Working Principle Of Electronic Relay

When the relay works, the electromagnet is energized, the armature is sucked down to make D and e contact, and the working circuit is closed. The electromagnet loses its magnetism when it is powered off, and the spring pulls up the holding iron and cuts off the working circuit. Therefore, the relay is a switch that uses electromagnets to control the on and off of the working circuit.

The advantages of using relays to control circuits: Using low voltage to control high voltage; Remote control; Auto-Control.

Main technical parameters of relay principle

It can be seen from the above principle that as a very common and safe electrical appliance, although it looks simple, its main technical parameters are not many. To sum up, there are the following items:

Rated working voltage of relay

It refers to the voltage required by the coil when the relay works normally. According to the model of relay, DC voltage is generally used, but AC relay can be AC voltage.

DC resistance of relay

It refers to the DC resistance of the coil in the relay, which can be measured by a three meter.

Contact resistance of relay

It refers to the resistance value after contact in the relay. This resistance is generally very small, so it is not easy to measure with a multimeter. It should be measured with a low resistance meter combined with a four wire measurement method. For many relays, infinite contact resistance or instability is the biggest problem.

Pull in current or voltage of relay

It refers to the minimum current or voltage that the relay can generate pull in action. In normal use, the given current must be slightly greater 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 the rated working voltage, otherwise a large current will be generated and the coil will be burned.

Release current or voltage of relay

It refers to the maximum current or voltage that the relay produces release action. When the current in the on state of the relay decreases to a certain extent, the relay will return to the non energized release state. At this time, the current is far less than the pull in current.

Contact switching voltage and current of relay

It refers to the voltage and current that the relay contact is allowed to carry. It determines the voltage and current that can be controlled by the relay, which cannot be exceeded during use, otherwise it is easy to damage the contact of the relay.

Contact of relay principle

The contact is the most important part of the relay. Their performance is greatly affected by the following factors, such as the material of the contact, the applied voltage and current value (especially the voltage and current waveform when the contact is excited and de excited), the type of load, working frequency, atmospheric environment, contact configuration and jump. If any of these factors cannot meet the predetermined value, problems such as metal electrowinning between contacts, contact welding, wear, or rapid increase in contact resistance may occur.

Contact voltage (AC, DC)

When the relay is disconnected and the inductive load is applied, a relatively high back emf will be generated in the contact circuit of the relay. The higher the back EMF, the greater the damage of the contact. This will cause a serious reduction in the switching capacity of the DC transfer relay. This is because, unlike AC transfer relays, DC transfer relays do not have zero crossing points. Once an arc is generated, it is not easy to weaken, thus prolonging the arcing time. In addition, the unidirectional flow of current in the DC circuit will also cause the contact to produce electrowinning and wear quickly.Although the data as the approximate switching power of the relay is specified in the catalogue or data sheet, it is always necessary to conduct tests under actual load conditions to determine the actual switching power.

Contact current

The amount of current passing through the contact directly affects the performance of the contact. For example, when relays are used to control inductive loads, such as motors or electric lamps, the wear of contacts will be faster, and metal electrowinning will occur more often between mating contacts due to the increase of surge current of contacts. Therefore, in some parts, the contact will not open.

Contact protection circuit

Contact protection circuits designed to prolong the expected life of relays are recommended. Another advantage of this protection is to suppress noise and prevent the generation of carbides and nitric acid, otherwise when the relay contact is opened, they will be generated on the contact surface. However, in addition to the correct design, the protection circuit will have the following adverse effects: such as extending the release time of the relay.

Electrical symbols and contact forms of relay principle

Because the relay is composed of coil and contact group, the graphic symbols of the relay in the circuit diagram also include two parts: a rectangular box represents the coil; A set of contact symbols represents a contact combination. When there are few contacts and the circuit is relatively simple, the contact group is often drawn directly on one side of the coil frame, which is called centralized representation.

If the relay has two coils, draw two parallel rectangular boxes. At the same time, the text symbol "J" of the relay shall be marked in or beside the rectangular box. There are two representations of relay contacts: 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 contact and coil of the same relay, and the contact group is numbered to show the difference. There are three basic forms of relay contacts:

1. When the dynamic closing (normally open) (H-type) coil is not energized, the two contacts are disconnected. After being energized, the two contacts are closed. It is represented by the phonetic prefix "H" of the combined character.

2. When the dynamic breaking (normally closed) (D-type) coil is not energized, the two contacts are closed, and the two contacts are disconnected after being energized. It is indicated by the phonetic prefix "d" of hyphenation.

3. Conversion type (Z type) this is the contact group type. This kind of contact group has three contacts, namely, the middle is a moving contact, and the upper and lower are static contacts. When the coil is not powered on, the moving contact and one of the static contacts are disconnected and the other is closed. After the coil is powered on, the moving contact moves, so that the originally disconnected ones become closed and the originally closed ones become disconnected, so as to achieve the purpose of conversion. Such contact groups are called transfer contacts. It is represented by the phonetic prefix "Z" of the word "Zhuan".

The difference between relay principle and contactor

When it comes to relays, someone will associate them with contactors, perhaps thinking they are the same thing. In fact, their working principle is the same, but there are also electrical differences. It can be distinguished simply by the following points:

First, contactors are used to connect or disconnect loads with high power. When used in the (power) main circuit, the main contact may have interlocking contacts to indicate the opening and closing state of the main contact. Generally, the current passing through the main circuit is larger than that of the control circuit. Contactors with large capacity are generally equipped with arc extinguishing covers.

Second, relays are generally used in electrical control circuits to amplify the contact capacity of miniature or small relays to drive larger loads. For example, the contact of the relay can be used to connect or disconnect the coil of the contactor. Generally, relays have more open and close contacts. Of course, relays can also realize some special functions, such as logic operation, through appropriate connection.

Third, the above two have the same thing: both of them drive the opening and closing of the contact by controlling whether the coil is energized or not, so as to disconnect or connect the circuit. It belongs to electrical appliances with contacts. The control circuit of the coil is electrically isolated from the electrical circuit where the contact is located.

Fourth, trigger generally refers to digital logic devices (such as integrated chips), which realize certain logic functions through external trigger conditions. Such as D trigger, t trigger, J-K trigger, R-S trigger, etc. Simple trigger can also be realized by separating electronic devices. There are many trigger methods, such as rising edge, falling edge, high level and low level.

Fifth, the contact capacity of the relay will generally not exceed 5a, the contact capacity of the small relay is generally only 1A or 2a, and the contact capacity of the contactor is also the smallest 9A; The contacts of contactors usually have three pairs of main contacts (the main contacts are normally open contacts) and several pairs of auxiliary contacts, while the contacts of relays are generally not divided into main and auxiliary contacts; The contacts of the relay are sometimes set in pairs, that is, the normally open contacts and normally closed contacts are combined together, while the contactors are not set in pairs; For specific requirements, the relay will be combined with other devices to design time relay, counter, pressure relay, etc., which has additional functions, while the contactor generally does not.