What is an Electrical Connector?

Catalog

I What is Electrical Connector?

Electrical Connector is an electromechanical component that connects the electrical circuit. It is composed of a fixed female contact (socket), and a free male contact (plug). According to the shape, there are circular electrical connectors and rectangular electrical connectors, and each type can be subdivided based on the connection method.

To select a suitable electrical connector, we should carefully consider their parameters, which include the electrical, safety, mechanical, and environmental parameters. In the future, electrical connectors will develop in many directions, especially in high-speed transmission.

II Electrical Connector Types

1. Circular Electrical Connector

According to the connection method of the electrical connector and the method of fixing the connection position, the connection form of the circular connector mainly includes types of the bayonet (fast), thread, automatic locking, push-pull, straight plug and pull.

Figure 1. Circular Electrical Connectors

(1) Thread Connection

The self-locking feature of the thread is used to connect the plug and the socket. In order to achieve fast connections, some use triple threads. And to ensure the anti-loosening under the vibration and shock after the connection, a fuse set screw or ratchet wheel structure is generally used.

This structure is reliable, easy for production and use, and can ensure the reliability of the product based on the anti-loosening structure.

(2) Bayonet Connection

This type of structure has three pins spaced 120° on the outer periphery of the socket. The matching plug connecting ring is equipped with a suitable three-curve groove, and a loaded spring to ensure safe self-locking.

This structure has two forms: inner three-curve and outer three-curve, as shown in the figure below. It's fast, reliable, and easy to use.

(3) Push-pull Connection

This type of structure has three steel balls with a 120° interval on the outer periphery of the plug. The matching socket has an arc groove. When inserted into place, the steel balls always in the arc groove under the action of the external force of the spring to ensure self-locking. This type of structure can achieve a fast connection.

2. Rectangular Electrical Connector

The connection method of the plug and the socket of the rectangular electrical connector generally has two forms: straight plug and pull and thread locking type.

Figure 2. Rectangular Electrical Connector

(1) Straight Plug and Pull Type

This type of structure mainly relies on the guide mechanism of the plug and socket to connect the product, and its locking structure mainly depends on the external structure. It's mainly used in the connection between printed boards or the connection between the chassis and cabinet.

(2) Thread-Locking Type

This structure adopts the leader pin bushing of the guide mechanism of the plug and socket to achieve the guidance and locking, which mainly used in the connection between printed boards and between printed boards and cables.

III Structure of Electrical Connector

The electrical connector is composed of a fixed female contact (socket), and a free male contact (plug). The socket is fixed(welded) to the electrical components through its square (circular) disk, and the plug is generally connected to the cable. The plug and socket are connected by connecting the nut.

The electrical connector is composed of three basic units: shell, insulator, and contact body.

1. Shell

The electrical connector shell refers to the housing, nut, and tail attachment of the plug and socket. The role of the shell is to protect the internal parts of the insulators and contact bodies from damage. The positioning keyway is for the plug and the socket, and the nuts are used to connect and disconnect the socket. The tail attachment is used to protect the connection point of the wire and the contact body from damage and also used to fix the cable.

The shell also has a certain electromagnetic shielding effect. It is generally made of aluminum alloy through machining, cold extrusion, die casting. Steel shells are mostly used for glass sealing and high-temperature resistant electrical connectors.

Figure 3. Aluminum Alloy Electrical Connector

2. Insulator

The insulator consists of a pin insulator, jack insulator, interface seal body, and line seal body, etc. It is used to keep the pin and jack at the set position and create electrical insulation among each contact body and between the contact body and the shell. The interface seal body and line seal body are added to the insulator to improve the environmental resistance of the electrical connector.

In order to achieve high-temperature and low-temperature resistance, flame retardancy, and ensure the stability of the geometric dimensions of parts, most insulators are molded from thermoset plastic, and the interface seal body and the line seal body are molded from silicone rubber.

Figure 4. Thermoset Plastic Moulding

3. Contact Body

The contact body includes a male contact piece and a female contact piece sometimes called a pin and a jack. And the connection methods are divided into the welding type, the crimping type, the press-in type, and the winding type, etc., to realize circuit connection.

The pin and the jack are the key component of the electrical connector, which directly affects reliability. Most of the pin and jack are machined from elastic copper alloy materials with good conductivity, and the surface is silver-plated or gold-plated to achieve low contact resistance and anti-corrosion. Generally, there are slotted jack, wire spring jack, crown spring jack, and punched jack.

Structural characteristics:

• environment resistance

• bayonet (fast) connection

• multi-key keyway (avoid misinsertion)

• crimp connection of the contact body and wire

The shielding ring ensures 360° electromagnetic interference shielding.

IV Electrical Connector Parameters

Electrical connectors are electromechanical components that connect electrical circuits. Therefore, the electrical parameters of the connector itself are the first issues to be considered when we select the connector.

1. Electrical Parameters

(1) Rated voltage

Rated voltage is also called working voltage. It mainly depends on the insulating material used by the machine and the distance between the contact pairs. Some components or devices may not be able to perform their proper functions when the voltage is below their rated value.

The rated voltage of the connector should in fact be understood as the highest working voltage recommended by the manufacturer. In principle, the connector can work normally under the rated voltage. To reasonably choose the rated voltage, we can consider the voltage resistance (dielectric strength) of the connector, the working environment, and safety requirements. That is to say, with the same withstand voltage index, according to different working environments and safety requirements, we should choose different maximum working voltages.

(2) Rated Current

It's also known as working current. As with the rated voltage, the connector generally works normally under the rated current.

In the connector design process, the thermal design is to meet the rated current requirements. When there is a current flowing through the contact pair, due to the conductor resistance and contact resistance, the contact pair will heat up. When its heat exceeds a certain limit, it will damage the insulation of the connector and soften the surface coating of the contact pairs, causing malfunctions. Therefore, to limit the rated current, in fact, is to limit the temperature rise inside the connector within the rated value.

Figure 5. Multi-Core Electrical Connection

For multi-core electrical connectors, the current must be derated, especially in the case of large currents. For example, the φ3.5mm contact pair is generally specified to have a rated current of 50A, but it needs to be derated by 33% when it is with 5 cores, that is, the rated current of each core is only 38A. The more core, the greater the derating. The derating range can refer to Table 1.

Table 1.

(3) Contact Resistance

Contact resistance refers to the resistance generated by the two conductors at the contact part. There are two issues to be noted.

First, the contact resistance index of the connector is actually the resistance of the contact pair, which includes the contact resistance and the conductor resistance of the connect pair. Usually, the conductor resistance is small, so the contact pair resistance is called contact resistance in many technical specifications.

Second, in the circuit that connects small signals, pay attention to the conditions under which the contact resistance index is tested.

There may be an oxide layer, oil stains, or other contaminants on the contact surface, and the film resistance will be produced on the surfaces of the two contacts. When the thickness of the film increases, the resistance increases rapidly, making the film layer become a poor conductor. However, the film will undergo mechanical breakdown under high contact pressure or electrical breakdown under high voltage and large current.

The contact pressure of some small-volume connectors is quite small, only in mA and mV. The film resistor is not easy to be broken down, which may affect the transmission of electrical signals.

(4) Shielding Property

In modern electrical and electronic equipment, the increasing density of components and related functions between them has imposed strict restrictions on electromagnetic interference. Therefore, the connector is often packed with a metal shell to avoid the internal electromagnetic radiation and the interference from the external electromagnetic field. At low frequencies, only magnetic materials can significantly shield the magnetic field.

2. Safety Parameters

(1) Insulation Resistance

Insulation resistance refers to the resistance value generated by the leakage current in or on the surface of the insulation part when a voltage is applied. It is mainly affected by insulation materials, temperature, humidity, stain, and other factors. The insulation resistance value provided on the electrical connector sample is generally the index value under standard atmospheric conditions. And in some environments, the value will decrease.

We should pay attention to the test voltage value of the insulation resistance. According to the formula:

insulation resistance (MΩ) = voltage applied to the insulator (V)/leakage current (μA),

different voltages will have different results. In the test, the applied voltage is generally 10V, 100V and 500V.

(2) Withstand Voltage

The withstand voltage is the critical voltage that the mutually insulating parts of the contact pairs or parts between the insulating parts and the ground can withstand without breakdown within a specified time, which is higher than the rated voltage.

It is mainly affected by the contact pair distance, creepage distance and geometric shape, insulator material, ambient temperature and humidity, and atmospheric pressure.

(3) Flammability

Electrical connectors can't work without the current, which may cause a fire. Therefore, the connector is required not only to prevent ignition, but also to self-extinguish in a short time once it ignites and catches fire. When choosing, pay attention to the electrical connector with flame retardant, self-extinguishing insulating material.

3. Mechanical Parameters

(1) One-foot Separation Force and Total Separation Force

The contact pressure is an important indicator of the electrical connector, which directly affects the contact resistance and the amount of abrasion loss. In most structures, it is quite difficult to directly measure the contact pressure, so we measure it through the separation force of one foot.

For contact pairs with circular pinholes, a standard pin with a specified weight is usually used to test the clamping ability of the jack. The diameter of the standard pin is -5μm shorter than the lower limit of the pin diameter.

The total separation force is generally twice the upper limit of the one-foot separation force. When the total separation force exceeds 50N, it is quite difficult to plug and pull manually. Of course, for some test equipment or occasions with special requirements, we can choose zero insertion force connector, self-releasing connector, and so on.

(2) Mechanical Life

The mechanical life of the electrical connector refers to the life for plug and pull, which is usually specified as 500 to 1000 times. When reaching the specified mechanical life, the contact resistance, insulation resistance and withstand voltage of the connector should not exceed the specified value.

Strictly speaking, mechanical life is a vague concept. It should have a certain relationship with time. 500 times in 10 years and 500 times in one year is obviously different. It's just that there is no more economical and scientific way to measure it by now.

(3) Number of Contact Pairs and Pinholes

First, the number of contact pairs depends on the needs of the circuit, and at the same time, the connector volume and the total separation force must be considered. The more contact pairs, the larger the volume, and the greater the total separation force. When high reliability is required and the volume is allowed, two contact pairs can be connected in parallel to improve the reliability of the connection.

In the plug and socket of the connector, the pin and the jack can generally be interchanged. For example, if the socket needs to be charged frequently, you can choose the socket with the jack. Because of the socket with the jack, the charged contact is buried in the insulator, which is relatively safe because we can not touch it easily.

Figure 6. The Plug and Socket

(4) Vibration, Impact, and Collision

We should mainly focus on the electrical continuity of the contact pairs under vibration, impact, and collision at specified frequency and acceleration. The contact pair will instantaneously turn off under this dynamic stress. The specified instantaneous turn-off time is generally 1μs, 10μs, 100μs, 1ms, and 10ms.

So how can we identify if there is an instantaneous turn-off failure? It is currently believed that when the voltage drop across the closed contact pair exceeds 50% of the power electromotive force, it can be determined that the contact pair has failed. In other words, there are two conditions for judging whether an instantaneous turn-off occurs: duration and voltage drop, both of which are indispensable.

4. Environmental Parameters

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

(1) Ambient Temperature

The metallic and insulating materials of the connector determine the working temperature of the connector. The high temperature will damage the material, causing a decrease in insulation resistance and pressure resistance. Besides, it can make contact pair lose elasticity, accelerating oxidation and plating deterioration. The general ambient temperature is -55-100℃, which may be more specific on special occasions.

(2) Humidity

Relative humidity greater than 80% is the main cause of the electric breakdown. The humid environment causes the absorption and diffusion of water vapor on the surface of the insulator, which can easily reduce the insulation resistance to below the MΩ level. Long-term exposure to high humidity will cause physical deformation, decomposition, and escape of products, resulting in respiratory effects, electrolysis, corrosion, and crack. Especially for the electrical connectors outside the equipment, moisture, water seepage, and pollution are considered.

(3) Rapid Temperature Change

The rapid temperature change test is to simulate the actual use of the connector device from a cold environment to a warm environment or to simulate the situation of a sudden change in the ambient temperature of a spacecraft or detector. Sudden changes in temperature may cause cracking or layering of the insulating material.

(4) Atmospheric Pressure

At high altitudes with thin air, the plastic emits gas to contaminate the contact pair, and the corona increases, the pressure resistance performance decreases, causing a short circuit fault. When the altitude reaches a certain value, the performance of the plastic deteriorates. Therefore, when unsealed electrical connectors are used at high altitudes, they must be derated. The recommended voltage derating factor at low pressure is shown in Table 2.

Table 2.

(5) Corrosive Environment

We should choose connectors with corresponding metal, plastic, and plating structures according to the corrosive environment of the electrical connector. For example, for connectors used in salt-spray environments, if there is no corrosion-resistant metal surface, the performance will deteriorate rapidly. And in an environment with a considerable concentration of SO2, it is not appropriate to use connectors with silver-plated contact pairs. In hot areas, mold is also an important issue.

V Butt Joint Method

The butt joint method refers to the connection method between the contact pair and the wire or cable. Reasonable selection and proper use of butt joint methods are also important aspects of using and selecting electrical connectors.

1. Welding

The most common type of welding is tin soldering, and the most important thing for it is the continuity between the solder material and the soldered surface. Therefore, for electrical connectors, solderability is important.

Tin alloys, silver, and gold are the most common coatings on the soldering end of connectors. The reed contact pair has a welding end of solder tab, punching solder tab, and notched solder tab type, while the pinhole contact has a drilled arc notch type for the common welding end.

2. Crimping

Crimping is a technique to connect wires to contact pairs for compressing and displacing metal within the limits. Good crimping can produce intermetallic fusion flow so that the wires and contacts material deform symmetrically. This kind of connection is similar to a cold welding connection, which can get better mechanical strength and electrical continuity, and can withstand more severe environmental conditions.

At present, it is generally believed that crimping is better than soldering, especially on high-current occasions. Special crimping tools or automatic or semi-automatic crimping machines must be used in crimping. And the wire barrel should be selected correctly according to the cross-section of the wire. It should be noted that crimping is a permanent connection and can only be used once.

3. Winding

Winding is to wind the wire directly on the winding post of the angular contact. The wire is wound under the tension, and pressed into and fixed at the corners of the winding post to form an airtight contact. There are several requirements for wire winding:

• the nominal value of the wire diameter should be in the range of 0.25mm -1.0mm;

• when the wire diameter ≤ 0.5mm, the elongation of the conductor material is ≥ 15%;

• when the wire diameter is > 0.5mm, the elongation of the conductor material is ≥ 20%.

Tools for winding include wire wrap tools and fixed winding machines.

4. Puncture Connection

Puncture connection, also known as insulation displacement connection(IDC), is a novel end technology invented in the United States in the 1960s with high reliability, low cost, and ease of use. It has been widely used in various electrical connectors for printed boards and is suitable for the connection of ribbon cables.

Figure 7. Example of IDC connectors in a small module

It is not necessary to strip the insulation layer of the cable during the connection, instead, penetrate the tip of the U-shaped contact spring of the connector into the insulation layer to make the cable conductor slide into the groove of the contact spring and be clamped so that a tight electrical connection is formed between them. It requires only simple tools but must use cables with a specified wire gauge.

VI Development Characteristics

The electrical connector is developing in the direction of

● miniaturization, high density, and high-speed transmission;

● high-performance and high-frequency technology;

● high-voltage, high-current electric

● anti-interference technology, modular technology, and lead-free technology.

In the case where the speed of the traditional parallel synchronous digital signal is about to reach its limit, the high-speed serial method is a good solution. This makes low voltage differential signaling (LVDS) become the primary next-generation high-speed signal level standard. The selection of high-speed electrical connectors has also become the main problem to be solved for high-speed signal interconnection.

Several key technologies used in the development of high-speed electrical connectors include:

● The differential signal, noise-free signal, and ground layer technology are used to reduce crosstalk;

● To adjust the lead of the connector, we can change the delay difference caused by the unequal physical distance between the input and output of the connector;

● In order to obtain maximum transmission efficiency, the characteristic impedance value of the connector should match that of the transmission circuit.