The Guide for Selecting Electrical Connectors

Electrical connectors can be classified in different ways. According to the frequency, there are high frequency connectors and low frequency connectors; there are circular connectors and rectangular connectors according to the shape; according to the purpose, there are connectors for printed boards, connectors for cabinets, connectors for audio equipment, power connectors, connectors for special purposes and so on. Next we will mainly discuss the selection method of low frequency connectors (the frequency is below 3MHZ). We need to choose the connectors from electrical parameters, safety parameters, mechanical parameters, environmental parameters, connection modes, installation modes and terminal modes.

1) The requirements for electrical parameters

Electrical connectors are electromechanical components that connect electrical circuits. Therefore, the electrical parameters of electrical connectors are the first consideration in choosing the perfect military electrical connector.

A. The rated voltage

Rated voltage is also known as working voltage, which mainly depends on the the insulation material used by the electrical connector and the spacing between contact pairs. Certain elements or devices may not perform their intended functions when the voltage is lower than their rated voltage. In fact, the rated voltage of electrical connectors is the maximum working voltage recommended by the manufacturers. In principle, electrical connectors can operate normally even the voltage is lower than the rated voltage. The author is inclined to choose the rated voltage reasonably according to the withstand voltage ( dielectric strength) index of the electrical connector and the requirements of the service environment and safety level. In other words, according to different use environments and safety requirements, different maximum working voltages can be used under the same withstand voltage index. This is also in line with objective service conditions.

B. The rated current

Rated current is also called working current. Like rated voltage, electrical connectors generally work normally even the current is lower than the rated voltage. In the design process of electrical connectors, the thermal design of electrical connectors is adopted to meet the requirements of the rated current, because when the current flows through the contact pairs, the contact pairs will get hot due to the conductor resistance and contact resistance. When the heat exceeds a certain limit, the insulation of the connector will be damaged and the surface coating of the contact pair will be softened, which will lead to the failure. Therefore, limiting the rated current is actually to the control the temperature rise inside standard power connectors below the specified value. It needs to be noted in the selection that the rated current must be reduced in the use for multicore connectors. It should be paid more attention in large current occasions. For example, generally the rated current is 50A for φ3.5mm contact pairs, but in 5 core connectors, the current should be reduced by 23% for use, that is, the rated current is only 38A per core. The more the cores are, the larger the reduction range is.

C. The contact resistance

Contact resistance refers to the resistance produced by two contact conductors at the contact part. Two points should be noted in the selection. First, the contact resistance index of the connectors is actually the resistance of contact pairs, which includes contact resistance and conductor resistance of contact pairs. The resistance of contact pairs is called contact resistance in many technical specifications because the conductor resistance is usually small.

Second, in circuits connecting with small signals, it is important to pay attention to the conditions under which the given contact resistance index is tested, because oxide layers, oil stains or other pollutants can attach to the contact surface, and there will be film resistance on the surface of the two contacts. As the thickness of the film increases, the resistance increases rapidly and the film becomes a bad conductor.

However, mechanical breakdown of the film may occur at high contact pressure, or electrical breakdown may occur at high voltage and high current. For some small size connectors, such as, the small circular connector, the designed contact pressure is quite small, so they are only suitable for mA and mV occasions. Therefore, the film resistance is not easy to be broken down, which may affect the transmission of electrical signals. In GB5095 Basic Test Procedures and Measurement Methods for Electromechanical Components Used in Electrical Equipment, the Contact Resistance -- Millivolt Method, one of the contact resistance test methods, stipulates that the DC or AC peak value of the open circuit electromotive force in the test loop shall not be greater than 20mV, and the DC or AC test current shall not be greater than 100mA to prevent the breakdown of the insulation film on the contact. This is actually a low level contact resistance test, and selectors who have this requirement should choose the connectors with low level contact resistance index.

D. The shielding property

In modern electrical and electronic equipment, it puts forward a strict limit on the electromagnetic interference due to the increasing density of components and their related functions, so the connectors are usually enclosed with a metal shell to prevent internal electromagnetic energy radiation or the interference from external electromagnetic fields. At low frequencies, only magnetic materials can significantly shield the magnetic field. At this time, there are certain provisions on the electrical continuity of the metal case, namely, the contact resistance of the shell.

2) Safety parameters

A. The insulation resistance

Insulation resistance refers to the resistance value presented by applying a voltage to the insulation part of a connector so that the leakage current will generate within or on the surface of the insulation part. It is mainly affected by the insulation material, temperature, humidity, dirt and other factors. The insulation resistance value provided on the connector sample is generally the index value under standard atmospheric conditions. In some environmental conditions, the insulation resistance value will decrease to varying degrees, and the test voltage value of the insulation resistance also should be noted. There will be different results if different voltages are applied according to the formula: insulation resistance (MΩ) = the voltage added on the insulator(V)/ the leakage current (μA). the voltages applied are generally 10V, 100V and 500V in connector tests.

B. The withstand voltage

The withstand voltage is the critical voltage between the insulation parts of contact pairs or between the insulation parts and the ground, which can withstand higher voltage than the rated voltage within a specified period of time without the breakdown phenomenon. It is mainly affected by the spacing of contact pairs, the creepage distance, the geometrical shape, insulation materials, ambient temperature and humidity, atmospheric pressure, etc.

C. The combustibility

No connector can operate without an electric current, which creates a risk of fire. Therefore, the connector is required not only to prevent the ignition, but also to be able to extinguish itself in a short time in case of an ignition and fire. Pay attention to choosing the electrical connectors of flame retardant and self-extinguishing insulation materials.

3) Mechanical parameters

A. Single separating force and total separating force

Contact pressure is an important index in connectors, which directly affects the magnitude of the contact resistance and the abrasion loss of contact pairs. In most structures, it is quite difficult to directly measure the contact pressure. Therefore, the contact pressure is usually measured indirectly through the single separating force. For round pinhole contact pairs, the standard pins with specified weights are usually adopted to test the clamping capacity of female contacts. Generally, the diameter of the standard pins is -5μm lower than the limit of the diameter of male contacts. The total separation force is generally twice of the sum of the upper limit of the single separating force. When the total separating force exceeds 50N, it is quite difficult to insert and remove the connector manually. Of course, zero-insertion force connectors and umbilical connectors can be used for some test equipment or some occasions with special requirements.

B. The mechanical life

The mechanical life of the connectors refers to the insertion and extraction life, which is usually stipulated for 500-1000 times. When it reaches the specified mechanical life, the contact resistance, insulation resistance and withstand voltage of the connector shall not exceed the specified value. Strictly speaking, the present mechanical life is a vague concept. The mechanical life should have a certain relationship with time. Inserting and extracting the connector for 500 times in 10 years and used up the 500 times in 1 year are obviously different situations, but there just isn't a more economical or scientific way to measure it.

C. The number of contact pairs and the pinhole property

Firstly, the number of contact pairs can be selected according to the need of the circuit. At the same time, the size of the connector and the total separating force should be considered. The more the contact pairs are, the larger the volume is, and the larger the the total separating force is. In some cases where high reliability is required and the volume allows, the parallel connection of two contact pairs can be used to improve the reliability of the connection. The plug, socket, pin (male contact) and jack (female contact) of the connector can be assembled interchangeably, and they can be selected according to the electrification condition at both ends of the plug and socket in actual use. If the socket needs to be always electriferous, you can choose the socket with jack, because the electriferous contact of the socket with jack is buried in the insulator, which is relatively safe because human bodies are not easy to reach it.

D. Vibration, impact and collision

It should mainly consider the electrical continuity of contact pairs during the vibration, impact and collision of connectors under the specified frequency and acceleration conditions. Momentary interruption will happen to contact pairs under this dynamic stress. The prescribed momentary interruption time is generally 1μs, 10s, 100μs, 1ms and 10ms. Attention should be paid to how to judge the momentary interruption of contact pairs. Now it is generally believed that when the voltage drop at both ends of the closed contact pairs exceeds 50% of the electromotive force of the power supply, the failure of closed contact pairs (contact) can be determined. In other words, there are two conditions for judging whether the momentary interruption occurs: the duration time and the voltage drop, and both of them are indispensable.

4) Connection modes

The connector is generally composed of a plug and a socket, and the plug is also called a free connector and the socket is also called a fixed connector. The connection and disconnection of circuits can be realized by means of the insertion and separation of plugs and sockets, therefore, a variety of ways of connecting plugs and sockets have occurred. There are mainly thread connection, bayonet connection and pin connection for circular connectors.

Thread connection is the most common mode among the three connection methods. It has the advantages of simple processing technology, low manufacturing cost, wide application range, etc. However, it is not suitable for the occasions requiring frequent insertion and fast connection due to its slow connection speed. Bayonet connection is faster due to the long lead of the three bayonet slots, but the cost is higher for its complicated manufacturing. Pin connection is the fastest connection of the three connection modes. It does not need rotation motion, and only linear motion can realize the function of connection, separation and locking. Pin connection belongs to the direct push-and-pull connection mode, so it is only suitable for the connectors with not large total separating force. Generally this connection mode is more commonly seen in small connectors.

5) Installation modes and the appearance

There are front-installation and post-installation modes, and the connector can be fixed by means of rivets, screws, clamps or fasteners of connectors. In some cases, the plug and the socket are both free connectors, which are also known as relay connectors.

Connectors vary in appearance. Users mainly choose the appearance from the straight shape, the curved shape, the outer diameter of wires or cables, the fixed requirements for shells, the volume, the weight and the connection necessity with metal hoses. The connectors used on face-plates should also be selected from the aspects of aesthetics, shape, color, etc.

6) Environmental parameters

Environmental parameters mainly include the ambient temperature, humidity, abrupt temperature change, atmospheric pressure and corrosive environment. The use, storage and transportation environment for the connectors have significant impacts on their performance, so appropriate connectors must be selected according to the actual environmental conditions.

A. The ambient temperature

The metal and insulating materials of the connector determines the operating environment temperature of the connector. The high temperature will damage the insulation material and cause the decrease of insulation resistance and voltage resistance; for metals, high temperature may cause the contact pairs to lose the elasticity, accelerate the oxidation and result in the coating deterioration. Generally the requirements are higher under special occasions where the ambient temperature is -55-100℃.

B. The humidity

It is the main cause of electrical breakdown when the relative humidity is greater than 80%. humid environment will lead to the absorption and diffusion of water vapor on the surface of the insulator, which is easy to reduce the insulation resistance below level MΩ. Prolonged exposure to high humidity will cause physical deformation, decomposition and the release of resultants, resulting in respiratory effects, electrolysis, corrosion and cracking. The environmental conditions of moisture, water seepage and pollution should be considered especially for external connector equipment, and hermetically sealed connectors should be selected in this case. For watertight and dust-tight connectors, GB4208 shell protection grade is generally adopted.

C. Temperature leaps

The temperature leap test is to simulate the actual use of connector equipment from cold environment to warm environment, or simulate the rapid change of ambient temperature of spacecrafts and detectors. Temperature leaps may cause the cracking or flaking of insulation materials.

D. Atmospheric pressure

At the airless high altitude, the gas given off by plastics can pollute contact pairs, increase the tendency of corona generation and decrease the withstand voltage, causing the short circuit. When it reaches a certain altitude, the property of plastics becomes worse, therefore, the voltage must be lowered when using unsealed connectors at high altitude. The recommended voltage drop coefficients at low pressure are shown in Table 2.

E. The corrosive environment

According to the different corrosion environments where connectors are used, corresponding metal, plastic and coating structure connectors should be selected. For example, the property of connectors used in salt mist environments can deteriorate rapidly without anti-corrosive metal surfaces. It is not advisable to use silver-plated contact pairs in environments with comparable concentrations of SO₂. Mold is also an important problem in hot areas.

7) Termination methods

Termination methods refer to the connection method of connecting the contact pairs of the connector with the wire or cable. Choosing the right termination method and using the right termination technology are also important aspects in using and selecting connectors.

A. Soldering

The most common type of soldering is tin soldering. The most important part of soldering is the continuity of the metal between the soldering material and the soldered surface. Therefore, the solderability is important for connectors. The most common coatings on the welding ends of connectors are tin alloy, silver and gold. Soldering lug type, punching soldering lug type and gap soldering lug type are common welding ends of leaf spring contact pairs. The common welding end of pin contact pairs is the drilling circular gap type.

B. Press connection

Press connection is a technique used to compress and displace the metal within the specified limits and to connect wires to contact pairs. Good press connection produces an intermetallic melt and flow to deform the wires and the materials of contact pairs symmetrically. This connection is similar to the cold welding connection, which can obtain better mechanical strength and electrical continuity and can withstand more severe environmental conditions. At present, it is generally accepted that the correct press connection is better than tin soldering, and press connection must be adopted especially in the large current situations. Special press pliers or automatic and semi-automatic press-connection machine shall be used for press connection. The choice of conductor barrels for contact pairs should be made correctly according the cross-section of conductors. It should be noted that the press connection is a permanent connection and can only be used once.

C. Wire-wrap connection

Wire-wrap connection is directly wrapping the wire around the wire-wrap column of angular contacts. During the wrapping, the wire is pressed into and fixed at the corner angles of the wire-wrap column to form an air-tight contact under a controlled tension. There are several requirements for wound wires: The nominal value of the wire diameter should be within 0.25mm-1.0mm; when the diameter of the conductor is not more than 0.5mm, the elongation of the conductor material should be no less than 15%; when the diameter of conductor is greater than 0.5mm, the elongation of conductor material should not be less than 20%. The tools for wire-wrap connection include the wire-wrap gun and fixed wire-wrap machine.

D. Piercing connection

Piercing connection is also called insulation displacement connection, which is a novel terminating technology invented by the United States in the 1960s with the characteristics such as high reliability, low cost and convenient use, and it has been widely used in a variety of printed board connectors. It is suitable for the connection of ribbon cables. Without removing the insulation layer of the cable, the U shape contact leaf spring of the connector can pierce into the insulation layer to make the cable conductor slide into the groove of the contact leaf spring and be clamped, thus forming a tight electrical connection between the conductor of the cable and the leaf spring of the connector. It only needs simple tools, but cables of specified wire gauge must be used.

E. Screw connection

Screw connection is the connection mode of adopting screw connection terminal. Pay attention to the maximum and minimum sections of the allowed connection wire and the maximum tightening torque of the screws of different specifications.

The selection method of connectors is discussed from electrical parameters, safety parameters, mechanical parameters, environmental parameters and terminal connection mode. However, due to the wide variety of connectors, it is inevitable to care for one thing and lose another, and some views are not necessarily correct and comprehensive. We hope that connector manufacturers and user factories can attach importance to the choice and use of connectors.