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Explain the working principle and circuit of relay in detail.

What is a relay?

Explain the working principle and circuit of relay in detail. Technical training 2021-11-16 07:301242 Browse 0 comments 0 likes  Wechat search search  Jicheng training Knowledge: Be familiar with the working principle and circuit of relay.  The steps of the electrical project: knowing the process-assigning IO points-selecting the list-drawing the circuit diagram (main circuit and control circuit)-programming and debugging-project handover. In the project process, we will select the relevant models of relays. The application of relays in our main circuit and control circuit is essential to realize the real safe voltage to control non-safe voltage and achieve the safety of people and equipment. Intermediate relay [KA] is mostly used at the output of PLC to protect PLC. It can also be used to convert high-voltage signals into low-voltage signals that can be borne by the input of PLC. The main contactor is mostly used for starting and stopping the main circuit, etc. Learning the principle, characteristics and type selection of relay will be handy in the project process.  01 Characteristics and working principle of relay principle  When the input quantity (such as voltage, current, temperature, etc.) reaches the specified value, the electrical appliances that make the controlled output circuit on or off can be divided into electrical quantity (such as current, voltage, frequency, power, etc.) relays and non-electrical quantity (such as temperature, pressure, speed, etc.) relays, which have the advantages of fast action, stable work, long service life and small size, and are widely used in power protection and automation.  Relay is an electronic control device, which has a control system (also called input loop) and a controlled system (also called output loop). It is usually used in automatic control circuits. It is actually an "automatic switch" that uses a smaller current to control a larger current. Therefore, it plays the role of automatic adjustment, safety protection, switching circuit, etc.  According to the working principle or structural characteristics, relays can be classified into electromagnetic relays, heat-sensitive dry-reed relays, solid-state relays (SSR), etc. These three types are common to us. Common brands of relays are Schneider, Omron, Zhengtai, Hongfa, Panasonic, etc. The common ones are 8-pin or 14-pin, 8-pin: two groups of normally open and two groups of normally closed, one coil contact, 16-pin: four groups of normally open, four groups of normally closed, one

1, the working principle and characteristics of electromagnetic relay  Electromagnetic relay is generally composed of iron core, coil, armature, contact reed, etc. As long as a certain voltage is applied to both ends of the coil, a certain current will flow through the coil, resulting in electromagnetic effect. Under the attraction of electromagnetic force, the armature will overcome the pulling force of the return spring and attract to the iron core, thus driving the moving contact of the armature to engage with the stationary contact (normally open contact). When the coil is powered off, the electromagnetic attraction will also disappear, and the armature will return to its original position under the reaction force of the spring, so that For the normally open and normally closed contacts of relay, it can be distinguished as follows: the static contact in the off state when the relay coil is not energized is called normally open contact; The stationary contact in the on state is called "normally closed contact".

2. Working principle and characteristics of thermal dry reed relay  Heat-sensitive dry reed relay is a new type of heat-sensitive switch that uses heat-sensitive magnetic materials to detect and control temperature. It is composed of a temperature-sensitive magnetic ring, a constant magnetic ring, a reed switch, a heat-conducting mounting piece, a plastic substrate and other accessories. Instead of coil excitation, the heat-sensitive dry reed relay is driven by the magnetic force generated by the constant magnetic ring. Whether the constant magnetic ring can provide magnetic force to reed switch is determined by the temperature control characteristics of the temperature-sensitive magnetic ring.

3. Working principle and characteristics of solid state relay (SSR)  Solid-state relay is a four-terminal device with two terminals as input terminals and the other two terminals as output terminals. The isolation device is used in the middle to realize the electrical isolation of input and output. Solid-state relay can be divided into AC type and DC type according to load power supply type, normally open type and normally closed type according to switch type, and hybrid type, transformer isolation type and photoelectric isolation type according to isolation type, with photoelectric isolation type being the most.  02  Technical parameters of relay main products 1. rated working voltage  It refers to the voltage required by the coil when the relay works normally. Depending on the model of the relay, it can be AC voltage or DC voltage. In practical projects, DC 24V and AC 220V are commonly used.

2. DC resistance  Refers to the DC resistance of the coil in the relay, which can be measured by a multimeter.  3, pull-in current  It refers to the minimum current that the relay can generate the pull-in action. In normal use, the given current must be slightly larger than the pull-in current, so that the relay can work stably. Generally, the working voltage applied to the coil should not exceed 1.5 times of the rated working voltage, otherwise, a larger current will be generated and the coil will be burned.

4. Release the current  It refers to the maximum current generated by the relay to release. When the current in the relay's pull-in state decreases to a certain extent, the relay will return to the non-energized release state, and the current at this time is far less than the pull-in current.

5, contact switching voltage and current  Refers to the voltage and current that the relay is allowed to load, which determines the magnitude of the voltage and current that the relay can control. When in use, it cannot exceed this value, otherwise it is easy to damage the contacts of the relay.

03 Relay test  1. Measure contact resistance.  Measure the resistance of normally closed contact and moving point with the resistance of multimeter, and the resistance should be 0 (the contact resistance can be measured within 100 mω in a more accurate way); However, the resistance between normally open contact and moving point is infinite, so it can be distinguished that one is normally closed contact and the other is normally open contact.

2. Measure the coil resistance.  The resistance of relay coil can be measured by universal meter R×10Ω, so as to judge whether the coil has an open circuit.  3. Measure the pull-in voltage and current.  Find an adjustable regulated power supply and ammeter, input a set of voltages to the relay, and connect the ammeter in series in the power supply circuit for monitoring. Slowly increase the power supply voltage. When you hear the closing sound of the relay, write down the closing voltage and current. For accuracy, you can try several times to get the average value.

4. Measure the release voltage and current.  It is also a connection test as mentioned above. After the relay is pulled in, the power supply voltage is gradually reduced. When the relay releases again, the voltage and current at this time are recorded. You can also try several times to obtain the average release voltage and current. In general, the release voltage of relay is about 10 ~ 50% of the pull-in voltage. If the release voltage is too small (less than 1/10 of the pull-in voltage), it will not work normally, which will threaten the stability of the circuit and make it unreliable.  04  Electrical symbols and contact forms of relays The relay coil is represented by a long square symbol in the circuit. If the relay has two coils, draw two parallel long squares. At the same time, mark the text symbol "J" of the relay in or beside the long box.  There are two ways to express the contact of relay:  One is to draw them directly on the side of the long box, which is more intuitive.  The other is to draw each contact into its own control circuit according to the needs of circuit connection. Usually, the same characters are marked next to the contacts and coils of the same relay, and the contact groups are numbered to show the difference.  There are three basic types of relay contacts:  1. The two contacts of the movable type (H-type) coil are open when they are not energized, and they are closed when they are energized. It is represented by the pinyin prefix "H" of the combined word.

2. When the D-type coil is not energized, the two contacts are closed, and the two contacts are disconnected after energization. It is represented by the phonetic prefix "D" of hyphenation.

3. Switching type (Z type) This is the contact group type. This contact group has three contacts, namely, a moving contact in the middle and a stationary contact on the upper and lower sides. When the coil is not energized, the moving contact and one of the stationary contacts will be disconnected and the other will be closed. After the coil is energized, the moving contact will move, so that the original one will be closed, and the original one will be in an open state to achieve the purpose of switching. Such a contact group is called a switching contact, and the ".  05 Selection of relay  1, first understand the necessary conditions.  A. control the power supply voltage of the circuit and the maximum current it can provide;  B. voltage and current in the controlled circuit;  C. How many groups of contacts are needed by the controlled circuit? When selecting a relay, the power supply voltage of the general control circuit can be used as the basis for selection, and the control circuit should be able to provide sufficient working current for the relay, otherwise the relay will be unstable.

2. After consulting relevant information to determine the conditions of use, you can look up relevant information and find out the model and specification number of the required relay. If you have the relay on hand, you can check whether it can be used according to the information, and finally consider whether the size is appropriate.

3. Pay attention to the volume of the appliance. If it is used for general electrical appliances, besides the chassis volume, the installation layout of circuit boards should be considered for small relays. For small electrical appliances, such as toys and remote control devices, ultra-small relay products should be selected.  06  Application in practical projects 1. Electromagnetic relay is commonly used in general occasions in practical projects, that is, intermediate relay, as it is often said; It is represented by KA in electrical drawings, for example: KA1 represents the first, KA2 represents the second, and so on; The mechanical action life of the contact is generally about 1 million times.

2. In frequent on-off action projects, solid-state relays are commonly used. Solid-state relays realize on-off and off-off switching functions by changing the working state of the circuit, and there are no electrical contact points. Therefore, they have many advantages that ordinary relays do not have, namely, high reliability, long service life, no noise, vibration resistance and mechanical impact resistance.