Connectors are electromechanical components that connect electrical circuits. Therefore, the electrical parameters of the connector itself are the first consideration when selecting a connector. Correct selection and use of electrical connectors is an important aspect of ensuring circuit reliability. Introduction Electrical connectors (hereinafter referred to as connectors), also known as plugs and sockets, are widely used in various electrical circuits, serving to connect or disconnect circuits. Improving connector reliability is primarily the responsibility of the manufacturer. However, due to the wide variety of connector types and applications, correct selection is also crucial for enhancing connector reliability. Only through the joint efforts of both manufacturers and users can the intended functions of connectors be maximized. Connectors can be classified in different ways. According to frequency, there are high-frequency connectors and low-frequency connectors; according to shape, there are circular connectors and rectangular connectors; according to application, there are connectors for printed circuit boards, connectors for cabinets, connectors for audio equipment, power connectors, special-purpose connectors, etc. The following mainly discusses the selection methods for low-frequency connectors (frequency below 3MHz). Electrical Parameter Requirements Connectors are electromechanical components that connect electrical circuits. Therefore, the electrical parameters of the connector itself are the first consideration when selecting a connector. Rated Voltage Rated voltage, also known as operating voltage, primarily depends on the insulation material used in the connector and the spacing between the contact pairs. Some components or devices may not function properly 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 below their rated voltage. However, it is generally advisable to select the rated voltage based on the connector's withstand voltage (dielectric strength) rating, the operating environment, and safety requirements. That is, for the same withstand voltage rating, different maximum operating voltages can be used depending on the operating environment and safety requirements. This aligns more closely with actual usage. Rated Current Rated current, also known as operating current, is similar to rated voltage. Connectors generally operate normally below their rated current. During connector design, the rated current requirement is met through thermal design. When current flows through the contact pairs, the contact pairs will heat up due to conductor and contact resistance. If this heating exceeds a certain limit, it will damage the connector's insulation and soften the plating on the contact pairs, causing malfunction. Therefore, limiting the rated current effectively means limiting the internal temperature rise of the connector to within the designed limit. A key point to consider when selecting a connector is that the rated current must be dated for multi-core connectors. This is especially important in high-current applications. For example, a φ3.5mm contact pair is generally rated at 50A, but with 5 cores, it must be dated by 33%, meaning each core's rated current is only 38A. The more cores, the greater the derating. See Table 1 for derating ranges. Contact Resistance Contact resistance refers to the resistance generated at the contact point between two conductors. Two points to note when selecting a connector: First, the contact resistance specification is actually the contact pair resistance, which includes both the contact resistance and the resistance of the contact pair conductors. Since conductor resistance is usually lower, the contact pair resistance is often referred to as contact resistance in many technical specifications. Second, in circuits connecting small signals, pay attention to the testing conditions under which the given contact resistance specification was tested. Oxide layers, oil, or other contaminants can adhere to the contact surfaces, creating a film resistance between the two contact surfaces. As the film thickness increases, the resistance increases rapidly, rendering the film a poor conductor. However, the insulating film can undergo mechanical breakdown under high contact pressure or electrical breakdown under high voltage and high current. For some small-volume connectors designed for very low contact pressure, typically in the mA and mV range, the film resistance is less prone to breakdown, potentially affecting signal transmission. GB5095, "Basic Test Procedures and Measurement Methods for Electromechanical Components in Electrical Equipment," specifies that one of the contact resistance test methods, the "Contact Resistance – Millivolt Method," requires that to prevent breakdown of the insulating film on the contact, the DC or AC peak value of the open-circuit electromotive force of the test circuit should not exceed 20mV, and the DC or AC test current should not exceed 100mA. This is essentially a low-level contact resistance test method; therefore, those requiring this should choose connectors with low-level contact resistance specifications. Shielding In modern electrical and electronic equipment, the increasing density of components and their interconnected functions impose strict limitations on electromagnetic interference. Therefore, connectors are often enclosed in metal housings to prevent internal electromagnetic radiation or interference from external electromagnetic fields. At low frequencies, only magnetic materials can provide significant shielding against magnetic fields. In this case, there are certain requirements regarding the electrical continuity of the metal casing, specifically the casing's contact resistance.
