Bosch Alternator Voltage Regulator

This regulator comes off a small Bosch alternator from a 1600 VW Fox (Golf / Rabbit). On these alternators, the voltage regulator looks a bit like a power transistor on the back of the brush assembly. My example has a plastic cover that snaps off.

The conformal coating is a joke -- it came off in paraffin when I was cleaning the oil off.

As you can see from the schematic, the regulator uses two transistors (one a power darlington), a zener, six resistors and three capacitors. There's also a diode to protect the power transistor from inductive spikes.

You will find an article on the web on how to boost the output of the alternator by raising the ground pin (D-) of the regulator by one diode drop (0.7 V). You can obviously achieve the same by fiddling with R4/R5/R6. Adding another parallel resistor to R5/R6 will boost the voltage, and I would also add a transistor (or small relay) to switch this resistor in and out, to prevent noise from the control wires.