Improved design of charging indicator circuit for vehicle alternator

With the improvement of comfort and intelligence level of modern vehicles, various intelligent electronic control systems are increasing, and the power load of the whole vehicle is gradually increasing. Therefore, it is more important to ensure the normal working state of the vehicle generator. Especially for vehicles using electronic control systems, once the power system fails, they can only park. The following analyzes the potential hazards and improvement measures of the charging indicator circuit of the currently widely used automotive alternator.

The circuit structure of domestic automotive alternators is basically the same, all of which are composed of power generation winding (stator), excitation winding (rotor, some models use permanent magnet rotor), rectifier, voltage regulator (can adjust the excitation current voltage, or directly adjust the output voltage through the switching element. The voltage regulator can be integrated into the generator or external). There are generally charging output (+b) terminals, indicator light (d+) terminals, neutral point (n) terminals, and ground (e) terminals on the shell, among which the neutral point terminal is not required. The circuit structure of the charging indicator light of the generator is similar: it relies on the output of DC voltage by the additional three diodes to control the charging indicator light. However, this widely used charging indicator circuit has defects, that is, when the generator fails to charge due to internal faults, its charging indicator circuit can be intact, so the indicator light is also normally off, indicating that it is charging. In fact, it is the battery that supplies electricity to the entire vehicle until the battery is exhausted and the vehicle cannot be started. The following is an analysis of the charging circuit of Dongfeng EQ1021H15Q gasoline pickup truck (generator part number: 31QM-01010/14V80A) and some improvements.

1. Work process analysis

As shown in Figure 1, diodes D1 to D6 are high-power main rectifiers, forming a three-phase star rectifier circuit to supply power and charge the entire vehicle. Medium-power diodes D7 to D9 are dedicated rectifiers for the voltage regulator and charging indication circuit, and the output 14V DC voltage (D+) is isolated from the main output (+B). When the generator is running, both the +B and D+ terminals output 14V DC voltage. As can be seen from Figure 1, when the main rectifiers D1 to D3 are disconnected due to a fault and other rectifiers are normal, the charging indication circuit can still work normally. That is, when the D+ terminal outputs a 14V DC voltage, the main output terminal (+B) may not have an output. In this way, once the battery is exhausted, the vehicle will not be able to start (the author's company has several pickup trucks that broke down in the wild due to battery exhaustion, and had to be sent to the rescue vehicle for emergency with new batteries).

II. Improvement measures

From the above analysis, it can be seen that the most direct and reliable way to determine whether the generator is generating electricity is to detect the output charging current of the main output (+b) terminal. Detecting the voltage at the d+ terminal is only an indirect method. Although the outputs of d+ and +b are synchronized in most cases (for example, when the stator winding or rectifier tubes d4 to d6 are damaged and no electricity is generated, d+ and +b will have no output voltage at the same time), the above analysis is not synchronized. Once the battery is exhausted, it may cause serious consequences. The following are several improvement methods that can be adopted.

1. Develop and use a new type of charging indication (reed) relay. The function of this (reed) relay is also to indicate the charging output of the generator, but the primary coil is connected in series in the +b terminal output circuit to directly detect the charging current (it can be called a current type reed relay. The commonly used ones are voltage type), see Figure 2. This relay has two key parameters: the rated voltage drop of the primary coil and the rated operating current of the primary coil. The resistor r provides the initial excitation current for the generator (the generator with independent power supply of the regulator does not need resistor r). This circuit is relatively simple.

2. Develop and use a dedicated charging indicator module. This module detects the generator charging current through an internal sampling resistor line (the magnetic field generated by the sampling resistor line can also be detected using a Hall element), and drives the indicator light after amplification and comparison. The principle block diagram is shown in Figure 3. This module can also be integrated into the regulator to simplify installation and improve reliability.

3. Use a charging indicating ammeter. The ammeter can indicate the charging and discharging current of the battery. It has the advantages of being intuitive and accurate, but it is difficult to arrange, has increased line loss, and has increased costs. It is rarely used in modern vehicles.

4. Use a voltmeter. The voltmeter can monitor the battery voltage at any time and can also intuitively reflect the charging and discharging conditions of the battery. According to actual measurements, the no-load voltage of a charged battery that can start the vehicle smoothly is 12.5v~13v, and the battery terminal voltage can rise to 14v after the engine is running (the 14v voltage is limited by the generator regulator and has nothing to do with the engine speed. At the same time, turning on the headlights and air conditioning will not cause the 14v voltage to drop). If the generator cannot generate electricity, then the battery terminal voltage will gradually decrease on the basis of the no-load voltage after the engine is started (the power supply of the entire vehicle is taken from the battery), and there will be no increase. Therefore, the change in the battery terminal voltage before and after the generator is running directly reflects whether the generator is generating electricity (if the voltage rises but cannot reach 14v, it means that the generator's power generation capacity has decreased and there is a fault). The voltmeter can be connected to any suitable power supply terminal, with flexible layout and will not increase line loss. At the same time, an alarm buzzer can be set inside the voltmeter to sound an alarm when the voltage is lower than 12v.

It can be seen from the internal structure of the generator that the charging circuit must not be disconnected when the generator is running. Otherwise, the sudden drop in large current (tens of amperes) will generate abnormally high voltage at both ends of the stator winding, breaking down the rectifier tube and regulator.

3. Practical Application

Considering the difficulty of production, development cycle, practical effect and other factors, the improvement measures 1 and 4 were adopted at the same time to make an additional indicator light module, the circuit of which is shown in Figure 4. The rated contact action current of the reed relay is set to 4A (if this current is too large, the charging indicator light will not go out when the engine is at low speed), which can sensitively indicate the charging current of the generator. However, this circuit using a reed relay has two disadvantages: ① When the generator's power generation capacity decreases (a rectifier tube failure causes it to output only a small current of less than 10A. It is not completely unable to charge), this circuit still indicates normal charging (when the vehicle's power load is very large, it will still cause the battery to lose power); ② When the battery is fully charged and the voltage is close to 14V, the charging current will drop to less than 4A. At this time, the charging indicator light indicates that there is no charging current, which will cause people to misunderstand that the generator cannot be charged. For this reason, a voltmeter is added to the instrument cluster, which together with the charging indicator light forms a complete charging indication circuit. The working process is analyzed as follows.

1. If the charging indicator light is on and the voltage on the voltmeter is lower than 13.8V, it means the generator cannot be charged (the generator is damaged).

2. The charging indicator light goes out and the voltage on the voltmeter is lower than 13.8V, indicating that the generator is charging normally.

3. The charging indicator light is on and the voltage on the voltmeter is higher than 13.8V, indicating that the battery is fully charged and the generator does not output excessive charging current (the generator is normal).

4. If the charging indicator light goes out and the voltage on the voltmeter is higher than 13.8V, it means that the generator regulator is damaged and cannot limit the charging voltage. The battery will be overcharged and damaged (generator damage).

This line received a good response after small-batch loading, eliminating potential accidents.