A Relay is an electrically operated switch designed by Joseph Henry in 1835 with the capability of controlling an output circuit of higher power than the input circuit. An electromagnetic relay is simply an adaptation of an electromagnet. Current passing through the relay coil creates a  agnetic field which attracts a lever and changes the switch contacts. A relay coil can be energized in following three ways:

1 Normally-open (NO) contacts They connect the circuit when the relay is activated and the circuit is disconnected when the relay is inactive. It is also called a Form A contact or “make” contact.                                                                                c  
2 Normally-closed (NC) contacts They disconnect the circuit when the relay is activated
and the circuit is connected when the relay is inactive. It is also called a Form B contact or “break” contact.
 3 Change-over (CO), or double-throw (DT),
They control two circuits i.e. one normally-open
contact and one normally-closed contact with a common terminal. It is also called a Form C contact or “transfer” contact (“break before make”). Another type of contact that utilizes “make before break” functionality is called a Form D contact.

Types of relay

  • Latching relay: These types of relays also known as ‘keep’ or ‘stay’ relays are actually Bistable having two relaxed states. A latching relay retains its last state as and when the current is switched off and consumes power only at the instant of switching.
  • Reed relay: These are faster switching speed relays having set of contacts inside a vacuum or inert gas filled glass tube that protects the  contacts against atmospheric corrosion. However, they have low switch current and voltage ratings which restricts their use in high power applications.
  • Mercury-wetted relay: A form of reed relay in which contacts are wetted with mercury to eliminates contact bounce which proves  advantageous in high-speed counting and timing applications. The relay’s low contact resistance makes it suitable to switch low voltage  signals only, say less than one volt. Mercury wetted relays being position-sensitive are always mounted vertically for proper functioning. These type of relays are hardly ever recommended for new equipment because of two reasons-a) Toxicity b)Cost
  • Polarized relay: Earlier in the mid of the 20th century, these relays were used in telephone exchanges for detecting faint pulses and  correcting telegraphic distortion. To boost sensitivity of a polarized relay, an armature was placed between the poles of a permanent  magnet.
  • Machine tool relay: These types of relays are typically apt for industrial control of machine tools, transfer machines, and other sequential control. Machine tool relays exemplified by fairly large number of contacts(readily convertible from normally-open to normally-closed status) and easily replaceable coils were once the backbone of automation industry but nowadays the programmable logic controller (PLC) has almost replaced machine tool relays from sequential control applications.
  • Contactor relay: It is an extremely heavy-duty relay employed for switching electric motors and lighting loads. Silver alloys are used for making contactor relay’s high-current contacts. One disadvantage on part of contactor relays is that they are entirely unsuitable for use within noise prone areas because of its loud operation.
  • Solid-state relay: It is a solid state electronic component comparable to an electromagnetic relay but unlike electromagnetic relays it does not have any moving components and can be falsely triggered by transients. With the improvement in transistor technology, high current SSRs have now become commercially available.
  • Solid state contactor relay: It is an exceptionally heavy-duty solid state relay activated by AC control signals or DC control signals from Programmable logic controller (PLCs), PCs, Transistor-transistor logic (TTL) sources, or other microprocessor controls. These are mainly used in applications where frequent switching cycles are involved e.g. electric heaters, small electric motors and lighting loads and hence possess indispensable heat sink. Moreover these type of relays are free from wear out (due to lack of moving parts) and contact bounce  caused due to vibration.
  • Buchholz relay: This type of
    relay acts as a sensing mechanism providing safety against accumulation of gas in large oil-filled transformers. The safety device will generate an alarm in case of slow accumulation of gas but will automatically shut down the transformer, in case the gas produces rapidly inside the transformer.
  • Forced-guided contacts relay: In this type of relay, all relay contacts are mechanically interlinked together in such a way that when the relay coil is energized or de-energized, all the linked contacts move together. Forced-guided contacts also known as “positive-guided contacts”, “captive contacts”, “locked contacts”, or “safety relays” are used to enable the safety circuit to check the status of the relay.
  • Overload protection relay: This type of relay is mainly operated by a heating element in series with the electric motor for overload protection.

Relays are used for

  • Controlling a high-voltage circuit with a low-voltage signal, for e.g. some types of modems or audio amplifiers. Control a high-current circuit  with a low-current signal, for e.g. the starter solenoid of an automobile,
  • Detection and isolation of faults on transmission and distribution lines by opening and closing circuit breakers.
  • Isolating the controlling circuit from the controlled circuit when the two are at different potentials, for example while controlling a mains-powered device from a low-voltage switch.
  • Realization of logic Functions Time delay functions: Relays can be modified to delay opening or delay closing a set of contacts.

Selection Criteria
Following factors need to be considered while selecting a suitable relay for a specific application:

  • Number and type of contacts
  • Contact sequence – “Make before Break” or “Break before Make”.
  • Rating of contacts
  • Voltage rating of contacts
  • Coil voltage
  • Coil current
  • Package/enclosure – open, touch-safe, double-voltage for isolation between circuits, explosion proof, outdoor, oil and splash resistant,  washable for printed circuit board assembly Assembly
  • Mounting – sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment
  • Switching time – considered where high speed is required
  • “Dry” contacts – when switching very low level signals, special contact materials may be needed such as gold-plated contacts
  • Coil and Contact protection
  • Isolation between coil circuit and contacts
  • Aerospace or radiation-resistant testing, special quality assurance
  • Expected mechanical loads due to acceleration
  • Accessories such as timers, auxiliary contacts, pilot lamps, test buttons
  • Regulatory approvals
  • Stray magnetic linkage between coils of adjacent relays on a printed circuit board.