A relay is an electrical control device. Its main working principle is as follows: primarily used in low-voltage circuits to control high-voltage circuits, it causes a predetermined step change in the controlled quantity in the electrical output circuit when the change in the input quantity (excitation quantity) reaches a specified requirement. It has an interactive relationship between the control system (also known as the input circuit) and the controlled system (also known as the output circuit). Relays can be classified according to their working principle into several types, including electromagnetic relays, solid-state relays, temperature relays, reed relays, time relays, high-frequency relays, polarized relays, optical relays, acoustic relays, thermal relays, instrument relays, Hall effect relays, and differential relays. We will introduce the working principles of the following types of relays.
1. Working principle of electromagnetic relay:
An electromagnetic relay is an electrical relay that works by utilizing the attraction force generated between the electromagnet core and the armature in the input circuit.
The working principle of an electromagnetic relay is that when the coil is energized, the iron core is magnetized to generate a sufficiently large electromagnetic force, which attracts the armature and drives the spring, causing the moving contact and the stationary contact to close or open. That is, the originally closed contact opens and the originally open contact closes. When the coil is de-energized, the electromagnetic attraction disappears, the armature returns to its original position, and the moving contact and the stationary contact return to their original closed or open state.
2. Working principle of solid-state relay:
A solid-state relay is a type of relay in which electronic components perform their functions without any moving mechanical parts, and the input and output are isolated.
Solid-state relays are a new type of contactless switching device composed entirely of solid-state electronic components. They utilize the switching characteristics of electronic components (such as switching transistors, triacs, and other semiconductor devices) to achieve the purpose of connecting and disconnecting circuits without contacts or sparks, hence they are also known as "contactless switches".
Most common solid-state relays are modular four-terminal active devices, with two terminals serving as input control terminals and the other two as output control terminals, as shown in the diagram above. Optocouplers are commonly used to achieve electrical isolation between the input and output. The output control terminals utilize the switching characteristics of semiconductor devices such as switching transistors and bidirectional thyristors to achieve contactless and spark-free connection and disconnection of external control circuits. The entire device has no moving parts or contacts, and can perform functions equivalent to those of conventional electromagnetic relays.
3. Working principle of temperature relay:
A temperature relay is a relay that activates when the ambient temperature reaches a given value.
The working principle of a temperature relay is to firmly bond two metals or alloys with significantly different coefficients of thermal expansion together to form a disc-shaped bimetallic strip. When the temperature rises to a certain value, the bimetallic strip will bend upwards due to the expansion of the lower metal layer and the smaller expansion of the upper metal layer. When it bends to a certain extent, it actuates the electrical contacts, thus connecting or disconnecting the load circuit. When the temperature drops to a certain value, the bimetallic strip gradually returns to its original shape. When it returns to a certain extent, it reverses direction and actuates the electrical contacts, thus disconnecting or connecting the load circuit.
4. Working principle of reed relay:
A reed relay is a type of relay that uses the action of a reed sealed inside a tube, which functions as both a contact spring and an armature magnetic circuit, to open, close, or switch circuits.
A reed relay is an electromagnetic relay with sealed contacts, mainly composed of dry reeds and an excitation coil. The dry reeds (contacts) are sealed and made of an iron-nickel alloy. The contact parts of the reeds are usually plated with precious metals (such as gold, rhodium, palladium, etc.), ensuring good contact and excellent conductivity. The contacts are sealed in a glass tube filled with inert gases such as nitrogen, effectively preventing dust contamination, reducing contact corrosion, and improving operational reliability. When the coil is energized, the free ends of the two reeds in the tube are magnetized into N and S poles respectively, attracting each other and thus connecting the controlled circuit. When the coil is de-energized, the reeds separate under their own elastic force, cutting off the circuit.
5. Working principle of time relay:
A time relay is a relay whose output section needs to be delayed or limited to a specified time before closing or opening its controlled circuit when an input signal is applied or removed.
There are generally two working principles for time relays:
① A charging circuit consisting of a large resistor and a large capacitor is used. When the voltage on the capacitor reaches a certain level, this voltage is used to trigger a certain circuit to achieve the purpose of telling time;
②Using a pulse source and a counter, when the count reaches the required value, a signal is sent to the monitoring circuit to achieve the purpose of delay action.
6. Other types of relays: such as optical relays, acoustic relays, thermal relays, instrument relays, Hall effect relays, differential relays, etc.
