Definition of relay: a relay is an automatic control device in which the output quantity will change by leaps and bounds when the input quantity (electricity, magnetism, sound, light and heat) reaches a certain value.

I. working principle and characteristics of the relay

An electrical device that causes the output circuit to be controlled to switch on or off when the input quantity (e.g., voltage, current, temperature, etc.) reaches a specified value. It can be divided into electric quantity relay (such as current, voltage, frequency, power, etc.) and non-electric quantity relay (such as temperature, pressure, speed, etc.). It has the advantages of quick action, stable operation, long service life and small size. Widely used in power protection, automation, movement, remote control, measurement and communication devices.

Relay is an electronic control device, it has a control system (also known as the input loop) and the controlled system (also known as the output loop), usually used in automatic control circuit, it is actually a small current to control the larger current of a "automatic switch". Therefore, it plays the role of automatic regulation, safety protection and conversion circuit in the circuit.

1. 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 added at both ends of the coil, a certain current will flow through the coil, thus generating electromagnetic effect. The armature will overcome the pull of the return spring to attract the core under the action of electromagnetic force, and thus drive the moving contact and the static contact (normally open contact) of the armature to attract.

When the coil is cut off, the electromagnetic suction also disappears, and the armature will return to its original position under the counterforce of the spring, so that the moving contact and the original static contact (normally closed contact) will be sucked together. In this way, the absorption, release, so as to achieve in the circuit on, off the purpose. For the "normally open and normally closed" contact of the relay, it can be distinguished as follows: the static contact in the disconnected state when the relay coil is not energized is called "normally open contact"; The static contact in the on state is called "normally closed contact".

2. Working principle and characteristics of thermally sensitive reed relay

Thermally sensitive reed relay is a new kind of thermally sensitive switch which USES thermally sensitive magnetic material to detect and control temperature. It is composed of a temperature-sensitive magnetic ring, a permanent magnetic ring, a dry reed pipe, a heat-conducting mounting piece, a plastic substrate and other accessories. Thermally sensitive reed relay does not use coil excitation, but the magnetic force generated by permanent magnetic ring drives the switch action. Whether the permanent magnetic ring can provide magnetic force to the reed tube 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 kind of four-terminal device with two terminals as the input terminal and the other two terminals as the output terminal.

Solid state relays can be divided into ac and dc type according to the type of load power supply. According to the type of switch, it can be divided into normally open type and normally closed type. According to the type of isolation can be divided into hybrid type, transformer isolation type and photoelectric isolation type, photoelectric isolation type for the most.

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 type of relay, it may be ac or dc voltage.

2. Dc resistance

It refers to the dc resistance of the coil in the relay, which can be measured by a multimeter.

3. Suction current

Refers to the minimum current that the relay can produce the suction action. In normal use, the given current must be slightly greater than the suction current in order for the relay to operate stably. And the working voltage that adds to to coil, do not exceed 1.5 times of rated working voltage commonly, can produce bigger current and burn out coil otherwise.

4. Release current

Refers to the maximum current generated by the relay for release action. When the current in the suction state of the relay decreases to a certain extent, the relay will return to the unenergized release state. The current is much less than the suction current.

5. Contact switching voltage and current

Refers to the voltage and current allowed to be loaded by the relay. It determines that the relay can control the size of the voltage and current, use can not exceed this value, otherwise it is easy to damage the relay contact.

3. Relay test

1. Test contact resistance

Use the resistance file of the multimeter to measure the resistance of the normally closed contact and the moving point, and the resistance value should be 0 (the resistance value of the contact can be measured within 100 milliohms in a more accurate way); The resistance of the normally open contact and the moving point is infinite. So we can tell which is the normally closed contact and which is the normally open contact.

2. Measure coil resistance

Available the multimeter R x 10 Ω gear measurement relay coil resistance, to judge whether the coil there exists a phenomenon of open circuit.

3. Measure the suction voltage and current

Find adjustable voltage regulator power supply and ammeter, input a set of voltage to the relay, and in the supply circuit into the ammeter for monitoring. Slowly adjust the power supply voltage. When you hear the sound of relay closing, write down the closing voltage and current. For accuracy, you can try it a few times and average it.

4. Measure the release voltage and release current

It is also a connection test like the above. When the relay is engaged, the supply voltage will be gradually reduced. When the relay is heard to emit sound again, write down the voltage and current at this time.

Generally, the release voltage of the relay is about 10 ~ 50% of the snap-in voltage. If the release voltage is too small (less than 1/10 of the snap-in voltage), it cannot be used normally, which will pose a threat to the stability of the circuit and make it unreliable.

4.Electrical symbol and contact form of relay

Relay coils are represented by a long box symbol in the circuit. If the relay has two coils, draw two parallel long boxes. At the same time, the text symbol "J" of the relay shall be marked in or beside the long box.

There are two representations of relay contacts: 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 need of circuit connection. Usually, the contact of the same relay and the coil are marked with the same text symbol, and the contact group is numbered to show the difference.

The contact of the relay has three basic forms:

1. The two contacts are disconnected when the moving and closing type (H type) coil is not energized. After energized, the two contacts will close. It is indicated by the prefix "H" in pinyin.

2. The two contacts are closed when the dynamic breaking type (type D) coil is not energized, and the two contacts are disconnected when the coil is energized. Use the hyphenated pinyin prefix "D".

This is the contact configuration. This kind of contact group has three contacts, namely, the moving contact in the middle and one static contact in the top and bottom. When the coil is not energized, the moving contact is disconnected from one of the static contacts and the other one is closed. After the coil is energized, the moving contact will move to make the original broken into a closed state and the original closed into a disconnected state to achieve the purpose of transformation. Such a set of contacts is called a transition contact. Use the pinyin prefix "z" for "zhuan".

5, the selection of relay

1. Understand the necessary conditions first:

Control circuit power supply voltage, can provide the maximum current;

Voltage and current in the controlled circuit;

The controlled circuit needs a few groups, what form of contact. When selecting the 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 enough working current to the relay, otherwise the relay is not stable.

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

3. Pay attention to the volume of the appliance. If used in general electrical appliances, in addition to consider the chassis volume, small relay mainly consider the circuit board installation layout. For small electrical appliances, such as toys, remote control devices should choose the ultra-small relay products.

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