The regulator excites the rotor via the slip rings causing the alternator to produce current, the regulator turns the alternator on and off. When voltage is applied to the rotor thru the slip rings by the regulator, the field windings produce alternating current that is rectified to direct current by the diodes connected to the field windings. The diodes are pressed into the alternator housing to dissipate heat from the current passing thru them. Re-builders replace diodes as necessary along with bearings, brushes and the regulator module.