Signal Relays – Understanding the Basics

Relays are a widely used and valuable electronic device, having been around since 1835. Despite their age, they continue to play an important role in various industries. They allow for remote control of a circuit from a distance, making them useful in widespread applications. Even early computers were built using a large number of relays for Boolean logic functions. One major subcategory of relays is signal relays, which have specific uses and often fill an important role in communications. This article will provide an introduction to signal relays, including what they are, their differences compared to other relays, key selection criteria, and more.

Signal relay basics

Signal relays are essentially electrically operated electromechanical switches that control the flow of current in a circuit. They work by using a magnetic force, produced by a control current running through a coil near the contacts, to move internal moving parts or contacts between open and closed positions. This allows a small signal to control a larger one. They are similar to power relays, but are designed to handle low voltage and current, typically under 2 amps, and switch low power signals with voltage ratings usually between 5 V_DC to 30 V_DC. Due to this, they are also known as "low signal relays".

As mentioned above, signal relays are an electromechanical relay type best suited for low voltage and current applications, with contacts specifically designed for low power. While power relays, which can handle higher current and voltage, may be more economical for certain applications, they can also corrupt low-power signals in audio or video circuits, making signal relays a more appropriate choice. Signal relays come in small packages, perfect for board-mounting and offer faster switching times. They are typically less expensive than solid state relays and are not affected by voltage or current transients, or susceptible to EMI/RFI. Because of their small power handling, they also generate less heat than solid state relays, and thus, do not typically require thermal management solutions in the circuit.

Figure 1: A basic comparison of low and high-level relays. (Image source: Same Sky)

Signal relay benefits

Signal relays, like other electromechanical relays, offer a variety of benefits in projects, including:

• Simplicity in design
• Electrical isolation
• Robust operation
• Cost savings in long-distance wiring
• Multiple packaging and feature options
• Immunity to EMI/RFI

When properly matched to the circuit's power requirements, signal relays also provide additional advantages such as:

• Ease of operation
• Small size
• Affordable cost
• Resistance to mechanical shock
• High insulation between the internal coil and contacts

Key specifications and selection of signal relays

When selecting a signal relay model for a design, there are several factors to consider, including:

• Voltage Rating: the maximum voltage that the relay can switch, typically measured in V_DC or V_AC.
• Current Rating: the maximum current that the relay can switch, measured in Amps.
• Contact Resistance: the resistance, measured in Ohms, added to the load circuit.
• Coil Voltage: the rated control voltage for the relay coil.
• Coil Current: the rated current that the coil takes with the rated coil voltage.
• Contact Form: the relay’s switch configuration (number of poles and normally open or normally closed configuration). For example, SPDT (1 Form C) and DPDT (2 Form C).
• Contact Rating: the current and voltage values for guaranteed relay performance. As an example, this is typically expressed as 1 A @ 30 V_DC.
• Switching Time: the operating speed of the relay from when control current is applied to when the contacts close, or vice versa.
• Mounting Type: the mounting method of the application. This is typically either through hole or surface mount on the PC board.
• Dielectric Strength: the maximum voltage a relay can tolerate without damage for a specified period.
• Operating Temperature: the specified range of temperatures a relay can safely and successfully operate in without degradation.

With the above list in mind, an engineer can then begin the process of choosing a signal relay for a project. To start, determine the required voltage and current ratings based on the maximum switching load of the circuit. It is important to keep in mind that signal relays typically switch 2 A or less.

Next, identify the required control voltage and type, either ac or dc, and specify the number of poles/circuits to be switched. Also, does the circuit/switching arrangement need to be normally open (NO) or normally closed (NC)?

Lastly, consider the method of mounting the device in the circuit, such as panel mount, DIN rail, surface mount, or through hole. By determining the specific parameters required, it is possible to identify a relay that meets the needs of the system without being over-specified.

Figure 2: The general steps of selecting a signal relay. (Image source: Same Sky)

Signal relay applications

Signal relays, due to their low power switching capabilities, have been implemented in a wide range of applications, both consumer and commercial. They provide an efficient solution for network devices that require long-distance signals with voltage and current higher than most electronics can handle natively. Signal relays are also useful in quick responsiveness applications that don't require as much power as a power relay. Additionally, in everyday settings such as homes and offices, a thermostat operates at a low voltage and current but needs to send a signal to the furnace or air conditioning (AC) unit. A signal relay can take the small, low-power control signal and use it to transmit the message to the heating or AC  unit, allowing them to take the appropriate action.

Conclusion

As electronic systems continue to become more advanced, the need for complete electrical isolation between a switching signal and the signal being switched often arises. Signal relays provide a solution for safely controlling current and voltage from a distance.

These devices are widely used in a variety of consumer and industrial products and systems to switch power in different types of circuits. After identifying the requirements of a design, Same Sky offers a range of signal relays to meet an engineer’s low-level current switching needs.