I'd imagine one could design a circuit with a two colour led, where its red
one way and green the other and use an op amp and a low value resistor as a
shunt and measure the polarity with the amp. Crude, but should work, but
might need to be set up.
Brian

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This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"Chris Green" wrote in message
...
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 Peugeot'007 self
Help FB group I'm on whose alternators have died, but their car's not
charging
light hasn't 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 "modern" Cars?

On the '007, I believe it's an LED and I SUSPECT it's 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.

--
Chris Green
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