[Introduction] When selecting a relay, it is essential to understand its technical parameters and characteristics before using it. Otherwise, the safety and reliability of the relay cannot be guaranteed, which may cause the circuit controlled by the relay to be out of control and potentially result in many unnecessary losses.
The selection method for electromagnetic relays is as follows:
① The rated load capacity of the relay contacts should be greater than the load of the controlled circuit. This is because the rated load of an electromagnetic relay is a purely resistive load. Therefore, when selecting one, the characteristics of the controlled circuit should be considered first, and different treatments should be given accordingly. For example, when the load is a small-power AC motor, the relay contact load should be selected to be at least 20% higher than the controlled load; when the load is an incandescent lamp, a purely resistive load, the contact capacity should be selected to be 15% higher than the controlled load; and when the load is a purely inductive or purely capacitive load circuit, the relay contact load should be selected to be 30% higher than the controlled load circuit.
② Types of relay contacts. Relay contact types include single-group contacts, double-group contacts, multi-group contacts, normally open contacts, normally closed contacts, etc. When selecting a relay, the choice should be based on the needs of the load circuit, rather than blindly pursuing multi-group contact relays.
③ Relay power. In actual use, never connect the low-power load contacts of a relay in parallel to control a high-power load circuit. This is because the time it takes for the relay contacts to open and close cannot be exactly the same. Therefore, the group of contacts with the shorter operating time will have to bear a larger power load after parallel connection, inevitably causing damage to that group of contacts.
④ Relay Voltage and Current. When selecting a relay, its rated voltage and pull-in current should meet the circuit design requirements. That is, the rated operating voltage of the relay coil should be selected based on the magnitude of the drive voltage and current. If the drive voltage and current are less than or equal to the rated voltage and current, the relay's normal operation cannot be guaranteed; if the drive voltage and current are greater than or equal to the relay's technical parameters, the relay coil is very likely to burn out.
⑤ Selection of relay power supply voltage. Relays are divided into AC and DC types. The choice between using an AC relay and a DC relay depends on the power supply conditions.
Figure 2 above shows a physical diagram of a reed relay. The structure and selection of reed relays will be explained below. Reed relays can be constructed with glass-encapsulated reeds, plastic-encapsulated reeds, or metal-encapsulated reeds.
Reed relays have two contact types: normally open (H) and changeover (Z). They also come in single, double, and triple control groups. The structure of the changeover contact is shown in Figure 2. Its working principle is very simple: using a permanent magnet or a current-carrying coil to generate a magnetic field, the normally open contact can be changed to a normally closed state to connect the control circuit. Generally, the rated capacity of reed relay contacts is very small, only sufficient for small control currents in control signal circuits. When used as a control circuit signal, there is no rated operating voltage parameter.
Figure 3 above shows a physical diagram of a solid-state relay. When selecting a solid-state relay, the power supply type, voltage, and current of the controlled circuit should be considered to determine the relay's power supply type and its load control capability. If the controlled circuit uses AC power, an AC solid-state relay should be selected; if the controlled circuit uses DC power, a DC solid-state relay should be selected accordingly. The load capacity of the solid-state relay should be determined based on the voltage and current of the controlled circuit. Generally, the output power of the solid-state relay should be 1.5 to 2 times greater than the load power of the controlled circuit. Please note the following points.
① The steady-state current at the output terminal of a solid-state relay must not exceed the rated output current. Almost all loads have inrush current when operating, such as the heating element of a so-called purely resistive load. However, it has a positive temperature coefficient, and its resistance is small at low temperatures, so the instantaneous current is relatively large when it is just starting up.
② Solid-state relays have an input voltage of DC 3~32V, providing a wide control range. The input current of a single-phase DC/AC solid-state relay's control circuit is typically around 10mA. The input current of a three-phase solid-state relay's control circuit is typically around 30mA.
③ The control operating frequency of AC solid-state relays should generally not exceed 10Hz, and the control signal period of DC solid-state relays should be greater than 5 times the sum of the relay's on and off times.
④ The load capacity of a solid-state relay (SSR) is directly related to the ambient temperature. As the ambient temperature rises, the SSR's load capacity decreases. Therefore, sufficient margin should be allowed when selecting the rated current of the SSR. This is especially important to consider when the ambient temperature is high. The above are personal opinions and for reference only.
