Construction of Relay Modules : A Comprehensive Overview

A relay module is a customized product manufactured according to specific requirements, making it challenging to differentiate between various types. However, for the sake of simplicity, relay modules can be classified based on their circuitry and mounting arrangements.

The module consists of the following components:

1. PCB (Printed Circuit Board)

2. Relay

? Electromechanical Relay

? Solid State Relay

3. Relay mounting socket (if required)

4. I/O terminals

5. LED indicators

6. Freewheeling diode

7. Mounting arrangement

8. Additional feature components

? Fuse socket & fuse, Fuse fail Indication circuitry

? Surge protection

1. PCB A printed circuit board, or PCB, is used to mechanically support and electrically connect the relay and I/O terminals using conductive pathways, tracks, or traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as a printed wiring board (PWB) or etched wiring board. The size of the PCB depends on the requirements and the number of modules used. Generally, single-sided or double-sided boards are used in relay modules. For more details, please click here for PCB.
2. Relay Normally, all PCB mounting types of relays are suitable for use in relay modules. The selection of the relay depends on the application and requirements. The relay is either directly soldered onto the PCB or mounted on a relay socket. Relay interface modules are manufactured with different channel densities, such as 1, 2, 4, 6, 8, 16, and 32 channels. (Modules with different numbers of channels are also manufactured based on the requirements.) The number of output contacts available depends on the number of changeover relays in the interface module:
3. Relay Socket (if required) The relay is either directly soldered onto the PCB or mounted on a relay socket. In the direct soldering method, the relay is permanently soldered onto the PCB, ensuring a secure connection under any condition (unless the relay is burnt down or mechanically damaged). Although it is an economical way to build the relay module, it is not suitable if the relay needs replacement in case of damage. To facilitate easy maintenance, a relay socket is used for mounting the relay on the PCB, and a relay clip ensures a secure and reliable contact.
4. I/O Terminal A terminal is the point at which a conductor from an electrical device or network comes to an end and provides a point of connection to the PCB. PCB-mounted screw or spring-type terminals are used in relay modules. To reduce the size of the relay module, double-decker type terminals are often used. The selection of terminals depends on the contact rating.
5. Flyback/Freewheeling Diode A flyback diode (sometimes called a snubber diode, freewheeling diode, suppressor diode, or catch diode) is used to eliminate flyback, which refers to the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed. A flyback/freewheeling diode is used across the relay.
6. LED for Indication LEDs are used for indication purposes. The selection of the LED is important as it indicates which relay is operating/energized. Depending on the application, either center glow or diffused types of LEDs are used. Normally, green LEDs are used for relay on indication, while red LEDs are used for power-on and fuse fail indications.
7. Mounting Arrangement If the module is to be permanently fixed in a system, screw mounting is used (although it is less commonly used nowadays). Otherwise, din rail mounting is preferred.
8. Additional Feature Components

8.1 Fuse Protection

To protect against short circuits and overloads, a fuse is placed on the relay contact side.

Normally, glass fuses are used in relay modules. Depending on the requirement or the size of the relay module, horizontal or vertical mounting fuse sockets are used to mount the fuse. Sometimes, resettable fuses are used, and a red LED is used for fuse fail indication.

8.2 Surge Protection

• §?Varister

A?varistor?is an electronic component with a significant non-ohmic?current–voltage characteristic. Varistors are used to protect?circuits?against excessive transient?voltages?by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. A varistor is also known as?Voltage Dependent Resistor?or?VDR. A varistor’s function is to conduct significantly increased current when voltage is excessive.

A varistor remains non-conductive as a?shunt?mode device during normal operation when voltage remains well below its “clamping voltage”. If a?transient?pulse (often measured in?joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed.

• §?Snubber circuit

Snubbers are used in electrical systems with an?inductive?load where the sudden interruption of?current?flow often leads to a sharp rise in?voltage?across the device creating the interruption. This sharp rise in voltage is a transient and can damage and lead to failure of the controlling device. A spark is likely to be generated (arcing), which can cause?electromagnetic interference?in other circuits. The snubber prevents this undesired voltage by conducting transient current around the device

• RC snubbers

A simple snubber comprising a small?resistor?(R) in?series?with a small?capacitor?(C) is used. Snubbers are used to prevent arcing across the contacts of relays and switches and the electrical interference and?welding/sticking of the contacts that can occur. An appropriately-designed RC snubber can be used with either?DC?or?AC?loads. This sort of snubber is commonly used with?inductive?loads such as?electric motors. The voltage across a capacitor cannot change instantaneously, so a decreasing transient current will flow through it for a small fraction of a second, allowing the voltage across the switch to increase more slowly when the switch is opened. While the values can be optimized for the application, a 100 ohm non-inductive resistor in series with a 100 nanofarad, or larger, capacitor of appropriate voltage rating is usually effective. Determination of voltage rating can be difficult owing to the nature of transient waveforms; the actual rating can be determined only by measuring temperature rise of the capacitor.

In conclusion, the construction of relay modules involves a range of components carefully assembled to meet specific requirements. Understanding the various elements and their functions is essential for developing reliable and efficient relay modules tailored to specific applications.