Relay basics
First, the relay structure principle Relay is an electronic control device, usually used in automatic control circuit, it is actually a small current to control a large current "automatic switch." Therefore, it plays a role in the circuit such as automatic adjustment, safety protection, and conversion circuit. There are many kinds of relays, such as electromagnetic relays, reed relays, start relays, time-limited relays, DC relays, and AC relays. However, the most widely used electronic circuit is the electromagnetic relay.
In general, electromagnetic relays generally consist of iron cores, coils, armature, contact reeds, etc. As long as a certain voltage is applied to both ends of the coil, a certain current will flow through the coil to generate an electromagnetic effect, and the armature will absorb the pulling force of the return spring against the core under the action of the electromagnetic force to attract the armature. The movable contact and the stationary contact (normally open contact) are attracted. When the coil is de-energized, the electromagnetic suction force also disappears, and the armature returns to the original position at the reaction force of the spring, so that the movable contact is attracted to the original stationary contact (normally closed contact). This sucks and releases so as to achieve the purpose of turning on and off in the circuit. For the relay's "normally open, normally closed" contacts, this can be distinguished by the fact that the relay coil is in the open state when the relay coil is not energized, and is called the "normally open contact"; the static contact in the on state is called It is a "normally closed contact".
In fact, electromagnetic relays can be divided into two kinds of DC and AC. The distinction is as follows: All AC electromagnetic relays have a copper short circuit ring embedded in their cores. The DC relay is not.
Second, the relay technical parameters 1, the rated working voltage is 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.
2. DC resistance refers to the DC resistance of the coil in the relay, which can be measured by a universal meter.
3. The pull-in current refers to the minimum current that the relay can generate 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. The working voltage applied to the coil should not exceed 1.5 times the rated operating voltage, otherwise it will generate a large current and burn the coil.
4. Release current refers to the maximum current generated by the relay to release the action. When the current in the relay pull-in state decreases to a certain degree, the relay will return to the unenergized release state. The current at this time is much less than the pull-in current.
5. Contact switching voltage and current refers to the voltage and current allowed by the relay. It determines that the relay can control the magnitude of the voltage and current and must not exceed this value during use, otherwise it is easy to damage the relay contacts.
Third, the relay test 1, measuring contact resistance with a universal table of resistance files, measuring normally closed contacts and moving point resistance, the resistance should be 0; and normally open contacts and moving points of the resistance is infinite. This can distinguish the normally closed contact, which is the normally open contact.
2. The coil resistance can be measured by the universal table R×10Ω to measure the resistance of the relay coil, so as to judge whether the coil has an open circuit.
3. Measure the pull-in voltage and the pull-in current to find the adjustable regulated power supply and current meter, input a set of voltage to the relay, and monitor the current meter in series in the power supply circuit. Slowly increase the voltage of the power supply. When you hear the sound of the relay, record the pull-in voltage and the pull-in current. For accuracy, try several times and average.
4. The measurement of the release voltage and the release current are also connected and tested as described above. When the relay is pulled in and then the supply voltage is gradually reduced, when the relay is heard to release the sound again, the voltage and current at this time will be recorded. Several more times to achieve 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. The work is not reliable.
IV. Relay's Electrical Symbols and Contact Form Relay coils are represented by a long box symbol in the circuit. If the relay has two coils, draw two parallel long boxes (see Figure 1a, Figure 1b respectively). At the same time, the relay's text symbol “J†is marked in the long box or beside the long box. There are two ways to represent the contacts of the relay: One is to draw them directly on the side of the long box. This representation is more intuitive. The other method is to draw each contact into its own control circuit according to the needs of the circuit connection. Usually, the same letter and symbol are marked on the contacts and the coil of the same relay, and the contact group is numbered. To show differences. There are three basic forms of relay contacts:
1. The two contacts are disconnected when the movable type (H type) coil is not energized. After the energization, the two contacts are closed. This is the Chinese phonetic alphabet "H". See Figure 2a.
2. The two contacts are closed when the disconnection type (D type) coil is not energized, and the two contacts are opened after the energization. Use the phonetic prefix "D" for hyphenation. See Figure 2b.
3. Conversion type (Z type) This is a contact type. This contact group has a total of three contacts, ie, the middle is a moving contact and each of the upper and lower stationary contacts. When the coil is not energized, the movable contact is disconnected from one of the stationary contacts and the other is closed. After the coil is energized, the movable contact is moved, so that the original disconnection is closed, and the original closed state becomes an open state, and the conversion is achieved. purpose. Such contact groups are called transition contacts. It is represented by the phonetic prefix "z" of the word "turn". See Figure 2C.
In addition, a relay can also have one or more contact groups, but none of the above three forms. In the circuit diagram, the method of drawing the contacts and contact sets is always drawn according to the state when no power is applied.
Relay selection:
1. First understand the necessary conditions: 1 control circuit power supply voltage, can provide the maximum current; 2 is the control circuit voltage and current; 3 controlled circuits need several groups, what form of contact. When the relay is selected, 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 operating current to the relay, otherwise the relay attraction is unstable.
2. After consulting the relevant data to determine the conditions of use, you can search for relevant information and find out the type and specification number of the required relay. If you already have a relay on hand, you can check whether it can be used based on the data. Finally consider whether the size is appropriate.
3. Pay attention to the volume of the appliance. If it is used for general electrical appliances, in addition to considering the volume of the chassis, the small relay mainly considers the circuit board installation layout. For small appliances, such as toys, remote control devices should choose ultra-small relay products.
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