Signal relays - Understand the basics

Since its appearance in 1835, relays have become a widely used and very important electronic device. Despite its age, relays still play an important role in various fields. Using signal relays, circuits can be controlled remotely, making them useful in a wide range of applications. Even early computers used a large number of relays to perform Boolean logic functions. Signal relay is a main subclass of relay and has a specific purpose. It usually plays an important role in the field of communication. This paper will introduce the signal relay, including the concept of signal relay, the difference with other relays and the key selection standards.

Basic knowledge of signal relays

Signal relays are essentially electrically operated electromechanical switches used to control the current in a circuit. A relay uses a magnetic force generated by a control current passing through a coil near a contact to move an internal moving part or contact between the pull and open positions. In this way, small signals can be controlled by large signals. Signal relays are similar to power relays, but are used to handle low voltages and small currents, usually below 2 A, and to switch low-power signals, typically rated between 5 VDC and 30 VDC. Therefore, this type of relay is also called "low signal relay".

As mentioned above, a signal relay is an electromechanical relay best suited for low voltage and low current applications, with contacts specifically designed for low power. While power relays that can handle higher currents and voltages may be more economical for some applications, such relays can destroy low-power signals in audio or video circuits, thus making signal relays a more appropriate choice. The signal relay comes in a small package that is ideal for board mounting and has faster switching times. Signal relays are generally much cheaper than solid state relays and are not affected by voltage or current transients and are not susceptible to EMI/RFI. Because of the low power handling capacity of signal relays, they generate less heat than solid-state relays, so there is usually no need to implement thermal management solutions in circuits.

                                          

                         Comparison of basic characteristics of low and high level relays

Advantages of signal relays

Signal relays, like other electromechanical relays, have a variety of advantages in projects:

● Simple design

● Electrical isolation

● Stable operation

● Save the cost of long-distance wiring

● Multiple packaging and functional options

● Resist EMI/RFI interference

Signal relays have other advantages when properly matched to the power requirements of the circuit, such as:

● Make it easy

● Small size

● Affordable

● Resistance to mechanical shock

● High insulation between internal coil and contact

● Main specifications and selection of signal relays

When selecting signal relay type for specific design, several factors need to be considered, including:

● Rated voltage: The maximum voltage at which the relay can be switched, usually in VDC or VAC.

● Rated current: the maximum current that the relay can switch, expressed in A.

● Contact resistance: Resistance in Ω added to the load circuit.

● Coil voltage: rated control voltage of the relay coil.

● Coil current: the rated current of the coil under rated voltage.

● Contact form: switch configuration of relay (pole number and normally open or normally closed configuration). For example, SPDT (1 type C) and DPDT (2 type C).

● Contact rating: Current and voltage values that guarantee relay performance. For example, relay ratings are usually expressed as 1 A @ 30 VDC.

● Switching time: The speed at which the relay operates from the applied control current to contact closure and vice versa.

● Installation type: application installation method. Usually in the PC board through hole or surface mount type installation.

● Dielectric strength: the maximum voltage that the relay can withstand within a specified time without causing damage to it.

● Operating temperature: The specified temperature range within which the relay can operate safely and normally without performance degradation.

Using the above list, the engineer can select signal relays for the project. First, the required voltage and current ratings are determined based on the circuit's maximum switching load. It is important to keep in mind that signal relays usually have A switching capacity of 2 A or less.

Next, determine the desired control voltage and type (whether AC or DC) and specify the number of poles/circuits to switch. Also, does the circuit/switch layout need to be normally open (NO) or normally closed (NC)?

Finally, consider how the relay is mounted in the circuit, such as panel mount, DIN rail mount, surface mount, or through-hole mount. By determining the specific parameters required, it is possible to determine a relay that meets the requirements of the system without selecting too high a specification.

                                

                                      General steps for selecting signal relays

Application of signal relay

Because of its low power switching capability, signal relays have been widely used in consumer and commercial fields. Signal relays provide an effective solution for networking devices that require long-distance signals at voltages and currents higher than most electronics are capable of processing. Signal relays are also useful in applications with fast response capabilities without the power of power relays. In addition, in everyday environments such as homes and offices, thermostats operate at low voltages and currents, but need to send signals to furnace or air conditioning (AC) units. Signal relays can accept small, low-power control signals and use that signal to relay information to heating or air conditioning units to make them work as required.