The relay is equivalent to an isolating switch. For example, 220V AC power is controlled by 5V DC power. The advantage is that when you press the 5V switch, it will not form a loop with 220V, thus ensuring the safety of the operator.
The common one is the voice control lamp in the corridor. The sound sensor module will generate a low level to the delay relay, and the light will be on for 10 seconds.
In short, the first circuit sends a level signal to the relay, so the relay can open or close the second circuit.
Introduction to working principle
There are usually many types of relays, including high-power relay and low-power relay. Their principles are the same. The common relays are shown in Figure 1:
Fig. 1 relays of various models
There are many relays in the market, such as four pin, five pin and even multi pin relays. Although they are used in different circuits, the principles are the same. For example, the internal principles of four pin and five pin relays are shown in Figure 2:
Fig. 2 internal structure principle of relay
As can be seen from the figure, one end of the relay is a coil and the other end is a contact point. Once the coil is energized, the magnetic force of the contact point will close the other end of the relay, so that it is in a working state. A relay with five pins is an additional set of contact points. If it is not energized, pins 4 and 5 are conductive. After the coil is energized, pins 4 and 5 will be disconnected and pins 3 and 4 will be conductive. The more pins a relay has, the more contacts it has. In short, a relay is an electromagnetic switch. As long as the coil is energized, it will generate magnetism, and then the two contacts will be conductive. To judge whether a relay is good or bad, first measure whether its coil is burnt. Use a multi meter to measure the resistance at both ends of the coil. If the resistance of the coil is infinite or the resistance is zero, it means that the coil has been burnt. Then measure whether the two pins of the contact are conductive. Under normal circumstances, if there is a normally open contact in the four pins, the two pins will not be connected. If it is conductive, It means that the contact inside has been burnt out and stuck together.
Practical application
There are many kinds of electromagnetic relays, which can be divided into three categories, namely voltage relay, current relay and intermediate relay. The voltage relay can be divided into over-voltage relay, under voltage relay, time relay, speed relay and so on. Current relay can be divided into over current relay and low current relay. Intermediate relays are divided into high current intermediate relays similar to contactors, ordinary intermediate relays and sealed intermediate relays. These are very common relays. The following is the classification and application of general control electromagnetic relay:
Type |
Action characteristics |
Main purpose |
Voltage relay |
It acts when the coil voltage in parallel with the measured circuit at the power supply side reaches the specified action value |
For over voltage protection and under voltage protection |
Current relay |
It acts when the coil current connected in series with the circuit under test at the power supply side reaches the specified action value |
It is used for overload protection or magnetic field control loss protection of DC motor |
Intermediate relay |
Its essence is voltage relay, but the number of contacts is large |
It is often used for amplifying and multiplexing switching signals |
Time relay |
When the touch signal of the coil is displaced, the action signal output by its contact has a certain delay |
Used for time related control and production process control |
Thermal relay |
Use the thermal bending generated when the overload current flows through the bimetallic sheet to push the control mechanism to act |
Used for overload protection and phase loss protection of AC motor |
Temperature relay |
Used for overload protection and phase loss protection of AC motor |
Used for temperature control and motor overheating protection control |
Speed relay |
Act when the rotation speed reaches the specified value |
Used for energy consumption braking and reverse connection braking of motor
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