5. Overvoltage protection

1. 5. Overvoltage protection with the battery correctly connected and under normal driving conditions, it is unnecessary to provide additional protection for the vehicle's electronic components. The battery's low internal resistance suppresses all the voltage peaks occurring in the vehicle electrical system. it is often advisable to install overvoltage protection as a precautionary measure in case of abnormal operating conditions.

2. 5.1 Reasons for overvoltage Overvoltage may occur in the vehicle electrical system as the result of: Regulator failure Influences originating from the ignition Switching off of devices with a predominantly inductive load Loose contacts Cable breaks

3. Such overvoltages take the form of very brief voltage peaks, lasting only a few milliseconds which reach a maximum of 350 V and originate from the coil ignition. Overvoltages are also generated when the line between battery and alternator is open-circuited with the engine running (this happens when an outside battery is used as a starting aid), or when high-power loads are switched off. For this reason, under normal driving conditions, the alternator is not to be run without the battery connected. Under certain circumstances though, short-term or emergency operation without battery is permissible. This applies to the following situations: Driving of new vehicles from the final assembly line to the parking lot Loading onto train or ship (the battery is installed shortly before the vehicle is taken over by the customer) Service work, etc.

4. 5.2 Types of protection 5.2.1 Z-diode protection Z-diodes can be used in place of the rectifier power diodes. They limit high-energy voltage peaks to such an extent that they are harmless to the alternator and regulator. Z-diodes function as a central overvoltage protection for the remaining voltage-sensitive loads in the vehicle electrical system. The limiting voltage of a rectifier equipped with Z-diodes is 25...30 V for an alternator voltage of 14 V, and 50...55 V for an alternator voltage of 28 V. Compact alternators are always equipped with Z-diodes.

5. 5.2.2 Surge-proof alternators and regulators The semiconductor components in surge-proof alternators have a higher electric-strength rating. For 14-V alternator voltage, the electric strength of the semiconductors is at least 200 V, and for 28-V alternator voltage 350 V. a capacitor is fitted between the alternator's B+ terminal and ground which serves for short-range interference suppression. The surge-proof characteristics of such alternators and regulators only protect these units, they provide no protection for other electrical equipment in the vehicle.

6. 5.2.3 Overvoltage-protection devices (only for 28 V alternators) These are semiconductor devices which are connected to the alternator terminals D+ and D- (ground). In the event of voltage peaks, the alternator is short-circuited through its excitation winding. Primarily, overvoltage-protection devices protect the alternator and the regulator, and to a lesser degree the voltage-sensitive components in the vehicle electrical system. Generally, alternators are not provided with polarity-reversal protection. If battery polarity is reversed (e.g. when starting with an external battery), this will destroy the alternator diodes as well as endangering the semiconductor components in other equipment.

7. 5.2.4 Overvoltage-protection devices, non-automatic This type of overvoltage-protection device is connected directly to the D+ and D- terminals on T1 alternators