Below is a schematic of an alternator/generator. I understand the rectification going to B+, but I don't understand why there are 3 diodes going to D+.

The way I understand the setup, D+ measures the voltage at the battery, and the delta between that and the preset voltage in the regulator drives the field to increase/decrease the output of the generator. I know why you need the diodes to charge the batteries, but why would you put the rectified current on the D+ sensor line. Seems like that would defeat the purpose of the sensor.

Maybe I'm completely misreading the diagram, if so, please point out my error.


You have a misconception about what the regulator of a car alternator does. It doesn't regulate the voltage of the battery, and so, it doesn't measure the voltage of the battery either.

The regulator is there so the voltage delivered to the battery to charge it is independent from the speed of the drive shaft.

The regulator does this (simply said) by controlling the field winding of the alternator. That's the winding on the rotor, in your schematic that one on the right. This field winding needs some power, and it has to be independent from the battery, so the speed the alternator runs at can be measured.

That's why these three leftmost diodes are there. They provide an additional DC output which is not filtered to "pure DC" by the battery. Instead, it still has an AC part with a frequency dependent on the drive speed.

  • \$\begingroup\$ So you are saying I should not be connecting D+ to the battery at all? \$\endgroup\$ – boatcoder Feb 18 '17 at 2:17
  • \$\begingroup\$ Right. Battery plus goes to B+. The purpose of the D+ contact is to connect the battery indicator lamp inside the instrument panel (against B+). When the alternator is stopped or broken, this lamp supplies current for the alternator field winding, and lights up to indicate the error condition. \$\endgroup\$ – Janka Feb 18 '17 at 2:19
  • \$\begingroup\$ Why is that smoothing capacitor drawn with a dashed line? \$\endgroup\$ – crowie Feb 18 '17 at 2:31
  • \$\begingroup\$ I much more think it's an EMC filtering cap. \$\endgroup\$ – Janka Feb 18 '17 at 2:36
  • \$\begingroup\$ +1 fascinating. But how does the whole system regulate the battery charging current? Does it limit it? Is there any feedback? \$\endgroup\$ – Ale..chenski Feb 18 '17 at 3:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.