Electrical Relay is an automatic control element with isolation switch function, which is widely used in electronic equipment such as remote control, monitoring, communication, electromechanical integration and so on. It can generally reflect a certain input variable (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.) sensing mechanism (input part); automatic control on-off (output part); between the input part and output part of the relay, there is also an intermediate mechanism (drive part) for coupling and isolating the input, functional processing and driving the output part.
common electrical relay structure
Fig 1. Common Electrical Relay Structure
As a control element, the relay has the following functions:
1) Expand the scope of control. For example, when the control signal of a multi-contact relay reaches a certain value, multiple circuits can be switched, disconnected, and connected at the same time according to different forms of contact groups.
2) Amplify. For example, sensitive relays, intermediate relays, etc., can control a circuit with a large power with a very small control quantity.
3) Integrated signal. For example, when multiple control signals are input to a multi-winding relay in a prescribed form, they are compared and integrated to achieve a predetermined control effect.
4) Automatic, remote control and monitoring. For example, the relay on the automatic device and other electrical appliances can form a program control circuit to realize automatic operation.

Mercury Relay
The mercury relay is composed of a coil, a plunger core, mercury and two electrodes. The contact is sealed in a sealed tube filled with mercury. When the relay is closed, the contact is conducted by mercury. That is, the mercury is installed in a long, sealed glass tube. When the glass tube is tilted, the electrical contacts at both ends of the glass tube are blocked. When the glass tube is horizontal, the electrical contacts at both ends are connected by mercury.
mercury relay structure
Fig 2. Mercury Relay Structure
1. Use a smaller current and voltage to control relay wire package, and its mercury contact can control a larger current and voltage load.
2. There will be arc when the current is large.
3. The frequency should not be too high, approximately in seconds.
4. Generally, it will not explode. If the seal is not tight, there will be leakage. The current sealing technology may be more reliable nowadays.
5. It is more impact resistant and has a longer life. If the current is not too high, it is better to use solid state relays.
6. If possible, you can also use low-power solid state relays to drive ultra-low resistance and high-power field effect transistors to replace mercury relay.

Voltage Relay
voltage relay structure
Fig 3. Voltage Relay Structure
Voltage relay is an electronic control device. It has a control system (also called an input loop) and a controlled system (also called an output loop). It is usually used in automatic control circuits. In fact, it is a kind of "automatic switch" using uses a smaller current to control high current. Therefore, it plays the role of automatic adjustment, safety protection, and conversion circuit in the circuit. Mainly used in relay protection devices of generators, transformers and transmission lines, as the starting element for over-voltage protection or low-voltage lockout. So voltage relay is a kind of starting relay.

Thermal Relay
Thermal relays generally have functions such as phase failure protection, overload protection, overload current adjustment, action indication, temperature compensation, and automatic/manual reset.
thermal relay structure
Fig 4. Thermal Relay Structure
The thermal relay is mainly used for overload protection of asynchronous motors. Its working principle is that after the overload current passes through the thermal element, the bimetallic sheet is heated and bent to push the action mechanism to drive the contact action, thereby disconnecting the motor control circuit to realize the motor disconnection, which play the role of overload protection. During the heating and bending process of the bimetallic strip, the heat transfer takes a long time. Therefore, the thermal relay cannot be used for short-circuit protection, but can only be used for overload protection of the thermal relay.
Andrewen 19 october 2021, 7:47