Quite why I didn't just check it on my own car at the time I can't remember now, must have been in pieces or something. Anyway for completeness I just measured the alternator excitation current fed through the LED in the tachometer on my car at 3.8mA! No wonder the alternators on these cars can be reluctant to start.
SV VVC 170 - 170.4 bhp @ 7100 rpm - 142.4 ft.lb. @ 4900 rpm
Is that the actual excitation current? I think the real excitation current is fed via the regulator pack and the current fed is dependent on the voltage read from the warning light circuit. At least that is my understanding.
@EEK - You may well be right.
But either way I've now seen 4 or 5 EU3 K Series cars where the current fed by the LED wasn't enough the get the alternator to start up and fitting a parallel resistor to more closely mimic the "load" of an incandescent bulb cured the issue. So one way or another they do seem to expect a certain level of current flow here to act as the signal to turn on and current passed by the LED with series resistor is marginal.
But as you say, it may not be that the actual current flow into this terminal provides the excitation directly.
Actually, after looking at the schematics for Bosch alternator regulators which all seem to be of this format:
In the case where the alternator is not charging, all of the diodes I have highlighted below will be reverse-biased and effectively out of the circuit, taking all of the stator wiring with them. The ones in green rectify the three-phase AC output from the stator to feed the battery but will be non-conducting when the stator is not generating a voltage. The ones in yellow (coupled with the three lower green ones) do the same to provide an excitation feed for the stator when running but again will be non-conducting when not generating. The ones in red look to be to suppress back-emf transients when switching the stator and again will not be conduction statically.
So the circuit effectively reduces to this in the non-charging state:
And from this you can see that the current through the warning lamp not only provides the actual excitation supply for the stator but also powers the regulator IC itself. The regulator IC switches the earth side of the rotor to regulate the excitation current DOWNWARDS but doesn't actually switch in any current from the battery side of things. The regulator IC has a supply connection form the warning lamp circuit, a ground connection, a sense input from the rotor and a switching output to control the switching transistor.
So unless I'm reading this wrong, yes the warning lamp feed IS the actual excitation current, AND the regulator supply.
Later regulators may add some extra functionality; I know these have a DFM output which is a PWM square wave which allows the ECU to monitor the alternator load, but to be honest this may well simply be taken from the transistor base signal in the above diagram as this would give a duty cycle proportional to the load.
Another exemplary piece of discovery, design and documentation.
Lovely bit of work, Andrew!
And from this you can see....
That's what my chemistry lecturer used to say when describing some arcane reaction. I was just as much in the dark then!