Electrical Relays

The relay is an electrically operated switch. It comprises of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals.

The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.

electromechanical relay is an electrical switch actuated by an electromagnet coil. As switching devices, they exhibit simple “on” and “off” behavior with no intermediate states. Relays are very useful devices, as they allow a single discrete (on/off) electrical signal to control much greater levels of electrical power, and/or multiple power or control signals that are otherwise isolated from each other. For example, a relay may be controlled by a low-voltage, low-current signal that passes through a delicate switch of some sort (e.g. limit switch, proximity switch, optical sensor), and then the switching contacts of that relay may be used to control a much higher-voltage, higher-current circuit, and even multiple circuits given multiple sets of switching contacts.
The electronic schematic symbol for a simple single-pole, single-throw (SPST) relay is shown here:

A coil of wire wrapped around a laminated ferrous core provides the magnetic field necessary to  actuate the switch mechanism. This electromagnet coil’s actuating influence on the relay’s contact(s)  is represented by the dashed line. This particular relay is equipped with normally open (NO) switch  contacts, which means the switch will be in the open (off) state when the relay coil is de-energized.  The “normal” status of a switch is the resting condition of no stimulation.  A relay switch contact will be in its “normal” status when its coil is not energized. A single-pole,  single-throw relay with a normally-closed (NC) switch contact would be represented in an electronic  schematic like this:

In the electrical control world, the labels “Form-A” and “Form-B” are synonymous  with  “normally open” and “normally closed” contacts,  respectively.  Thus, we could have labeled the  SPST relay contacts as “Form-A” and “Form-B,” respectively:

An extension of this theme is the single-pole, double-throw (SPDT) relay contact, otherwise  known as a “Form-C” contact. This design of switch provides both a normally-open and normally-  closed contact set in one unit, actuated by the electromagnet coil:

A further extension on this theme is the double-pole, double-throw (DPDT) relay contact. This  design of switch provides two sets of Form-C contacts in one unit, simultaneously actuated by the  electromagnet coil:

DPDT relays are some of the most common found in industry, due to their versatility. Each  Form-C contact set offers a choice of either normally-open or normally-closed contacts, and the two sets (two “poles”) are electrically isolated from each other so they may be used in different circuits.  A common package for industrial relays is the so-called ice  cube relay,  named  for its clear  plastic  case allowing inspection of the working elements. These relays plug into multi-pin base sockets for  easy removal and replacement in case of failure. A DPDT “ice cube” relay is shown in the following  photographs, ready to be plugged into its base (left) and with the plastic cover removed to expose both sets of Form-C contacts (right):

These relays connect to the socket with eight pins: three for each of the two Form-C contact set,  plus two more pins for the coil connections. Due to the pin count (8), this style of relay base is often  referred to as an octalbase.

A closer view of one Form-C contact shows how the moving metal “leaf” contacts one of two  stationary points, the actual point of contact being made by a silver-coated “button” at the  end of  the leaf. The following photographs show one Form-C contact in both positions:

Industrial control relays usually have connection diagrams drawn somewhere on the outer shell  to indicate which pins connect to which elements inside the relay.  The style of these diagrams may  vary somewhat, even between relays of identical function.  Take for instance the diagrams shown  here, photographed on three different brands of DPDT relay:

Bear in mind that these three relays are identical in their essential function (DPDT switching),  despite differences in physical size  and  contact  ratings (voltage  and  current  capacities).  Only two  of the three diagrams shown use the same symbols to represent contacts, and all three use unique  symbols to represent the coil.

Types of Relays

  • Electromechanical Relays: These relays use an electromagnet to mechanically operate a switch. They are commonly used in applications requiring high reliability and robustness.
  • Solid-State Relays (SSRs): SSRs utilize semiconductor devices such as thyristors or transistors to perform switching operations. They offer faster response times, longer lifespans, and are suitable for high-frequency switching.
  • Reed Relays: Reed relays consist of a set of reed switches enclosed within a coil. They are compact, have low power consumption, and are ideal for applications requiring high-speed switching.
  • Protective Relays: These relays are designed to detect abnormalities or faults in electrical systems and initiate protective actions to prevent damage. Examples include overcurrent relays and differential relays.

Working Principle

In electromechanical relays, when a low-power signal is applied to the coil, it generates a magnetic field, which attracts an armature connected to the switch contacts. This action closes or opens the contacts, allowing or interrupting the flow of current in the controlled circuit.

Solid-state relays operate by controlling the flow of current through semiconductor devices, eliminating the need for mechanical parts. They achieve this by using a small control signal to trigger the switching action of the semiconductor component.

Quiz on Relays

What is the primary function of a relay in electrical circuits?

Answer: The primary function of a relay is to control the flow of current in a circuit using a low-power signal.

Name one advantage of solid-state relays over electromechanical relays.

Answer: Solid-state relays offer faster response times compared to electromechanical relays.

What are protective relays used for?

Answer: Protective relays are used to detect abnormalities or faults in electrical systems and initiate protective actions.

Describe the working principle of an electromechanical relay.

Answer: When a low-power signal is applied to the coil of an electromechanical relay, it generates a magnetic field that attracts an armature, thereby closing or opening the switch contacts.

What is the distinguishing feature of a reed relay?

Answer: Reed relays are characterized by their use of reed switches enclosed within a coil, offering compactness and low power consumption.

Also see:

  1. What is motor contactor
  2. Why motor drives use input and output line reactors
  3. What is AC Motor Braking and What are some methods for Induction motor Braking
  4. Why PT Secondary Winding should never be Short-Circuited
  5. Thyristors in Power Electronics

Article text extracted (with modifications and addition carried out by Electrical Engineering XYZ Team) from Lessons In Industrial Instrumentation by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public license

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