For upwards of 180 years, the relay has been one of the most valuable devices in the electrical and electronic industry. Their major function is remote control of a circuit from a distance, and this makes them significantly useful in a wide variety of applications. For instance, early computers had innumerable relays to conduct Boolean logic functions. A signal relay is a major subcategory of relays, with a specific and important function in the communications industry.

Like regular relays, signal relays are also electrically operated electromechanical switches. Their function is typically to control the current flow in a circuit. A control current flowing through a coil near the contacts generates a magnetic force, and this moves internal parts to open or close the contacts controlling a secondary circuit. This allows a small current in the coil to control a larger current in the secondary circuit.

Although the above functions are similar to those of a power relay, the design of a signal relay makes it more suitable for handling low currents and voltages, typically lower than 2 A, and voltage ratings between 5-30 VDC. The design of their contacts is suitable for handling low power.

Coming in small packages, signal relays are eminently suitable for mounting on PCBs or printed circuit boards. As their mechanical design makes them light, they offer significantly faster switching times as compared to power relays. Signal relays are far less expensive than solid-state relays and are impervious to voltage and current transients. They are also not susceptible to EMI or RFI. Since they are small and handle low power, they generate significantly lower amounts of heat than solid-state relays do, thereby requiring very few thermal management solutions in the PCB.

Like other electromechanical relays, signal relays also offer several benefits. These include simple design, robust operation, electrical isolation, cost savings, multiple feature options, and immunity to EMI and RFI. With a proper matching to meet the power requirements of the circuit, signal relays can offer additional benefits. These include affordable cost, small size, ease of use and operation, ability to withstand mechanical shock and vibrations, and high insulation between primary and secondary circuits.

For selecting a signal relay for a specific circuit, the designer must consider multiple factors. These include the maximum voltage that the relay must switch, the maximum current that the relay must switch, the contact resistance, the relay coil voltage, the relay coil current, the contact form, switching time, mounting type, operating temperature, and dielectric strength.

The above list is the minimum requirement for an engineer to start choosing a signal relay for their project. For instance, they can determine the necessary secondary voltage and current ratings from the maximum load that the circuit must switch. For a signal relay, it is essential that it switches a current lower than 2 A. Next, they must identify the number of circuits the relay must switch. That is, the number of poles on the relay contacts, and whether the arrangement should be normally closed or open. The next point to identify is the primary or control voltage that operates the relay, and whether this is AC or DC.