How to select connectors in design?

Electrical connectors (hereinafter referred to as connectors), also known as plug sockets, are widely used in various electrical circuits to connect or disconnect circuits. Improving the reliability of connectors is primarily the responsibility of the manufacturer. However, due to the wide variety and application range of connectors, choosing the right connector is also an important aspect of improving connector reliability.
There are different classification methods for connectors.
According to frequency, there are high-frequency connectors and low-frequency connectors;
According to their appearance, there are circular connected machines and rectangular connected machines;
According to their usage, there are connectors for printed boards, cabinets, audio equipment, power supplies, special purpose connectors, and so on.
Today's main discussion is on how to choose low-frequency connectors (with frequencies below 3MHz)
1, Electrical parameters
A connector is an electromechanical component that connects electrical circuits. Therefore, the electrical parameters of the connector itself are the first consideration when choosing a connector.
rated voltage
The rated voltage, also known as the working voltage, mainly depends on the insulation material used in the connector and the spacing between the contact pairs.
Some components or devices may not be able to perform their intended functions below their rated voltage. The rated voltage of a connector should actually be understood as the maximum operating voltage recommended by the manufacturer. In principle, connectors can operate normally at voltages lower than the rated voltage.
Therefore, we need to select the rated voltage reasonably based on the withstand voltage (electrical strength) index of the connector, the usage environment, and the safety level requirements.
rated current
Rated current, also known as operating current. Like the rated voltage, connectors can generally operate normally below the rated current.
In the design process of connectors, the rated current requirements are met through thermal design of the connector, because when current flows through the contact pair, the contact pair will generate heat due to the presence of conductor resistance and contact resistance. When its heat generation exceeds a certain limit, it will damage the insulation of the connector and cause softening of the surface coating due to contact, resulting in malfunction.
Therefore, to limit the rated current, it is necessary to limit the temperature rise inside the connector to not exceed the specified design value. The issue to note when choosing is that for multi-core connectors, the rated current must be reduced for use.
This should be given more attention in high current situations, such as in the case of a 3.5mm contact pair with a rated current of 50A. However, when using 5 cores, the rated current should be reduced by 33%, meaning that each core has a rated current of only 38A. The more cores there are, the greater the reduction in rating.
contact resistance
Contact resistance refers to the resistance generated by two contacting conductors at the contact area.
When selecting, two issues should be noted:
Firstly, the contact resistance index of a connector is actually the contact to resistance, which includes contact resistance and contact to conductor resistance. Usually, conductor resistance is relatively small, so contact resistance is referred to as contact resistance in many technical specifications.
Secondly, in circuits connecting small signals, it is important to note under what conditions the given contact resistance index is tested, as the contact surface may adhere to oxide layers, oil stains, or other pollutants, resulting in film resistance on the surfaces of the two contact components. As the thickness of the film layer increases, the resistance rapidly increases, making the film layer a poor conductor. However, the film layer may undergo mechanical breakdown under high contact pressure, or electrical breakdown under high voltage and high current.
Shielding (anti-interference)
In modern electrical and electronic equipment, the increasing density of components and their related functions pose strict limitations on electromagnetic interference. So connectors are often enclosed with metal shells to prevent internal electromagnetic radiation or interference from external electromagnetic fields.
At low frequencies, only magnetic materials can provide significant shielding against magnetic fields. At this point, there are certain regulations regarding the electrical continuity of the metal casing, namely the contact resistance of the casing.
2, Safety parameters
insulation resistance
Insulation resistance refers to the resistance value presented by applying voltage to the insulation part of a connector, resulting in leakage current on or within the surface of the insulation part.
It is mainly affected by insulation materials, temperature, humidity, fouling and other factors. The insulation resistance values provided on the connector samples are generally indicators under standard atmospheric conditions, and under certain environmental conditions, the insulation resistance values may decrease to varying degrees.
Also, pay attention to the test voltage value of insulation resistance. Applying different voltages based on insulation resistance (M Ω)=voltage applied to the insulator (V)/leakage current (μ A) results in different outcomes. In connector testing, the applied voltage is generally divided into three levels: 10V, 100V, and 500V.
pressure resistance
Voltage resistance refers to the critical voltage that a contact pair can withstand within a specified time between mutually insulated parts or between insulated parts and ground, which is higher than the rated voltage without causing breakdown. It is mainly affected by the contact distance, creepage distance, geometric shape, insulation material, environmental temperature and humidity, and atmospheric pressure.
flammability
Any connector cannot operate without current, which poses a risk of fire. Therefore, connectors are not only required to prevent ignition, but also to be able to self extinguish in a short period of time in the event of ignition or fire. When selecting, attention should be paid to choosing electrical connectors made of flame-retardant and self extinguishing insulation materials.
3, Mechanical parameters
Contact pressure (single leg separation force and total separation force)
The contact pressure in the connector is an important indicator, which directly affects the size of the contact resistance and the wear of the contact pair.
In most structures, directly measuring contact pressure is quite difficult. Therefore, contact pressure is often indirectly measured by the separation force of a single foot. For circular pinhole contact pairs, standard pins with specified weight weights are usually used to test the ability of the female contact piece to hold the weight. Generally, the diameter of the standard pin is -5 μ m, which is the lower limit of the diameter of the male contact piece.
The total separation force is generally twice the sum of the upper line separation forces of a single foot. When the total separation force exceeds 50N, it is already quite difficult to manually plug and unplug. Of course, for some testing equipment or special requirements, zero insertion force connectors, automatic dropout connectors, and so on can be used.
mechanical life
The mechanical lifespan of connectors refers to the insertion and extraction lifespan, usually specified as 500-5000 times.
When reaching the specified mechanical lifespan, the contact resistance, insulation resistance, and withstand voltage of the connector should not exceed the specified values.
Strictly speaking, the current mechanical lifespan is a vague concept. The mechanical lifespan should have a certain relationship with time, with 500 uses per 10 years and 500 uses per 1 year, obviously the situation is different. However, there is currently no more economical and scientific method to measure it.
Contact with number and pinhole properties
The number of contact pairs can be selected according to the needs of the circuit, while considering the volume of the connector and the total separation force. The larger the number of contact pairs, the larger their volume, and the relatively larger the total separation force. In some cases where high reliability is required and the volume allows, the method of parallel connection of two pairs of contact pairs can be used to improve the reliability of the connection.
In the plugs and sockets of connectors, the pins (male contacts) and sockets (female contacts) are generally interchangeable for assembly. In actual use, the choice can be made based on the live status of the plug and socket at both ends. If the socket needs to be constantly charged, you can choose a socket with a plug-in hole, because the live contacts of the socket with a plug-in hole are buried in the insulation, making it relatively safe for the human body to touch the live contacts.
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Vibration, impact, collision
The main consideration is the electrical continuity of the contact pairs during vibration, impact, and collision of the connector under specified frequency and acceleration conditions.
The phenomenon of instantaneous circuit breaking occurs when in contact with this dynamic stress condition. The prescribed instantaneous break time generally includes 1 μ s, 10 μ s, 100 μ s, 1ms, and 10ms. It is important to pay attention to how to determine the occurrence of instantaneous break faults in contact pairs.
It is now generally believed that when the voltage drop across a closed contact pair (contact) exceeds 50% of the electromotive force of the power supply, it can be determined that the closed contact pair (contact) has malfunctioned. That is to say, there are two conditions for determining whether a transient interruption has occurred: duration and voltage drop, both of which are indispensable.
connection method
A connector is generally composed of a plug and a socket, where the plug is also known as a free end connector and the socket is also known as a fixed connector. The connection and disconnection of circuits are achieved through the insertion and disconnection of plugs and sockets, thus giving rise to various connection methods for plugs and sockets.
For circular connectors, there are mainly three methods: threaded connection, bayonet connection, and self-locking (pin) connection.
Among them, threaded connections are the most common, which have the advantages of simple processing technology, low manufacturing cost, and wide applicability. However, the slow connection speed is not suitable for situations that require frequent insertion and rapid connection.
Due to the longer lead of its three bayonet slots, the bayonet connection has a faster connection speed, but its manufacturing is more complex and the cost is also higher.
The self-locking (pin) connection is the fastest among the three connection methods. It does not require rotational motion, but only linear motion to achieve the functions of connection, separation, and locking. Due to its push-pull connection method, it is only suitable for connectors with low total separation force. It is generally more common in small connectors.
Installation method and appearance
The installation of connectors includes front installation and rear installation, and the installation and fixing methods include rivets, screws, snap rings, or quick locking of the connector itself. There is also a type of plug and socket that are both free end connectors, known as relay connectors.
The appearance of connectors varies greatly, and users mainly choose from straight, curved, outer diameter of wires or cables, requirements for fixing with the housing, volume, weight, and whether to connect metal hoses. For connectors used on panels, users also need to choose from aesthetics, shape, color, and other aspects.
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4, Environmental parameters
The environmental parameters mainly include ambient temperature, humidity, sudden temperature changes, atmospheric pressure, and corrosive environment. The environment in which connectors are used, stored, and transported has a significant impact on their performance, so it is necessary to select appropriate connectors based on actual environmental conditions.
ambient temperature
The metal and insulation materials of the connector determine the working environment temperature of the connector. High temperature can damage the edge material, causing a decrease in insulation resistance and withstand voltage performance; For metals, high temperatures can cause contact to lose elasticity, accelerate oxidation, and cause coating deterioration. The usual ambient temperature is -55~100 ℃, which may be higher in special situations.
damp
Relative humidity greater than 80% is the main cause of electrical breakdown. The humid environment causes the absorption and diffusion of water vapor on the surface of insulators, which can easily reduce the insulation resistance to below the M Ω level. Long term exposure to high humidity environments can cause physical deformation, decomposition, and release of products, resulting in breathing effects, electrolysis, corrosion, and cracking. Especially for external connectors of equipment, environmental conditions such as humidity, water infiltration, and pollution often need to be considered, in which case sealed connectors should be selected. For water tight and dust tight connectors, the shell protection level of GB4208 is generally used to indicate.