The coil in the relay is energized, and the moving iron core acts under the action of electromagnetic force to drive the moving contact to separate the normally closed contacts and close the normally open contacts; when the coil is powered off, the moving iron core is driven by the spring The moving contact is reset, and the working principle of the relay is that when a certain input amount (such as voltage, current, temperature, speed, pressure, etc.) reaches a predetermined value, it is made to act to change the working state of the control circuit, thereby achieving the predetermined control Or the purpose of protection. In this process, the relay mainly plays the role of transmitting signals. 
The function of the intermediate relay is usually divided into two parts: the main circuit and the control circuit. The relay is mainly used for the control circuit, and the contactor is mainly used for the main circuit. The relay can realize the function of controlling one or several signals with one control signal. To complete start, stop, and linkage control, the main control object is the contactor; the contactor of the contactor is relatively large and has a strong carrying capacity. It can be used to control weak to strong electricity, and the control object is electrical appliances.
1. The contacts that replace the middle relay of the small contactor have a certain load capacity. When the load capacity is relatively small, it can be used to replace the small contactor, such as the control of electric rolling gates and some small household appliances. This has the advantage that it can not only serve the purpose of control, but also save space and make the control part of the appliance more refined. 
Second, increase the number of contacts This is the most common use of intermediate relays. In the circuit control system, the contact A of a contactor needs to control multiple contactors or other components (contact A in the figure needs to control one contactor and two indicator lights), generally not connected to the form of Figure 1a, because this is not It is good for maintenance (sometimes the capacity of one contact is not enough), but adding an intermediate relay in the circuit will not only change the control form, but also facilitate maintenance.
3. Increasing the contact capacity. We know that although the contact capacity of the intermediate relay is not very large, it also has a certain load capacity. At the same time, the current required for its drive is very small, so the intermediate relay can be used to expand the contact capacity. For example, it is generally not possible to directly use the output of an inductive switch or a transistor to control electrical components with a relatively large load. Instead, an intermediate relay is used in the control circuit, and other loads are controlled by the intermediate relay to achieve the purpose of expanding the control capacity.
4. Conversion contact type In industrial control circuits, such a situation often occurs. The control requires the use of the normally closed contact of the contactor to achieve the control purpose, but the normally closed contact carried by the contactor itself has been used up and the control cannot be completed. task. At this time, an intermediate relay can be connected in parallel with the original contactor coil, and the normally closed contact of the intermediate relay can be used to control the corresponding component, and the contact type can be converted to achieve the required control purpose.
V. Used as a switch In some control circuits, the switching of some electrical components often uses intermediate relays, which are controlled by the opening and closing of their contacts, as shown in Figure 4. Such as the automatic degaussing circuit common in color TV or display, the triode controls the on and off of the intermediate relay, so as to control the degaussing coil on and off.
Sixth, the conversion voltage in the industrial control circuit control circuit voltage is DC24V. The contactor KM2 needs to control the on and off of the solenoid valve KT, and the coil voltage of the solenoid valve is AC220V. If the coil of the solenoid valve is directly connected to the contact of the contactor according to the circuit shown in FIG. 5a, it is not impossible in principle, but the maintenance habits and safety issues are considered. An intermediate relay should be installed in another place to control the solenoid valve. As shown in Figure 5b. In this way, DC and AC, high voltage and low voltage can be separated. It is convenient for future maintenance and safe for use.
7. Eliminate the interference in the circuit In industrial control or computer control circuit, although there are various interference suppression measures, the interference phenomenon still exists more or less. As shown in Figure 6a, the input point 10.1 of the PLC has a line coupling phenomenon, and there is a very small induced current in the line, and the current required for the PLC input is also very small. When the induced current is greater than the current required for the PLC input, it will Make the PLC control malfunction. That is to say, although there is no operation button SB at this time. But the PLC will also output the corresponding action. At this time, a small intermediate relay can be connected in series in the control circuit, and the corresponding control circuit is changed as shown in FIG. 6b. The general induced current will not cause the action of the intermediate relay. Only when the button in the original 10.1 line is operated will the intermediate relay act, giving the PLC a normal input signal, which achieves the purpose of eliminating interference. 
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