How to choose the right connector?

Connector is an electromechanical component that connects electrical lines. Therefore, the electrical parameters of the connector itself are the first considerations for selecting a connector. Proper selection and use of electrical connectors is an important aspect of circuit reliability.
Connectors are widely used in a variety of electrical circuits to function as a connection or disconnect circuit. Improving the reliability of the connector is first and foremost the responsibility of the manufacturer. However, due to the wide variety of connectors and a wide range of applications, proper selection of connectors is also an important aspect of improving connector reliability. Only through the joint efforts of both the manufacturer and the user can the maximum function of the connector be maximized.
The connector has different classification methods according to the frequency, there are high-frequency connectors and low-frequency connectors; according to the shape, there are circular connected machines, rectangular connected machines; according to the use, there are connectors for printed boards, connectors for cabinets , connectors for audio equipment, power connectors, special purpose connectors, etc. The following mainly discusses the selection method of the low frequency connector (the frequency is below 3MHZ).
Electrical parameter requirements
A connector is an electromechanical component that connects electrical lines. Therefore, the electrical parameters of the connector itself are the first considerations for selecting a connector.
Rated voltage

The rated voltage, also known as the operating voltage, depends mainly on the insulation material used in the machine and the spacing between the contact pairs. Some components or devices may not perform their intended functions below their rated voltage. The rated voltage of the connector is in fact understood to be the highest operating voltage recommended by the manufacturer. In principle, the connector works well below the rated voltage. The author tends to reasonably select the rated voltage according to the pressure resistance (electrical strength) of the connector, according to the use environment and safety level requirements. That is to say, the same withstand voltage indicators, according to different use environments and safety requirements, can use different maximum working voltage. This is also in line with objective use.

Rated current

The rated current is also called the operating current. As with the rated voltage, the connector generally works properly at less than the rated current. In the design process of the connector, the rated current requirement is met by the thermal design of the connector, because when there is current flowing through the contact pair, the contact pair will generate heat due to the conductor resistance and the contact resistance. When the heat exceeds a certain limit, the insulation of the connector is broken and the contact is softened to the surface plating, causing malfunction. Therefore, to limit the rated current, in fact, to limit the temperature rise inside the connector does not exceed the design specified value. The problem to be aware of when selecting is: For multi-core connectors, the rated current must be derated. This should be taken more attention in the case of high current. For example, a φ3.5mm contact pair generally requires a rated current of 50A, but a derating of 5 cores. % use, that is, the rated current per core is only 38A, the more the number of cores, the greater the derating. Derating range can be seen below:

Number of cores	1	2	3	4	5	6	7	8	9	10	11	12	13	14	>15
Reduction rate	100	94.3	88.6	82.9	77.1	71.4	65.7	60	54.3	48.6	42.9	37.1	31.4	25.7	20

Contact resistance

Contact resistance refers to the resistance generated by the two contact conductors at the contact portion. Two problems should be noted when selecting. First, the contact resistance index of the connector is actually the contact resistance, which includes the contact resistance and the contact resistance of the conductor. Usually the conductor resistance is small, so the contact pair resistance is called contact resistance in many technical specifications. Second, in the circuit connecting small signals, it should be noted under what conditions the contact resistance index is given, because the contact surface will be attached with oxide layer, oil stain or other contaminants, and the surface resistance of the two contact parts will be generated. . When the thickness of the film layer increases, the resistance rapidly increases, and the film layer becomes a poor conductor. However, the film may undergo mechanical breakdown under high contact pressure, or electrical breakdown may occur at high voltage and high current. The contact pressure of some small-volume connectors is quite small. The use is only mA and mV. The resistance of the film is not easily broken down, which may affect the transmission of electrical signals. One of the contact resistance test methods in GB5095 "Basic test procedures and measurement methods for electromechanical components for electrical equipment" contact resistance - millivolt method "specified, in order to prevent the insulation film on the contact member from being broken down, the open circuit electromotive force of the test circuit is DC Or the AC peak should be no more than 20mV, the DC or AC test current should be no more than 100mA. In fact, this is a low-level contact resistance test method. Therefore, the selector with this requirement is selected by the low level. Connector that contacts the resistance indicator.

Shielding
In modern electrical and electronic equipment, the density of components and the increasing correlation between them have imposed strict limits on electromagnetic interference. Therefore, the connector is often closed with a metal casing to prevent internal electromagnetic energy radiation or interference from external electromagnetic fields. At low frequencies, only magnetic materials can significantly shield the magnetic field. At this time, there is a certain regulation on the electrical continuity of the metal casing, that is, the casing contact resistance.

Safety parameter
    Insulation resistance

The insulation resistance refers to a resistance value which is exerted by applying a voltage to the insulating portion of the connector to cause a leakage current in or on the surface of the insulating portion. It is mainly affected by insulation materials, temperature, humidity, and fouling. The value of the insulation resistance provided on the connector sample is generally an index value under standard atmospheric conditions. Under certain environmental conditions, the insulation resistance value may be reduced to an unnecessary degree. Also pay attention to the test voltage value of the insulation resistance. According to the insulation resistance (MΩ) = voltage (V) / leakage current (μA) applied to the insulator, different voltages are applied, and there is an unnecessary result. In the test of the connector, the applied voltage is generally 10V, 100V, 500V third gear.

Withstand voltage

Withstand voltage is the critical voltage between the mutually insulated parts of the contact pair or between the insulated part and the ground, which can withstand higher than the rated voltage within a specified time without causing breakdown. It is primarily affected by contact versus spacing and creepage distance and geometry, insulator materials as well as ambient temperature and humidity, and atmospheric pressure.

Flammability

Any connector can't be insulated from current when it's working, which creates a fire hazard. Therefore, the connector is required not only to prevent ignition, but also to be self-extinguishing in a short time upon ignition and fire. When selecting, pay attention to the choice of electrical connectors with flame-retardant and self-extinguishing insulation materials.

Mechanical parameters
Single foot separation force and total separation force

Contact pressure in the connector is an important indicator that directly affects the size of the contact resistance and the amount of wear on the contact pair. In most configurations, direct measurement of contact pressure is quite difficult. Therefore, the contact pressure is often measured indirectly by the separation force of one foot. For circular pin contact pairs, the standard pin with a specified weight is usually used to verify the ability of the female contact to hold the weight. Generally, the diameter of the standard pin is the lower limit of the diameter of the male contact, which is -5 μm. The total separation force is generally twice the sum of the single-leg separation forces. When the total separation force exceeds 50N, manual insertion and removal has been quite difficult. Of course, for some test equipment or some special requirements, you can use a zero insertion force connector, automatic disconnect connector and so on.

Mechanical life
The mechanical life of the connector refers to the insertion and removal life, usually specified as 500 to 1000 times. When the specified mechanical life is reached, the contact resistance, insulation resistance and withstand voltage of the connector should not exceed the specified values. Strictly speaking, the current mechanical life is a vague concept. Mechanical life should have a certain relationship with time. After running out of 500 times in 10 years and 500 times in 1 year, it is obviously different. But there is no more economical and scientific way to measure it.
The number of contact pairs and the number of signals (the system is not allowed to be modified)
It is preferred to select the number of contact pairs according to the needs of the circuit, taking into account the size of the connector and the total separation force. The number of contact pairs is large, of course, the volume is large, and the total separation force is relatively large. In the case where certain reliability requirements are high and volume is allowed, two pairs of contacts can be used in parallel to improve the reliability of the connection.
The connector plug, the socket, the pin (male contact) and the jack (female contact) are generally interchangeable. In actual use, it can be selected according to the charging condition at both ends of the plug and the socket. If the socket needs to be charged frequently, you can choose the socket with the jack. Because the socket with the socket is buried in the insulator, the human body is not easy to touch the live contact, which is relatively safe.

Vibration, shock, collision

Mainly consider the electrical continuity of the contact, vibration, impact, and contact pair of the connector under specified frequency and acceleration conditions. Contact is a phenomenon that instantaneous disconnection occurs under the condition of dynamic stress. The specified momentary break times are typically 1 μs, 10 μs, 100 μs, 1 ms and 10 ms. It is important to note how the contact pair is faulted. It is generally believed that when the voltage drop across the closed contact pair (contact) exceeds 50% of the source's electromotive force, it can be determined that the closed contact pair (contact) has failed, that is, whether there is a transient or not. There are two conditions: duration And voltage drop, both are indispensable.

Connection method

The connector generally consists of a plug and a socket, wherein the plug is also called a free end connector, and the socket is also called a fixed connector. The connection and disconnection of the circuit is achieved by plugs, sockets and plug-in and disconnection, thus creating various connections for the plug and socket. For round connectors, there are three main types: threaded connection, bayonet connection and marble connection. Among them, the threaded connection is the most common, and it has the advantages of simple processing technology, low manufacturing cost, wide application range, etc., but the slow connection speed is not suitable for occasions requiring frequent insertion and removal and rapid connection. The bayonet type connection has a faster lead because the lead of the three bayonet slots is longer, but it is more complicated to manufacture and higher in cost. The pin-type connection is the fastest connection type among the three connection modes. It does not require a rotary motion, and the function of connection, separation and locking can be realized by linear motion. Since it is a straight push-pull connection, it is only suitable for connectors with a small total separation force. Generally more common in small connectors.

Installation method and shape
The connector is installed with front and rear mounting. The mounting and fixing methods include rivets, screws, collars or quick locks on the connector itself. There is also a plug and socket which are both free end connectors, so-called relay connectors.

The shape of the connector is ever-changing. The user mainly chooses from the straight shape, the curved shape, the outer diameter of the wire or cable and the fixing requirements of the outer casing, the volume, the weight, whether the metal hose needs to be connected, etc., and is used on the panel. Connectors should also be selected from the aspects of aesthetics, shape, color and so on.

Environmental parameters

Environmental parameters mainly include ambient temperature, humidity, temperature change, atmospheric pressure and corrosive environment. The connector is used and stored, and the environment in which it is transported has a significant impact on its performance, so the corresponding connector must be selected according to the actual environmental conditions.

Ambient temperature

The metal material and the insulating material of the connector determine the working environment temperature of the connector, which will damage the edge material and cause the insulation resistance and pressure resistance to decrease. For the metal, the high temperature can lose the elasticity of the contact pair, accelerate the oxidation and cause the deterioration of the coating. The usual ambient temperature is -55 to 100 °C and may be required for higher occasions.

Moist

Relative humidity greater than 80% is the cause of electrical breakdown. The humid environment causes the absorption and diffusion of water vapor on the surface of the insulator, which tends to reduce the insulation resistance below MΩ. If it is exposed to high humidity for a long time, it will cause physical deformation, decomposition, escape of products, respiratory effects and electrolysis, corrosion. And cracks. Especially for connectors on the outside of the equipment, it is often necessary to consider the environmental conditions of moisture, water seepage and pollution. In this case, a sealed connector should be used. For watertight, dust-tight connectors are generally indicated by the enclosure rating of GB4208.

Rapid temperature change

The humidity jerk test simulates the actual use of a connector device in a cold environment to enter a warm environment, or simulates a spacecraft, where the ambient temperature of the detector changes drastically. Rapid temperature changes may cause cracks or delamination of the insulating material.

Atmospheric pressure

In the air at a thin air, the plastic emits gas to contaminate the contact pair, and the tendency of the corona is increased, the withstand voltage performance is lowered, and the circuit is short-circuited. When a certain value is reached at a high altitude, the performance of the plastic deteriorates. Therefore, when using a non-sealed connector at high altitude, it must be derated.

Corrosive environment

Depending on the connector used in a corrosive environment, the choice of metal, plastic, coated connectors, like connectors used in salt spray environments, can quickly deteriorate if there is no corrosion-resistant metal surface. In environments containing a significant concentration of SO2, it is not advisable to use a silver-plated contact pair connector. Mold is also an important issue in hot flashes.

Termination method

Termination refers to the way the connector's contact is connected to a wire or cable. Proper selection of termination methods and proper use of termination techniques are also an important aspect of using and selecting connectors.

Welding

The most common soldering is soldering. The most important thing for solder joints is the continuity of the metal between the solder and the surface to be soldered. Therefore, it is important for the connector to be solderable. The most common coating on the soldered end of the connector is tin alloy. Silver and gold reed contacts have solder tabs for common solder joints, punched and notched solder tabs: ZK contacts have drilled holes for common solder joints Arc notch type.

Crimp

Crimp is a technique for compressing and displacing metal within specified limits and connecting the wires to the contact pairs. A good crimp connection creates a metal interflow flow that causes the wires and contacts to symmetrically deform the material. This connection is similar to a cold welded joint and provides better mechanical strength and electrical continuity, and it can withstand harsher environmental conditions. It is generally accepted that the correct crimp connection is better than soldering, especially in high current applications where crimping is required. Special crimping pliers or automatic, semi-automatic crimping machines are required for crimping. The wire barrel of the contact pair should be correctly selected according to the wire cross section. It should be noted that the crimp connection is a permanent connection and can only be used once.

Winding

Winding is to wrap the wire directly around the angular contact wrap. When wound, the wire is wound under tension, pressed and fixed at the corners of the contact wrap to form gas. There are several requirements for dense contact wrap wire: the nominal diameter of the wire diameter should be in the range of 0.25 mm to 1.0 mm; when the wire diameter is not more than 0.5 mm, the elongation of the conductor material is not less than 15%; the wire diameter is greater than 0.5 mm. When the conductor material has an elongation of not less than 20%. Wound tools include a gun and a stationary wrap.

Piercing

Puncture connection, also known as insulation displacement connection, is a novel end technology invented by the United States in the 1960s. It has the characteristics of high reliability, low cost and convenient use. It has been widely used in various connectors for printed boards. . It is suitable for the connection of ribbon cables. It is not necessary to strip the insulation of the cable when connecting, relying on the tip of the "U" shaped contact spring of the connector.