Every electrical system in the car requires harness to enable circuit function. Circuits include power supply, switch control, electrical equipment or actuator, iron, etc. The harness equipment in each electrical system circuit needs to be selected by the harness engineer. According to the combing of harness design process, the weak links are summarized, which can provide a reference for the design and development of automobile harness in the future. The harness in this article refers to low-voltage automobile harness terminal.

1. Brief description of automobile wiring harness terminal

In terms of function, the automobile harness terminal is the main body of automobile circuit, connecting the electrical equipment of automobile to make it work.
In terms of composition, the form of harness is basically the same, composed of wires, connecting plug-ins and packaging tape. It is necessary not only to ensure the transmission of electrical signals, but also to ensure the reliability of connecting circuits, to provide specified current value to electrical equipment, prevent electromagnetic interference of surrounding circuits and eliminate short circuit of equipment.

The whole automobile system generally has power system, braking system, lighting system, multimedia system, etc. The circuits of all systems are built into automobile networks. Harness is the network body of automobile circuit, which connects the electrical and electronic components of automobile to make it work. Because there are many circuits and large number of harness components needs to be selected, the matching of components is very important for vehicle safety. This section is discussed in this article.

2. Main contents of automobile harness terminal design

First, let's start with the basic circuit. Start with the power supply.

Circuit 1: power → circuit breaker.

Circuit 2: fuse → controller.

Circuit 3: controller → horn power circuit.

Loop 4: horn wiring → controller.

Circuit 5: controller → iron wire.

Circuit 6 & 7: CAN line control.

According to the above circuits for physical split, it can be roughly divided into fuse wire, electrical end connector / terminal, wire harness end connector / terminal, wires.

Therefore, when the current flowing through the electrical equipment is known, how to select the appropriate circuit equipment is the center of this test.

3. Selection principle

3.1 Terminal

3.1.1 Female terminal

Generally speaking, the common terminal is set at the device end, and the model is selected by the electrical performance of the device. Female terminal connected to the device end is usually used in combination with the common terminal without separate selection. When selecting the female terminal of harness connection, you must first understand the current value of circulation circuit.

The allowable current value of a terminal with width of 0.64mm is generally lower than 3A. For circuits with small current value, 0.64mm terminal are generally selected (Figure 2). If the dynamic current value of motor and other equipment is large, generally choose 0.64mm/1.0mm or above terminals. The allowable current value of the terminal is related to the number of pins, wire size, power on time, operating temperature, etc. It is necessary to view the drawing of the terminal to make a selection.

The above is the selection of terminal dimensions. Once the terminal dimension is determined, the coating requirements shall also be considered. Common coating examples: the airbag system requires gold-plated terminals to ensure the stability of relatively small current.

3.1.2 Male terminal

In addition to the device end, the wiring harness docking part also has a male terminal. Details of selection of male terminal are identical with that of female terminal.

3.2 Connector

After the basic selection of terminals, it is necessary to consider whether the selection of connectors needs to be waterproof. Need to consider the number of holes, etc. The number of holes can affect the current attenuation coefficient and plugging force, which needs to be verified repeatedly in the actual selection.

3.3 Selection of fuse capacity

Fuse is designed to protect the equipment and mainly plays the role of overload protection. If the fuse is correctly placed in the circuit, when the current abnormally rises to a certain height and heat, the fuse will automatically cut off the current to protect the safe operation of the circuit. Therefore, it is necessary to determine the capacity of the fuse based on the current flowing through the fuse by the downstream equipment. Considering the durability of the fuse, as shown in Figure 3, the total rated current should be less than 70% of the fuse capacity as the preferred condition. In addition, we also need to consider whether there is a burst current, an oscillation current, etc. Consider the durability of the fuse according to the actual current.

3.4 Electric wire

3.4.1 Wire type

The wire line needs to consider whether heat resistance is required， whether special lines such as twisted pair or shielded lines are required, and whether it is used for bending parts, etc. After considering the above special needs, select the corresponding material.

3.4.2 Wire dimension

In the power supply circuit, after determining the fuse capacity, select the wire dimension, i.e. thickness.

The smoke characteristic curve of the wire should be higher than that of the fuse, so that the fuse wire can be burned out first in case of short circuit.

In the non-power circuit, since there is no fuse, the corresponding wire dimension is directly selected according to the circuit current value.

For example, at 20 ℃, the allowable current value of general wire 0.35mm2 is 9A. The allowable current value decreases with the increase of temperature to about 3A at 70℃, so it needs to be selected according to actual use position of the vehicle.

The wire size is 0.13mm2, and the common wire is 0.35mm2, 0.5mm2, 0.75mm2, 2.2mm2, 2.0mm2.

Select according to actual current value and the wire product manual of each manufacturer.

3.4.3 The color of the line

It is usually selected according to product manual, with no special requirements. In terms of operation, try to avoid using the same diameter color in the same plug-in to prevent misinsertion during operation or misidentification during maintenance.

3.5 Overall circuit selection principle

In fact, many basic methods of physical selection are not block but whole. In power circuit, the fuse is actually there to protect the entire circuit from problems, thus the fuse must be the weakest in the circuit.

The wire selected through the fuse characteristic of the fuse and the smoke characteristic curve of the wire needs to match each terminal in the circuit. At this time, the allowable current of the terminal is likely to be less than the capacity of the fuse. In the case of short circuit, because the compressive part of the terminal is relatively weak, the fuse is burnt out and deformed before the fuse is blown, and the fuse also loses its protection function. For example, the combination of 15A fuse, 0.5 mm2 wire and 0.64 terminal.

Therefore, after selecting the final part, a loop check should be carried out to ensure that no similar situation will occur.

In the non-power circuit, terminals and wires are selected according to the above contents. Since there is no power connection, there will be no large current access even if there is a short circuit, so the above inspection is not required.

4. Conclusion

The whole vehicle of automobile harness terminal has at least hundreds of and thousands of circuits. A good circuit design is: in any case, all circuits will not have problems, but this is very difficult to achieve. Even if there are circuit breakers, some circuits will have improper matching due to improper selection of parts, which may eventually lead to terrible consequences such as smoke and combustion. Therefore, the designer can verify one by one according to the method of single circuit to ensure the safety of each circuit. In addition, select the excess circuit diameter. Wire or large terminal can ensure the safety of some circuits, but it is not conducive to cost reduction. Therefore, appropriate parts selection can not only ensure the safety of the circuit, but also can reduce the cost.