boatbuilder wrote:Makes a bit more sense now. So when the ignition is switched on current flows through the 470ohm, then led, then diode to alt. Alt acts as a ground I presume in that case.
Then when the engine is running and the alternator is generating power, it pushes back current in the opposite direction which is stopped by the diode and hence the led goes out.
I've read somewhere that alternators sometimes have an exciter connection so maybe the alternator needs a voltage applied in order to start it up.
Yep that's pretty much it - the voltage travelling through the LED is the 'exciter' (and the 150ohm 'route' provides an additional or alternate path to that exciter). Although to confuse things I think most modern alternators are self-exciting (oouu-errr!)!!
Either way - the alternator, when it's not spinning, provides a ground for that circuit and the LED lights up; when the alternator spins it outputs 14.5 volts-ish on that line which causes the LED to turn off.
boatbuilder wrote:If it just relied on the voltage after the led, the 2v might not be enough to excite the alternator so that could be another reason for the 150 ohm bypass...It would provide a higher voltage to the alt.
Not saying I'm right about all of that, just thinking out loud!
FYI - The LED 'drops' the voltage by 2 volts, it doesn't 'leave' only 2 volts.