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How the Relay Works

A relay is an electronic control device that has a control system (also called an input loop) and a controlled system (also called an output loop). It is usually used in an automatic control circuit. It actually uses a small current to control the larger An “automatic switch” of current. Therefore, it plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.

What is a relay?

Subject: Electricity (a subject); Relay protection and automation (two subjects)

A relay is an electronic control device that has a control system (also called an input loop) and a controlled system (also called an output loop). It is usually used in an automatic control circuit. It actually uses a small current to control the larger An “automatic switch” of current. Therefore, it plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.

Definition of relay: A relay is an automatic control device that changes the output when the input quantity (electricity, magnetism, sound, light, heat) reaches a certain value.

A relay is an electronic control device that has a control system (also called an input loop) and a controlled system (also called an output loop). It is usually used in an automatic control circuit. It actually uses a small current to control the larger An “automatic switch” of current. Therefore, it plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.

Several functions of the relay:

  • 1) Expand the scope of control. For example, when the multi-contact relay control signal reaches a certain value, the multi-circuit circuit can be switched, disconnected, and turned on at the same time according to different forms of the contact group.
  • 2) Zoom in. For example, sensitive relays, intermediate relays, etc., can control very high power circuits with a very small amount of control.
  • 3) Integrated signal. For example, when a plurality of control signals are input to a multi-winding relay in a prescribed form, the comparison is integrated to achieve a predetermined control effect.
  • 4) Automatic, remote control, monitoring. For example, a relay on an automated device, together with other appliances, can form a program control circuit for automated operation.

The working principle and characteristics of electromagnetic relay

Electromagnetic relays are generally composed of a core, a coil, an armature, a contact spring, and the like. As long as a certain voltage is applied to both ends of the coil, a certain current flows in the coil, thereby generating an electromagnetic effect, and the armature will absorb the pulling force of the return spring against the iron core under the action of the electromagnetic force attraction, thereby driving the armature. The moving contact is in contact with the stationary contact (normally open contact). 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 static contact (normally closed contact). This is sucked and released, thereby achieving the purpose of turning on and off in the circuit. For the “normally open, normally closed” contacts of the relay, it can be distinguished as follows: a static contact that is in an open state when the relay coil is not energized, called a “normally open contact”; a static contact that is in an on state It is a “normally closed contact”.

Principle and structure of solid state relay

SSR can be divided into AC type and DC type according to the use occasion. They are used as load switches on AC or DC power supply, and cannot be mixed.

The AC SSR is taken as an example to illustrate its working principle. Figure 1 is a block diagram of its working principle. The components 1-4 in Figure 1 form the main body of the AC SSR. As a whole, the SSR has only two inputs ( A and B) and two outputs (C and D) are four-terminal devices.

Principle and structure of solid state relay
Principle and structure of solid state relay

As long as a certain control signal is added to A and B during operation, the “on” and “off” between C and D terminals can be controlled to realize the function of “switch”, wherein the function of the coupling circuit is A, The control signal input at the B terminal provides a channel between the input/output terminals, but electrically disconnects the (electrical) connection between the input terminal and the output terminal in the SSR to prevent the output terminal from affecting the input terminal, and the coupling circuit The component used is an “optocoupler” which is sensitive, has a high response speed, and has a high level of insulation (withstand voltage) between the input/output terminals. Since the load at the input is a light-emitting diode, this makes the input of the SSR easy. It matches the input signal level and can be directly connected to the computer output interface when used. It is controlled by the logic levels of “1” and “0”. The function of the trigger circuit is to generate a suitable trigger signal to drive the switch circuit 4 to work, but since the switch circuit does not add a special control circuit, it will generate radio frequency interference and pollute the power grid with high harmonics or spikes. “Zero crossing control circuit”. The so-called “zero crossing” means that when the control signal is added and the AC voltage crosses zero, the SSR is in the on state; and when the control signal is turned off, the SSR waits for the junction of the positive half cycle and the negative half cycle of the alternating current (zero potential). At this time, the SSR is in an off state. This design prevents interference from higher harmonics and pollution to the grid. The snubber circuit is designed to prevent spikes and surges (voltages) from the power supply from impacting and interfering (or even malfunctioning) the bidirectional thyristor of the switching device. Generally, the “RC” series snubber circuit is used. Linear resistance (varistor). Figure 2 is an electrical schematic diagram of a typical AC-type SSR.

Compared with the AC type SSR, the DC type SSR has no zero-crossing control circuit and does not need to provide an absorption circuit. The switching device generally uses a high-power switching transistor, and the other working principles are the same. However, the DC type SSR should be noted when using: 1 When the load is an inductive load, such as a DC solenoid valve or an electromagnet, a diode should be connected in parallel across the load. The polarity is shown in Figure 3. The current of the diode should be equal to the working. Current, voltage should be greater than 4 times the working voltage. 2SSR should work as close as possible to the load, and its output leads should meet the load current requirements. 3 The power supply is obtained by AC step-down rectification, and the filter electrolytic capacitor should be large enough.

Introuduction of Solid State Relay and Its Classification

Solid State Relays (SSR) is a non-contact electronic switch consisting of discrete components, a membrane-mounted resistor network and a chip that are assembled using a hybrid process to implement the control loop (input circuit) and load loop (output circuit). The electrical isolation and signal coupling, the on-off switching function of the load is realized by the solid-state device, and there is no movable component inside. Although the types of solid state relays on the market are numerous, their operation is basically similar. Mainly composed of input (control) circuit, drive circuit and output (load) circuit.

The input circuit of the solid state relay provides a loop for the input control signal, making it the trigger source for the solid state relay. The input circuit of the solid state relay is mostly DC input, and the individual is the AC input. The DC input circuit is further divided into a resistive input and a constant current input. The input control current of the resistive input circuit varies linearly with the input voltage. The constant current input circuit, when the input voltage reaches a certain value, the current no longer increases significantly with the increase of the voltage. This relay can be applied to a relatively wide input voltage range.

The driving circuit of the solid state relay may include three parts: an isolation coupling circuit, a function circuit and a trigger circuit. Isolation coupling circuit, currently uses two types of circuit forms: optocoupler and high frequency transformer. Commonly used optocouplers are light-transistors, light-two-way thyristors, light-diode arrays (light-volts), and the like. The high-frequency transformer is coupled to form a self-oscillating oscillation of about 10 MHz at a certain input voltage, and the high-frequency signal is transmitted to the secondary of the transformer through the transformer core. The functional circuit can include various functional circuits such as detection and rectification, zero-crossing, acceleration, protection, and display. The function of the trigger circuit is to provide a trigger signal to the output device.

The output circuit of the solid state relay realizes the on/off switching of the solid state relay under the control of the trigger signal. The output circuit is mainly composed of an output device (chip) and an absorption circuit that acts as a transient suppression function, and sometimes includes a feedback circuit. At present, the output devices used in various solid state relays mainly include Transistor, thyristor (Syristor or SCR), Triac, MOS MOSFET, and insulated gate bipolar. Transistor (IGBT), etc.

Solid State Relay (Solid State Relay, SSR) is a new type of contactless switch consisting of solid-state electronic components that utilizes the switching characteristics of electronic components (such as switching transistors, bi-directional thyristors, etc.) to achieve non-contact It has no spark, but can be switched on and off, so it is also called “contactless switch”. Compared with the previous “Electromechanical Relay” (EMR), SSR does not have any movable mechanical parts, and there is no mechanical action in the work. It has the advantage of surpassing EMR, such as fast response and reliability. High and long life (SSR switching times up to 108″ 109 times, 100 times higher than the general EMR 106), no action noise, shock resistance, mechanical shock resistance, good moisture resistance, mildew and corrosion resistance. These characteristics make SSR It is widely used in military, chemical, and various industrial civil electronic control equipment. The power required for control signals of solid state relays is extremely low, so weak currents can be used to control strong currents. At the same time, AC-type SSRs use zero-crossing trigger technology. The SSR can be safely used in the computer output interface, does not cause a series of interference to the computer like the EMR, and even leads to serious crash. The more commonly used DIP package type. The control voltage and load voltage can be divided according to the use occasion. There are two types of AC and DC, so there are four types of DC-AC, DC-DC, AC-AC, and AC-DC, which are respectively loaded on AC or DC power. Off, can not be mixed.

The SSR can be divided into an AC solid state relay (AC-SSR) and a DC solid state relay (DC-SSR) depending on the type of load power supply. AC-SSR is a solid-state relay used to turn on or off the AC load power supply with a bidirectional thyristor as a switching device. The AC-SSR has different control trigger modes, and can be divided into two types: zero-crossing trigger type and random conduction type. The zero-crossing trigger type AC-SSR is turned on when the AC power supply passes near zero voltage when the control signal is input, so the interference is small. The random-on AC-SSR is turned on or off at any phase of the AC power supply, so it may cause large interference at the turn-on instant.

Thermal relay working principle and structure

Thermal relays are protective devices used for overload protection of electric motors or other electrical equipment and electrical circuits. In the actual operation of the motor, such as dragging the production machine to work, if the machine is abnormal or the circuit is abnormal, the motor encounters overload, the motor speed decreases, the current in the winding will increase, and the winding temperature of the motor will be increased. Raise. If the overload current is not large and the overload time is short, the motor winding does not exceed the allowable temperature rise. This overload is allowed. However, if the overload time is long and the overload current is large, the temperature rise of the motor winding will exceed the allowable value, which will cause the motor winding to age, shorten the service life of the motor, and even burn the motor winding in severe cases. Therefore, this overload is unacceptable to the motor. The thermal relay is a protection device that uses the principle of the thermal effect of the current to cut off the motor circuit in the event of an overload that the motor cannot withstand, providing overload protection for the motor.

The structure and working principle of electromagnetic relay

Electromagnetic relays are the earliest and most widely used relays in relays. Electromagnetic relays are generally composed of a core, an electromagnetic coil, an armature, a return spring, a contact, a support, and a pin.

The working principle of the electromagnetic relay is not complicated, and it mainly works by using the principle of electromagnetic induction. When the coil is energized, the coil generates a magnetic field, and the core in the middle of the coil is magnetized to generate a magnetic force, so that the armature is attracted to the core under the action of electromagnetic attraction. At this time, the armature drives the strut to push the leaf spring away, so that two often The closed contact is broken. When the current of the relay coil is disconnected, the core loses its magnetic force, and the armature returns to the initial state under the action of the leaf spring, and the contact is closed again.

The form of the contact is generally divided into three types: one is a static contact that is in an on state when the relay coil is not energized, and is called a normally closed contact. It is represented by the letter H; the second is a static touch in an open state. The point, called the normally open contact, is indicated by the letter D; the other is a moving contact and a static contact normally closed, while at the same time being normally open with a static contact, forming an open and closed switching contact form. . It is indicated by the letter I. The normally closed contact is disconnected from the closed state when the coil is energized, so it is also called the dynamic break contact, and the normally open contact is called the dynamic contact changeover contact. There are two cases, that is, the conversion after the first break and the break. Contact and break-before-make conversion contacts Figure 7-2 shows the circuit in the form of contacts. In a relay, there may be one or several (group) normally open points, normally closed contacts and corresponding switching contact forms.

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