DefectiveLemon wrote: RSB wrote:
DefectiveLemon has a good point - what type of alternator do you have? If you have a 2-wire device, then maybe you are damaging the diodes in the excitation input, and maybe not the diodes in the output. Here is the wiring for our car (1986 Audi Coupe GT), note that switch "Z" disconnects the field wire (D+) from the alternator output (B+), so any transients generated at B+ will not make it to D+.
From your diagram, it doesn't look like D+ is the field input. It looks like it's just a charge-fail warning light. Are you sure the alternator output stops if you disconnect D+?
Absolutely. The circuit going to D+ serves 2 functions: 1) excitation of the alternator and 2) charge-fail indication. If the 12V going to the D+ terminal is removed the alternator will not 'start'. Proven this to ourselves many times.
Here is what the inside of a 2 wire alternator looks like: https://electronics.stackexchange.com/q … -generator
When the car is not running, the battery provides 12V (12.0 to 12.6, depending on its condition) to the D+ terminal. This is connected inside the alternator to the voltage regulator. When the car is not running, the regulator provides essentially a ground reference. This results in a voltage drop across the warning light, causing it to illuminate.
The D+ voltage also provides excitation to the regulator when the motor begins to spin. After the revs reach a high enough value, the alternator will output its normal voltage (13.8 - 14.2V range) out of both the B+ and the D+ terminals. Now that the voltage is the same across the alternator warning light, there is no current flow and the light goes out.
The diodes for the B+ output are heavy duty and sized to source the high currents demanded by the car's systems. There are 6 diodes, so the AC voltage coming out of the alternator is subject to 'full wave rectification' and will be a pretty good (but not perfect) DC. https://www.electronics-tutorials.ws/diode/diode_6.html
The diodes for the D+ output are kinda wimpy since they only have to handle the current for a small light bulb. There are only 3 diodes here, so that output is half-wave rectified. The DC is a bit lumpy but it doesn't matter.
We Audi Be Faster
'85 Audi Coupe G(in &) T(onic)