Connectors are electromechanical components that connect electrical circuits. Therefore, the electrical parameters of the connector itself is the first question to consider when choosing a connector. Accurate selection and use of electrical connectors is a major aspect of ensuring circuit reliability.

preface 

Electrical connectors (hereinafter referred to as connectors) can also be called plug sockets, which are widely used in various electrical circuits to connect or disconnect circuits. Improving the reliability of the connector is first of all the responsibility of the manufacturer. However, because of the complex variety of connectors and the wide range of applications, accurate selection of connectors is also a major aspect of improving the reliability of connectors. Only through the joint efforts of both the producer and the user can the function of the connector be displayed to the maximum limit.

Connectors have different classification methods. According to the frequency, there are high-frequency connectors and low-frequency connectors; according to the shape, there are circular connectors and rectangular connectors; according to the usage, there are connectors for printed boards, connectors for cabinets, and connectors for audio equipment. Power connector, special-purpose connector, etc. The following first discusses the selection method of low-frequency connectors (frequency below 3MHZ).

Electrical parameter request

Connectors are electromechanical components that connect electrical circuits. Therefore, the electrical parameters of the connector itself is the first question to consider when choosing a connector.

Additional voltage

The additional voltage is also called the operating voltage, which mainly depends on the insulation material used by the connector and the size of the interval between the touch pairs. Some components or devices may not perform their functions when they are lower than their additional voltage. The extra voltage of the electrical connector should in fact be understood as the highest operating voltage recommended by the manufacturer. According to the guidelines, the electrical connector can operate normally at a voltage lower than the additional voltage. The author tends to select the extra voltage reasonably according to the withstand voltage (electric strength) target of the electrical connector, and the operating environment and safety level requirements. In other words, the same withstand voltage target can be applied to different maximum operating voltages according to different application environments and safety requirements. This also contrasts with the objective application situation.

Additional current

The extra current is also called operating current. As with the additional voltage, the electrical connector usually works normally under the condition of lower than the additional current. In the planning process of the electrical connector, the thermal planning of the connector is used to satisfy the additional current request, because the current flow expires in the touch pair, and the touch pair will heat up because of the presence of conductor resistance and touch resistance. When its heat exceeds a certain limit, it will damage the insulation of the electrical connector and the softening of the outer surface coating that constitutes the touch, causing problems. Therefore, to constrain the extra current, in fact, the temperature rise inside the electrical connector should be constrained not to exceed the planned rule value. The question to pay attention to when selecting is: For multi-core electrical connectors, the additional current must be derated. This should be paid more attention to in the case of high current, such as φ3.5mm touch pair, the extra current is usually 50A, but when it is 5 cores, it should be derated by 33%, that is, the extra current per core is only 38A. The more the number, the greater the ups and downs in derating.

Touch resistance

Touch resistance refers to the resistance that occurs when the two touch conductors are touching. Two questions should be noticed when selecting. First, the touch resistance target of the electrical connector is actually the touch resistance, which includes the touch resistance and the touch resistance of the conductor. Usually the conductor resistance is small, so the touch-to-resistance is called touch resistance in many technical specifications. Second, in the circuit connecting small signals, pay attention to the conditions under which the given touch resistance target is tested, because touching the surface will add oxide layer, oil or other contaminants, and the two touch parts will have film on the surface. resistance. When the film thickness is increased, the resistance quickly increases, and the film becomes a poor conductor. However, the film will experience mechanical breakdown under high touch pressure, or electrical breakdown under high voltage and large current. For some small-volume connectors, the planned touch pressure is appropriately small, and the application occasions are only mA and mV levels. The film resistance is not easy to be broken down, which may affect the transmission of electrical signals. One of the touch resistance test methods in GB5095 "Electromechanical Components Fundamental Experimental Procedures and Measurement Methods for Electrical Equipment" is the "touch resistance-millimetric penalty" rule. In order to prevent the insulating film on the touch part from being broken down, the open electromotive force of the circuit is tested. The DC or communication peak value should not be greater than 20mV, and the DC or communication test current should not be greater than 100mA. In fact, this is a test method for low-level touch resistance. Therefore, those who have this request choose electrical connectors with low-level touch resistance targets.

Shielding

In modern electrical and electronic equipment, the density of components and the increasing number of functions among them impose severe restrictions on electromagnetic interference. Therefore, the electrical connector is usually closed with a metal shell to block the internal electromagnetic energy radiation or be disturbed by the external electromagnetic field. At low frequencies, only magnetic materials can significantly shield the magnetic field. At this time, there are certain rules for the electrical continuity of the metal shell, that is, the shell touch resistance.

Safety parameters

Insulation resistance

Insulation resistance refers to the resistance value that a voltage is applied to the insulation of the electrical connector, so that leakage current occurs in or on the outer surface of the insulation. It is mainly affected by factors such as insulation materials, temperature, humidity, and contamination. The insulation resistance value provided on the electrical connector sample is usually the target value under the regulated atmospheric conditions. Under certain environmental conditions, the insulation resistance value may be reduced to an unnecessarily degree. Also pay attention to the experimental voltage value of the insulation resistance. According to the insulation resistance (MΩ) = the voltage applied to the insulator (V)/leakage current (μA), if a different voltage is applied, there will be unnecessary results. In the experiment of electrical connectors, the applied voltage usually has three levels of 10V, 100V, and 500V.

Withstand voltage

The withstand voltage is the critical voltage that is higher than the additional voltage that can be accepted within a regular time between the insulated parts of the touch pair or between the insulated parts and the ground without causing a breakdown appearance. It is mainly affected by the touch interval and creepage interval and geometry, insulator data, ambient temperature and humidity, and atmospheric pressure.

Combustibility

Any electrical connector is inseparable from current during operation, which presents the risk of fire. Therefore, the electrical connector is requested not only to prevent ignition, but also to self-extinguish in a short period of time once it is ignited and fired. When choosing, pay attention to choosing flame-retardant electrical connectors with self-extinguishing insulation materials.

Mechanical parameters

Single-leg separation force and total separation force

The touch pressure in the electrical connector is a main target, which directly affects the size of the touch resistance and the amount of wear of the touch pair. In most configurations, it is reasonably difficult to directly measure touch pressure. Therefore, the touch pressure is usually directly measured by the separation force of a single foot. Regarding the circular pinhole touch pair, a standard pin with a regular weight weight is usually used to test the ability of the female touch to hold the weight. The diameter of the standard pin is usually the lower limit of the diameter of the male touch-5μm. The total separation force is usually twice the sum of the separation force on a single foot. When the total separation force exceeds 50N, manual insertion and removal is now appropriately difficult. Of course, for some test equipment or occasions with special requirements, you can choose zero-plug force electrical connectors, active disconnect electrical connectors, and so on.

Mechanical life

The mechanical life of an electrical connector refers to the mating life, and the usual rule is 500 to 1000 times. When the mechanical life of this rule is reached, the touch resistance, insulation resistance and withstand voltage of the electrical connector should not exceed the rule value. Strictly speaking, today's mechanical life span is a vague concept. The mechanical life should have a certain relationship with time. After 500 times in 10 years and 500 times in 1 year, the situation is obviously different. It's just that there is no more economical and more scientific way to measure it.

Number of touch pairs and pinhole

Firstly, the number of touch pairs can be selected according to the needs of the circuit. At the same time, the size of the electrical connector and the size of the total separation force should be considered. The number of touch pairs is large, of course, the volume is large, and the total separation force is relatively large. In some situations where the reliability is high and the volume is acceptable, two pairs of touch pairs can be used in parallel to improve the reliability of the connection.

In the plug and socket of the electrical connector, the pin (male touch piece) and the jack (female touch piece) can usually be interchangeable devices. In practice, it can be selected according to the live conditions of the plug and socket. If the socket needs to be charged frequently, you can choose a socket with a socket, because the socket with a socket has the live touch parts buried in the insulator, and the human body is not easy to touch the live touch parts, which is relatively safe.

Vibration, shock, bump

The first thing to consider is the electrical continuity of the touch pair when the electrical connector oscillates, impacts, and bumps under regular frequency and acceleration conditions. The touch pair will appear instantaneously disconnected under this dynamic stress condition. The regular instantaneous moments are usually 1μs, 10μs, 100μs, 1ms and 10ms. What should be paid attention to is how to judge the touch to the onset of instantaneous faults. Nowadays, it is generally believed that when the voltage drop at both ends of the closed touch pair (contact) exceeds 50% of the electromotive force of the power supply, it can be judged that the closed touch pair (contact) is faulty. That is to say, there are two conditions for judging whether a transient occurs: duration and voltage drop, both of which are indispensable.

connection method 

The electrical connector is usually composed of a plug and a socket. The plug is also called a free-end electrical connector, and the socket is also called a fixed electrical connector. The connection and disconnection of the circuit is completed through plugs, sockets, insertion and separation, and various connection methods of plugs and sockets have emerged. For circular electrical connectors, there are three main methods: threaded connection, bayonet connection and marble connection. Among them, threaded connection is the most common. It has the advantages of simple processing skills, low production cost, and wide application scale. However, the slow connection speed is not suitable for occasions that require frequent plug-in and fast connection. The bayonet type connection is faster because of the longer lead of the three bayonet slots, but it is more messy to make and the cost is higher. The marble connection is the fastest of the three connection methods. It does not need to be rotated, but only needs to be linearly moved to complete the functions of connection, separation and locking. Because it belongs to the direct push-pull connection method, it is only suitable for electrical connectors with little total separation force. Usually more common in small electrical connectors.

Device method and appearance

The device of the electrical connector includes a front device and a rear device, and the device fixing methods include rivets, screws, clamp rings, or the connector's own bayonet and so on. There is also a kind of plug and socket that are both free-end electrical connectors, so-called relay connectors.

The shape of the connector is ever-changing, and the user first chooses from the straight shape, the curved shape, the outer diameter of the wire or cable, the fixing request with the shell, the volume, the weight, whether it needs to be connected to the metal hose, etc., and the application on the panel The electrical connector should be selected from the aspects of beauty, appearance, color and so on.

Environmental parameters 

Environmental parameters mainly include environmental temperature, humidity, temperature drastic changes, atmospheric pressure and corrosive environment. The environment in which the electrical connector is used, stored, and transported has a significant impact on its function, so it is necessary to select the corresponding electrical connector according to the actual environmental conditions.

Ambient temperature

The metal material and insulation material of the electrical connector determine the operating environment temperature of the electrical connector. High temperature will damage the edge data, resulting in the reduction of insulation resistance and voltage withstand function; for metals, high temperature can make the touch pair lose elasticity, accelerate oxidation and cause plating transformation. The usual ambient temperature is -55~100 ℃. In special occasions, higher temperature may be required.

Moist

Relative humidity greater than 80% is the main cause of electrical breakdown. The humid environment causes the absorption and dispersion of water vapor on the surface of the insulator, which simply reduces the insulation resistance below MΩ. Long-term exposure to high humidity will cause physical deformation, decomposition and escape of products, respiratory effects, electrolysis, and corrosion. And cracks. Especially for electrical connectors outside the equipment, environmental conditions such as humidity, water seepage and pollution are often considered. In this situation, sealed electrical connectors should be selected. Regarding water-tight and dust-tight electrical connectors, GB4208 shell protection grade is usually used to indicate.

Temperature drastic change

Humidity drastic change experiment is to imitate the practical application of using electrical connector equipment in a cold environment to a warm environment, perhaps imitating a situation where the ambient temperature of a spacecraft and detector changes sharply. Sudden changes in temperature may cause cracks or delamination of insulation materials.

Atmospheric pressure 

At high altitudes where the air is thin, plastic emits gas to pollute the touch pair, and increase the tendency of corona to occur, reduce the voltage withstand function, and cause short circuit problems in the circuit. When reaching a certain value at high altitude, the function of the plastic deteriorates. Therefore, when unsealed electrical connectors are used at high altitude, derating is necessary. Refer to Table 2 for the recommended voltage derating factor under low air pressure.

Corrosive environment

According to the different corrosive environment of the electrical connector, the electrical connector of the corresponding metal, plastic, and plating structure is selected, like the electrical connector used in the salt spray environment, if there is no anti-corrosive metal appearance, the function will be quickly deteriorated. In an environment rich in SO2 with a proper concentration, it is not suitable to use silver-plated touch-pair electrical connectors. In hot flash areas, mold is also a major problem.

Termination method

The termination method refers to the method of connecting the touch pair of the electrical connector with the wire or cable. Reasonable selection of termination methods and accurate use of termination skills are also a major aspect of the use and selection of electrical connectors.

Welding

The most common type of soldering is soldering. The most important thing in solder connection is the continuity of the metal between the solder and the soldered surface. Therefore, for electrical connectors, the main thing is solderability. The most common coatings on the soldered ends of electrical connectors are tin alloy, silver and gold. The reed type touch is more common in the welding end including the welding piece type, the punching welding piece type and the notched welding piece type: the pinhole type touch is the most common welding end with a drilled arc notch type.

Crimp

Crimping is a technique for tightening and displacing the metal within the limits of the rules and connecting the wires to the touch pair. A good crimp connection can produce metal mutual melting activity, so that the wire and the touch are deformed symmetrically to the data. This kind of connection is similar to the cold welding connection, which can obtain better mechanical strength and electrical continuity, and it can accept more severe environmental conditions. Nowadays, it is generally believed that it is better to choose an accurate crimping connection than soldering, especially when it is necessary to use crimping in high current situations. Special crimping pliers or active and semi-active crimping machines must be used for crimping. According to the cross section of the wire, the wire barrel with the right touch should be selected accurately. It should be noted that the crimp connection is a permanent connection and can only be used once.

Wrap around

Winding is to wind the wire directly on the winding post with sharp touch parts. When winding, the wire is wound under the condition of controlled tension, and pressed into and fixed on the edge of the winding post of the touch piece to form an air-tight touch. There are several requirements for winding wires: the nominal value of the wire diameter should be within the scale of 0.25mm~1.0mm; when the wire diameter is not greater than 0.5mm, the elongation of the conductor data is not less than 15%; when the wire diameter is greater than 0.5mm, the conductor The elongation of the data is not less than 20%. Winding things include winding guns and fixed winding machines.

Piercing one after another

Piercing connection, also known as insulation displacement connection, is a novel terminal technology created by the United States in the 60s. It has the characteristics of high reliability, low cost, and convenient operation. Now it has been widely used in various electrical connectors for printed boards. in. It is suitable for the connection of ribbon cables. There is no need to strip the insulation layer of the cable during connection. Relying on the top of the "U"-shaped touch reed of the electrical connector to penetrate into the insulation layer, the conductor of the cable slides into the groove of the touch reed and is clamped. The cable conductor and the reed of the electrical connector form a tight electrical connection. It only needs simple things, but it is necessary to use cables with regular wire gauges.

Screw connection

Screw connection is a connection method using screw-type terminals. Pay attention to the maximum and minimum cross-sections of the connecting wires and the maximum tightening torque allowed by different standard screws.