I. What are the uses of relays?
A relay is an electrical control device that causes a predetermined step change in the controlled variable 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). It is commonly used in automated control circuits, and is essentially an "automatic switch" that uses a small current to control a large current. Therefore, it plays a role in automatic adjustment, safety protection, and circuit switching in circuits.
Relays are automatic switching elements with isolation functions, widely used in remote control, telemetry, communication, automatic control, mechatronics and power electronic equipment, and are one of the most important control elements.
Relays generally have a sensing mechanism (input section) that can reflect certain input variables (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.); an actuator (output section) that can control the controlled circuit to "on" and "off"; and an intermediate mechanism (drive section) between the input and output sections of the relay to couple and isolate the input quantity, process the function, and drive the output section.
As a control element, relays generally have the following functions:
1) Expand the control range: For example, when the control signal of a multi-contact relay reaches a certain value, it can simultaneously switch, disconnect, and connect multiple circuits according to different forms of contact groups.
2) Amplification: For example, sensitive relays and intermediate relays can control circuits with a very small control quantity.
3) Combined signals: For example, when multiple control signals are input to a multi-winding relay in a specified manner, they are compared and combined to achieve the intended control effect.
4) Automatic, remote control, and monitoring: For example, relays on automatic devices, together with other electrical appliances, can form a program control circuit to achieve automated operation.
II. High-frequency relays
(I) Working principle of high frequency relay
The working principle of a high-frequency relay: It generally consists of an iron core, coil, armature, and contact springs. When a certain voltage is applied across the coil, a certain current flows through the coil, generating an electromagnetic effect. Under the attraction of the electromagnetic force, the armature overcomes the tension of the return spring and is attracted to the iron core, thus causing the moving contact of the armature to engage with the stationary contact (normally open contact). When the coil is de-energized, the electromagnetic attraction disappears, and the armature returns to its original position under the reaction force of the spring, causing the moving contact to engage with the original stationary contact (normally closed contact). This engagement and release achieves the purpose of connecting and disconnecting the circuit.
(II) Characteristics of High-Frequency Relays
A relay used for switching high-frequency circuits. Its windings can be powered by DC or AC power. Its characteristics are:
1. The circuit has very high insulation resistance and low dielectric loss; the contact system is fixed on a high-frequency ceramic, high-frequency plastic, or glass substrate.
2. The distributed capacitance between contacts is small, and the contact springs are small in size and arranged at 45°, 90°, and 180° angles to each other;
3. The contacts are far from other conductive parts or grounding components, or the contact circuit is shielded. When high-speed switching of high-frequency circuits is required, or when switching high-voltage high-frequency circuits is needed, the contacts are sealed in a high vacuum or gas-filled chamber.
(III) Structure of High-Frequency Relays
The electromagnetic system of a small high-frequency relay is the same as that of a regular electromagnetic relay, except that its conductive contact system has been significantly modified to meet the needs of high-frequency circuits. To reduce the distributed capacitance between the contact springs, the contact pieces are made in different shapes so that the planes of the contact pieces do not overlap (e.g., the distributed capacitance between the contacts of the JP-4 type is only 3 picofarads under environmental conditions of 20°C and 98% relative humidity). The spring surface is silver-plated to improve its conductivity under high-frequency conditions. The insulating pads between the contacts should be made of materials with low dielectric loss (e.g., the JP-4 type relay rivets three non-overlapping contact spring groups onto a high-frequency ceramic block) to reduce dielectric loss under high-frequency conditions. The figure shows structural examples of two types of high-frequency relays.
