Tag Archives: Relays

Understanding Signal Relays

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.

What is Industrial Connectivity?

Engineers include any component involved in the path of delivering control signals or power for doing useful work as part of industrial connectivity. Typically, components such as terminal blocks, connectors, motor starters, and relays are part of industrial connectivity.

Engineers divide industrial connectors into four categories depending on the environments in which they operate—commercial, industrial, military, and hermetic. Commercial applications do not consider temperature and atmosphere as critical operating factors affecting performance. Industrial applications require connectors capable of handling more rugged environments involving hazards such as sand, dust, physical jarring, vibration, corrosion, and thermal shock.

Most general connectors use low-cost materials to merely maintain electrical continuity. However, designers have a large variety of materials from which to choose for making connectors. These include brass, beryllium copper, nickel-silver alloys, gold, gold-over-silver, gold-over-nickel, silver, nickel, rhodium, rhodium-over-nickel, and tin.

No wire preparation is necessary for use in terminal blocks. The user only needs to strip the insulation and install the wire using a screwdriver. One can use a wide range of wire sizes with terminals that provide an easy way to hookup wires from different components, ensuring fast connection/disconnection during troubleshooting and maintenance.

Manufacturers make terminal bodies from a copper alloy with the same expansion coefficient as the wire it connects. This prevents uneven expansion from causing loosening between the connector screws and the wire, avoiding an increase in contact resistance. Using similar metals also avoids corrosion, usually with two different metals in contact, as a result of electrolytic action between them.

SSRs or Solid-State Relays control load currents passing through them. For this, they use power transistors, SCRs, or silicon-controlled rectifiers, or TRIACs as switching devices. Engineers use isolation mechanisms such as optoisolators, reed-relays, and transformers for coupling input signals to the switching devices to control them.

To reduce the voltage transients and spikes that load-current interruptions typically generate, engineers use zero-crossing detectors and snubber circuits, incorporating them within solid-state relays.

Semiconductor switches generate significant amounts of waste power, and engineers must minimize their operating temperature using heat sinks attached to solid-state relays. SSRs can operate in rapid on/off cycles that would wear out conventional electromechanical relays quickly.

Electromechanical relays physically open and close electrical contacts for operating other devices. In general, they cost much less than equivalent electronic switches. They also have some inherent advantages over solid-state devices. For instance, the input circuit in electromechanical relays is electrically isolated from the output circuits, and one relay can have more than one output circuit, each electrically isolated from the others.

Furthermore, the contact resistance offered by electromechanical relays is substantially lower than that offered by a solid-state relay of a similar rating. The contact capacitance is lower as well, benefitting high-frequency circuits. Compared to solid-state relays, electromechanical relays are far less sensitive to transients and spikes, not turning on as frequently as SSRs do. Brief shorts and overloads also damage electromechanical relays to a far less extent than the damage they cause to SSRs.

Improved manufacturing technology is now making available electromechanical relays in small packages suitable automated soldering for PCB mounting and surface mounting.

Using Relays to Detect Faults

Different types of relays are in use in every-day life. These include relays constructed from electromechanical elements such as from solenoids, induction discs, hinged armatures, or from solid-state elements such as from transistors, magnetic or operational amplifiers, silicon-controlled-rectifiers (SCRs), diodes, or digital computers using microprocessors and analog-to-digital converters.

Development of protection with relays began with the electromagnetic types, and most descriptions of relay characteristics still retain the electromagnetic terms. Although the construction of a relay does not inherently alter the concept of protection, each type has its own advantages and disadvantages.

General faults are often short circuits, where the current increases in magnitude, while the voltage goes down. Apart from the changes in magnitude, the AC field may also undergo changes in parameters such as system frequency, active/reactive power, harmonic components, phase angles of the current and voltage phasor, and more.

Operating principles of detection of faults with relays are based on detecting the above changes and identifying whether the changes exist within the predefined zone of protection or outside. Depending upon the operating principle of the relay, detection can be categorized based on which of the input quantities the specific relay will respond. This leads to eight major types of faults that relays can detect:

  • Frequency Sensing
  • Harmonic Content
  • Pilot Relaying
  • Distance Measurement
  • Phase Angle Comparison
  • Differential Comparison
  • Magnitude Comparison
  • Level Detection

Most power systems operate at a normal frequency of 50 or 60 Hz, depending on the country. Deviating from the normal frequency indicates an existing problem or an imminent one. Engineers use frequency-sensing relays to detect and take corrective action to bring the system frequency back to normal.

Power systems usually operate with a sinusoidal waveform of the fundamental frequency. Abnormal system conditions create harmonics that are typically associated with heat and loss in efficiency. Electromechanical or solid-state relays can detect these harmonics, based on which control action may be required.

Sometimes information is required from a remote location, and a pilot relay provides it in the form of contact status, open or closed. Usually, this information is carried over a channel of communication using telephone, microwave, or carrier circuits.

An impedance relay determines the distance or length of the line based on a given spacing and diameter of the conductor. The relay compares the local voltage with the local current, and gives a measurement of the line impedance as seen from its terminals.

A phase angle comparison relay compares the relative angle of phase between the AC voltage and the AC current, measuring the power factor angle. This comparison determines the direction of flow of the current with respect to the voltage, with the magnitude of the angle measured giving an indication of faults.

Under normal operating conditions, current entering one end of an electrical equipment should equal the current exiting from the other end. However, in case of any fault within the equipment, this balance is no longer maintained. A differential relay detects the difference in the two currents, and provides protection.

Relays can compare the magnitude of current in one circuit with the magnitude of current in another and detect abnormalities based on whether they should have been equal or proportional.

Finally, relays can be designed to trip the circuit breakers should the operating current level crosses a specific setting.