Relay knowledge that you must know when working with electronics

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Relay knowledge that you must know when working with electronics [Copy link]

1. Working principle and characteristics of relay When the input quantity (such as voltage, current, temperature, etc.) reaches the specified value, the controlled output circuit is turned on or off. It can be divided into two categories: electrical quantity (such as current, voltage, frequency, power, etc.) relays and non-electrical quantity (such as temperature, pressure, speed, etc.) relays. It has the advantages of fast action, stable operation, long service life and small size. It is widely used in power protection, automation, motion, remote control, measurement and communication devices. Relay is an electronic control device. It has a control system (also known as input circuit) and a controlled system (also known as output circuit). 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, and conversion circuit in the circuit. 1. Working principle and characteristics of electromagnetic relay Electromagnetic relays are generally composed of iron core, coil, armature, contact spring, etc. As long as a certain voltage is applied to both ends of the coil, a certain current will flow through the coil, thus generating an electromagnetic effect. The armature will overcome the pulling force of the return spring under the action of electromagnetic attraction and be attracted to the iron core, thereby driving the moving contact of the armature to close with the static contact (normally open contact). When the coil is de-energized, the electromagnetic attraction disappears, and the armature returns to its original position under the reaction force of the spring, causing the moving contact to close with the original static contact (normally closed contact). In this way, the closing and releasing can achieve the purpose of conducting and cutting off in the circuit. The "normally open and normally closed" contacts of the relay can be distinguished as follows: the static contact in the disconnected state when the relay coil is not energized is called the "normally open contact"; the static contact in the connected state is called the "normally closed contact". 2. Working principle and characteristics of thermal reed relay Thermal reed relay is a new type of thermal switch that uses thermal magnetic materials to detect and control temperature. It consists of a temperature-sensitive magnetic ring, a constant magnetic ring, a reed switch, a heat-conducting mounting plate, a plastic substrate and some other accessories. The thermal reed relay does not use coil excitation, but the magnetic force generated by the constant magnetic ring drives the switch action. Whether the constant magnetic ring can provide magnetic force to the 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. Isolation devices are used in the middle to achieve electrical isolation of input and output. Solid-state relays can be divided into AC and DC types according to the type of load power supply. According to the switch type, they can be divided into normally open type and normally closed type. According to the isolation type, they can be divided into hybrid type, transformer isolation type and photoelectric isolation type, with photoelectric isolation type being the most common. 2. Main technical parameters of relay products 1. Rated working voltage refers to the voltage required by the coil when the relay is working normally. Depending on the model of the relay, it can be AC voltage or DC voltage. 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 refers to the minimum current that the relay can produce a 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. As for the working voltage applied to the coil, it should generally not exceed 1 of the rated working voltage.5 times, otherwise a large current will be generated and the coil will be burned. 4. Release current It refers to the maximum current of the relay to produce a release action. When the current in the relay's energized state decreases to a certain extent, the relay will return to the unenergized release state. The current at this time is much smaller than the energized current. 5. Contact switching voltage and current It refers to the voltage and current that the relay is allowed to load. It determines the magnitude of the voltage and current that the relay can control. It should not exceed this value during use, otherwise it is easy to damage the contacts of the relay. III. Relay test 1. Measure contact resistance Use the resistance range of the multimeter to measure the resistance of the normally closed contact and the moving point. The resistance should be 0 (the contact resistance can be measured within 100 milliohms in a more accurate way); while the resistance of the normally open contact and the moving point is infinite. This can distinguish which is the normally closed contact and which is the normally open contact. 2. Measure the coil resistance You can use the multimeter R×10Ω to measure the resistance of the relay coil to determine whether the coil has an open circuit. 3. Measure the pull-in voltage and pull-in current Find an adjustable voltage-stabilized power supply and an ammeter, input a set of voltages to the relay, and connect an ammeter in series in the power supply circuit for monitoring. Slowly increase the power supply voltage, and when you hear the relay pull-in sound, write down the pull-in voltage and pull-in current. For accuracy, you can try several times and calculate the average value. 4. Measure the release voltage and release current Also connect and test as above. After the relay is energized, gradually reduce the supply voltage. When you hear the relay release sound again, record the voltage and current at this time. You can also try several times to obtain the average release voltage and release current. Under normal circumstances, the release voltage of the 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 cannot be used normally, which will threaten the stability of the circuit and make the work unreliable. 4. Electrical symbols and contact forms of relays The relay coil is represented by a rectangular symbol in the circuit. If the relay has two coils, draw two parallel rectangular boxes. At the same time, mark the relay text symbol "J" in the rectangular box or next to the rectangular box. There are two ways to represent the contacts of the relay: one is to draw them directly on one side of the rectangular 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 text symbols are marked next to the contacts and coils of the same relay, and the contact groups are numbered to distinguish them. There are three basic forms of relay contacts: 1. Dynamic make type (H type) When the coil is not energized, the two contacts are disconnected. After power is turned on, the two contacts are closed. It is represented by the pinyin initial "H" of the word "合". 2. Dynamic break type (D type) When the coil is not energized, the two contacts are closed. After power is turned on, the two contacts are disconnected. It is represented by the pinyin initial "D" of the word "断". 3. Conversion type (Z type) This is a contact group type. This contact group has three contacts, namely the moving contact in the middle and a static contact on the top and bottom. When the coil is not energized, the moving contact is disconnected from one of the static contacts and closed from the other. When the coil is energized, the moving contact moves, making the originally disconnected contact closed and the originally closed contact disconnected, thus achieving the purpose of conversion. Such a contact group is called a conversion contact. It is represented by the pinyin prefix "z" of the word "turn". V. Selection of relays 1. First understand the necessary conditions ① The power supply voltage of the control circuit and the maximum current that can be provided; ② The voltage and current in the controlled circuit; ③ How many groups of contacts and what form of contacts are required for the controlled circuit. When selecting a relay, the power supply voltage of the general control circuit can be used as the basis for selection. The control circuit should be able to provide sufficient working current to the relay, otherwise the relay will be unstable when it is closed. 2. After consulting relevant information to determine the conditions of use, you can search for relevant information to find out the model and specification of the required relay. If you already have a relay, you can check whether it can be used according to the information. Finally, consider whether the size is appropriate. 3. Pay attention to the volume of the device. If it is used for general electrical appliances, in addition to considering the volume of the chassis, small relays mainly consider the layout of the circuit board installation. For small electrical appliances, such as toys and remote control devices, ultra-small relay products should be selected. Classification of relays: There are many ways to classify relays. They can be classified according to the working principle, appearance size, protection characteristics, contact load, product use, etc. 1. Classification by working principle 1. Electromagnetic relay A relay that produces a predetermined response by the relative movement of mechanical parts under the action of the current in the input circuit. It includes DC electromagnetic relays, AC electromagnetic relays, magnetic latching relays, polarized relays, reed relays, and energy-saving power relays. (1) DC electromagnetic relay: An electromagnetic relay whose control current in the input circuit is DC. (2) AC electromagnetic relay: an electromagnetic relay whose control current in the input circuit is AC. (3) Magnetic latching relay: a magnet is introduced into the magnetic circuit. After the relay coil is powered off, the relay armature can still maintain the state when the coil is powered on. It has two stable states. (4) Polarized relay: a DC relay whose state change depends on the polarity of the input excitation. (5) Reed relay: a relay that uses the action of a reed sealed in a tube and has the dual functions of a contact reed and an armature magnetic circuit to open, close or switch the circuit. (6) Energy-saving power relay: an electromagnetic relay whose control current in the input circuit is AC, but it has a large current (generally 30-100A), a small size, and a power-saving function. 2. Solid-state relay A relay whose input and output functions are completed by electronic components without mechanical moving parts. 3. Time relay When the input signal is added or removed, the output part needs to delay or limit to a specified time before closing or disconnecting the controlled circuit. 4. Temperature relay A relay that operates when the external temperature reaches a specified value. 5. Wind speed relay When the wind speed reaches a certain value, the controlled circuit will be connected or disconnected. 6. Acceleration relay When the acceleration of the moving object reaches a specified value, the controlled circuit will be connected or disconnected. 7.Other types of relays Such as photo relays, acoustic relays, thermal relays, etc. 2. According to the size Micro relay: relay with the longest side dimension not exceeding 10mm Ultra-small relay: relay with the longest side dimension greater than 10mm, but not greater than 25mm Small relay: relay with the longest side dimension greater than 25mm, but not greater than 50mm 3. According to the contact load Micro power relay: relay with less than 0.2A. Weak power relay: relay with 0.2~2A. Medium power relay: relay with 2~10A. High power relay: relay with more than 10A. Energy-saving power relay: 20A-100A relay 4. Classification by protection characteristics Sealed relay: A relay with a low leakage rate that uses welding or other methods to seal the contacts and coils in a metal cover. Plastic-sealed relay: A relay with a high leakage rate that uses a sealing method to seal the contacts and coils in a plastic cover. Dust-proof cover relay: A relay that uses a cover to seal the contacts and coils for protection. Open relay: A relay that does not use a protective cover to protect the contacts and coils. 5. Classification by use Communication relay (including high-frequency relay): The contact load range of this type of relay is from low level to medium current, and the environmental conditions for use are not high. Machine tool relay: The relay used in machine tools has large contact load power and long life. Home appliance relay: The relay used in home appliances requires good safety performance. Automotive relay: The relay used in automobiles has large switching load power and high impact and vibration resistance. Small DC relay parameters The main parameters of small DC electromagnetic relays are: 1-Coil DC resistance, which refers to the resistance value of the coil measured by a multimeter. 2-Rated working voltage or rated working current, which refers to the voltage or current value of the coil when the relay is working normally. Sometimes, the manual only gives the rated working voltage or rated working current. At this time, Ohm's law can be used to calculate the rated current or rated voltage value that is not given: that is, /=U/R, U=IxR, R is the DC resistance of the relay coil. 3-Pull-in voltage or current, which refers to the minimum voltage or current when the relay is pulled in. If only the pull-in voltage is applied to the relay coil, the pull-in is not reliable. Generally, the pull-in voltage is about 75% of the rated working voltage. 4-Release voltage or current, which refers to the voltage value when the voltage at both ends of the relay decreases to a certain value and the relay changes from the pull-in state to the release state. The release voltage is much smaller than the pull-in voltage, and generally the release voltage is about 1/4 of the pull-in voltage. 5-Contact load, which refers to the voltage and current value that the relay contacts can withstand when switching.