- Voltage Regulator (Fig. F & G)
In order to regulate the tension, it is necessary to carry out micro cuts on the excitation of the rotor to decrease “the power of the magnet”.
The voltage regulator comprises transistors which allow fast cuts and a zener diode which has the particularity of being conductive in the opposite direction when it reaches a breakdown voltage of 14.5 V.
We find :
- DZ: zener diode.
- PNP 1: PNP transistor (drives the closing of the 2nd transistor).
- PNP 2: PNP transistor (powers the rotor).
- R1 & R2: Limiting resistors (limits overcurrents in the transistors).
- D1: Free-wheeling diode (protects the transistors from choke effects).
Reminder of PNP transistor operation:
When the base is grounded and the emitter is powered by a positive (+), the current flow is from the emitter to the collector.
In Fig. F, the rotor is powered by the PNP transistor 2. The zener diode remains closed because the voltage is lower than the breakdown voltage (14.5V).
In Fig. G, the voltage reaches the breakdown value of the zener diode. Diode DZ becomes conductive and connects PNP transistor 1 to ground, which activates. When activated, a positive (+) is brought to the base of PNP transistor 2 which closes the passage between E2 and C2. The rotor is no longer powered, which allows the magnetization to decrease and lower the output voltage of the alternator.
The current induced in the windings of the rotor creates a large reverse voltage in the electrical circuit when the excitation stops. This phenomenon is called self-effect.
Diode D1 is a free-wheeling diode installed to protect transistors from electric arcs.