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The principle of operation and characteristics of relay (relay)


One, relay (relay) working principle and characteristics

When an input value (such as voltage, current, temperature, etc.) reaches set value, controlled output circuit turns on or off. It can be divided into two categories: electrical (such as relays for current, voltage, frequency, power, etc.) and non-electrical (such as relays for temperature, pressure, speed, etc.). It has advantages of fast action, stable performance, long service life and small size. Widely used in power protection, automation, motion, remote control, measurement and communication applications.

A relay is an electronic control device that has a control system (also known as an input circuit) and a controlled system (also known as an output circuit), commonly used in automatic control circuits, is actually a small kind of "automatic switch" that controls large current. Therefore, it plays role of automatic adjustment, protective protection and conversion circuit in circuit.

1.The principle of operation and characteristics of electromagnetic relay

An electromagnetic relay usually consists of an iron core, a coil, an armature, a contact tongue, and so on. As long as a certain voltage is applied to both ends of coil, a certain current will flow through coil, thereby creating an electromagnetic effect. The moving contact and static contact (normally open contact) stick together. When coil is de-energized, electromagnetic attraction will also disappear and armature will return to its original position under reaction force of spring, so that moving contact and original static contact (normally closed contact) will attract. Thus, it is attracted and released to achieve purpose of conducting and breaking in circuit. For "normally open and normally closed" relay contacts, it can be distinguished in this way: a static contact in off state, when relay coil is not energized, is called a "normally open contact"; static contact in on state The condition is called "Normally Closed Contact".

2.The principle of operation and characteristics of thermal reed switch

Thermal Reed Switch is a new type of thermal switch that uses thermomagnetic materials to detect and control temperature. It consists of a temperature-sensitive magnetic ring, a permanent magnetic ring, a dry reed switch, a thermally conductive mounting sheet, a plastic substrate and other accessories. The thermal reed switch does not use coil excitation, but magnetic force generated by permanent magnetic ring controls switch action. Whether permanent magnetic ring can impart magnetic force to reed switch is determined by temperature control characteristics of thermosensitive magnetic ring.

3.The principle of operation and characteristics of a solid state relay (SSR)

The solid state relay is a four-terminal device with two terminals as input terminals and two other terminals as output terminals. In middle, an isolating device is used to ensure electrical isolation of input and output. Solid state relays can be divided into AC types and DC types according to type of load power supply. According to type of switch, it can be divided into normally open and normally closed. According to insulation type, it can be divided into hybrid type, transformer insulation type and photovoltaic insulation type, and photovoltaic insulation type is most common.

2. Technical parameters of main relays

1.Rated operating voltage

refers to voltage required by coil when relay is operating normally. Depending on type of relay, it can be AC ​​voltage or DC voltage.

2.DC resistance

refers to DC resistance of relay coil, which can be measured with a multimeter.

3.Trip current

refers to minimum current that relay can generate with a pull-in action. In normal use, set current should be slightly larger than pull-in current so that relay can work stably. As for operating voltage applied to coil, generally, it should not be more than 1.5 times rated operating voltage, otherwise a large current will be generated and coil will burn out.

4.Current version

refers to maximum current at which relay has an opening action. When current in relay pull-in state decreases to a certain extent, relay will return to open state without power. At this time, current is much less than pull-in current.

5.Contact switching voltage and current

refers to voltage and current that relay can load. It determines amount of voltage and current that relay can control, and cannot exceed this value when used, otherwise it is easy to damage relay contacts.

3. Relay test

1.Contact resistance measurement

Use multimeter resistance file to measure resistance of normally closed contact and moving point, and resistance value should be 0. The resistance value of point and moving point is simply infinite. From here it is possible to distinguish which contact is normally closed and which is normally open.

2. Coil resistance measurement

The resistance value of relay coil can be measured with an R×10 ohm multimeter to determine if there is an open circuit in coil.

3.Measuring input voltage and input current

Find an adjustable regulated power supply and an ammeter, enter a set of voltages into relay, and connect ammeter in series with power circuit for monitoring. Slowly increase supply voltage and record operating voltage and current when you hear sound of relay operating. For accuracy, you can try several times and calculate average value.

4.Release voltage and current measurement

The connection test is same as described above. When relay is activated, reduce supply voltage gradually. When you hear relay release sound again, record voltage and current at that time. You can also try several times to get average dropout voltage and dropout current. In general, release voltage of relay is about 10-50% of pull-in voltage. If release voltage is too low (less than 1/10 of pull-in voltage), it cannot be used normally, which will endanger stability of circuit. , works unreliably.

Fourth, electrical symbol and relay contact form

First, electric relay symbol

The relay coil is indicated on diagram by a rectangular rectangle. If relay has two coils, draw two parallel rectangular rectangles. At same time, mark text symbol "J" of relay in or next to rectangular box. There are two ways to draw relay contacts: one is to draw them right on side of rectangular box, which is more intuitive. The other is to involve each pin in its own control scheme according to needs of circuit connection. Usually, same text symbols are marked on contacts and coils of same relay, and groups of contacts are numbered. To show difference.

Second, three basic forms of relay contacts

1. Moving Closing Type (Type H) Two contacts open when coil is not energized and two contacts close when coil is energized. It is represented by pinyin "H" prefix of ligature.

2. Dynamic interrupt type (type D). Two contacts are closed when coil is not energized, and two contacts open after energization. It is represented by word-breaking pinyin prefix, "D".

3. Conversion type (type Z) This is contact group type. This contact group has only three contacts, that is, a moving contact in middle and a static contact at top and bottom. When coil is not energized, moving contact and one of static contacts open while other closes. After coil is energized, moving contact moves, causing original disconnected to become closed and initially closed to become open, reaching switch point Target. Such a set of contacts is called a switching contact.ohm. It is represented by pinyin "z" prefix of word "turn".

Five, relay selection

1. Read prerequisites first

①Power supply voltage of control circuit and maximum current that it can provide;②Voltage and current in controlled circuit;③How many and what forms of contacts are needed for controlled circuit. When choosing a relay, supply voltage of common control circuit can be used as basis for selection. The control circuit must provide sufficient operating current for relay, otherwise relay operation will be unstable.

2. After reading relevant materials and confirming terms of use, you can search for relevant materials and find out what you need for model and specification number of desired relay. If relay is already at hand, you can check if it can be used according to data. Finally, consider if size is right.

3. Pay attention to volume of device. If it is used for general electrical appliances, in addition to taking into account volume of case, small relay mainly takes into account location of printed circuit board. For small electrical appliances such as toys and remote control devices, ultra-small relay products should be used.

6. Relay classification

There are many methods for classifying relays, which can be classified according to principle of operation, external dimensions, protection functions, contact load, product use, etc.

1. According to principle of action

1.Electromagnetic relay

Under influence of current in input circuit, a relay is activated, which forms a given response due to relative movement of mechanical parts. It includes DC electromagnetic relays, AC electromagnetic relays, magnetic latching relays, polarized relays, reed relays and energy-saving power relays.

DC electromagnetic relay: an electromagnetic relay whose control current in input circuit is constant.

AC electromagnetic relay: an electromagnetic relay whose control current in input circuit is alternating.

Magnetic latching relay: magnetic steel is introduced into magnetic circuit. After relay coil is disconnected, relay armature can maintain state when coil is energized and has two stable states.

Polarization Relay: A DC relay whose state changes depending on polarity of input excitation.

Reed Switch: A relay that utilizes action of a reed switch sealed in a tube that doubles as a contact reed switch and an armature magnetic circuit to open, close, or switch circuit.

Energy-saving power relay: control current in input circuit is an electromagnetic relay with alternating current, but its current is large (generally 30-100A), small in size and energy-saving functions.


A relay whose input and output functions are performed by electronic components with no mechanical moving parts.

3.Time relay

When an input signal is added or removed, output part must delay or time limit to a specified time before closing or opening controlled circuit relay.

4.Temperature switch

Relay that is activated when outdoor temperature reaches set value.

5.Wind speed switch

When wind speed reaches a certain value, controlled circuit will be connected or disconnected.

6.Acceleration relay

When acceleration of moving object reaches set value, controlled circuit will turn on or off.

7.Other relay types

For example, photo relays, acoustic relays, thermal relays, etc.

Second, by size

Miniature Relay: A relay whose longest side does not exceed 10mm.

Subminiature Relays: Relays whose longest side is greater than 10mm but not greater than 25mm.

Small Relays: Relays whose longest side is greater than 25mm but not greater than 50mm.

Three, depending on load on contacts

Micro power relay: less than 0.2A relay. Low power relay: 0.2~2A relay. Medium Power Relay: Relay 2~10A. Powerful relays: relays over 10A. Energy saving power relay: relay 20A-100A.

Fourthly, on protection functions

Sealed Relay: A low leakage relay that uses welding or other methods to seal contacts and coils in a metal case.

Relay with plastic seal: contact and coil are sealed in a plastic cover with sealing adhesive, and relay has a high leakage rate. Dust Relay: A relay that uses a cover to protect contacts and coils. Open relay: a relay that does not use a protective cover to protect contacts, coils, etc.

Five, by use

Communication relays (including high frequency relays): The contact load range of this type of relay is from low level to medium current, and operating conditions are not demanding. Machine relay: The relay used in machine tool has a large contact load capacity and a long service life. Relays for household appliances: Relays used in household appliances require a good safety record. Automotive relays: Relays used in automobiles, this type of relay has a high switching load, high shock resistance and vibration resistance.

Seven small DC relay parameters

Basic parameters of a small DC electromagnetic relay:

1. DC coil resistance refers to coil resistance value measured by a multimeter. 2. Rated operating voltage or ratedrated operating current, which refers to voltage or current value of coil when relay is operating normally. Sometimes in manual only rated operating voltage or rated operating current is given, and then value of rated current or rated voltage that is not given can be calculated using Ohm's law: i.e. / \u003d U / R, U \u003d IxR, R is resistance of relay coil to direct current .

3. Pull-in voltage or current, which refers to minimum voltage or current when relay generates pull-in. If only pull-in voltage is applied to relay coil, pull-in at this time is not reliable. Typically, starting voltage is about 75% of rated operating voltage.

4. Release voltage or current refers to voltage value when relay changes from pull-in state to release state when voltage across relay decreases to a certain value. The release voltage is much less than pull-in voltage, typically release voltage is about 1/4 of pull-in voltage. 5.Contact load refers to amount of voltage and current that relay contacts can withstand when switching.