light hasnt illuminated (despite its passing its POST before the problem
and after its been rectified (see what I did there).

Got me to thinking, how were these bulbs driven in the past (i.e, what
electrics / electronics / circuit lit them when charging stopped)?

Most alternators (and also dynamos as were used in 60s and before) had
a specific terminal to drive the 'charging' lamp. It monitored the
voltage between the alternator/dynamo output and the battery.


And how are they supposed to work in €œmodern€� Cars?

On the €˜007, I believe its an LED and I SUSPECT its fed from the ECU??

On modern 'all electronic' cars it needs to do exactly the same job
but is presumably down to the designer exactly how it's driven.

The bulb was connected between +12V after the ignition switch and the
special terminal on the alternator or dynamo. If the latter was working
there was 12V on that terminal so the light went out. If there was no
output from the alternator or dynamo the terminal would drop to 0V
(chassis). That would bring the bulb on.

This meant that a connection to the alternator/dynamo side of the bulb
could be used to feed a relay that could connect a second battery so it
got charged when the engine was running but was otherwise isolated.

Bill