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Monday, March 21, 2011


            A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. The first relay was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered, in a broad sense, to be a form of an electrical amplifier. The name relay derives from the french noun relais that indicates the horse exchange place of the postman. Generally a relay is an electrical hardware device having an input and output gate. The output gate consists in one or more electrical contacts that switch when the input gate is electrically excited. It can implement a decoupler, a router or braker for the electrical power, a negation, and on the base of the wiring, complicated logical functions containing and or, and flip-flop. In the past relays had a wide use, for instance the telephone switching or the railway routing and crossing systems. 
There are a wide variety of relays. A first distinction is :
·         Electromechanical Relay: It consists in a fixed coil (a) and a moving armature (b) mechanically linked (c) to a moving contact (d). Feeding the coil by means of electrical current a magnetic field rises. Then the moving armature is attracted to the coil and, consequentially, the contact can be moved. The movement of the contact either makes or breaks an electrical connection with a fixed contact (e). When the feeding current of the coil is removed, the armature and the feed contact return to their relaxed position by means of a spring or of the elasticity of the contact. An electromechanical relay has galvanic insulation between the input and output gate and between the terminals of the contacts when they are in break position.
·         Solid State Relay: In this type of relay there aren't moving components, so typically a solid state relay has a longer life than an electromechanical relay. Basically, it consists in a TRIAC (a) on the output gate excited  by a photo-decoupler (b) on the input gate. Respect to the electromechanical relay, the solid state relay  has only galvanic insulation between the input and output gate while there isn't galvanic insulation between the terminals of the contacts.

A second distinction is based on the type of the contacts:
·         normally open (NO): also called Form A or make contact. In the relaxed position, which means that the input gate is not excited, the output contact is open. When the input gate is excited then the output contact is close.
·         normally close (NC): also called Form B or break contact. In the relaxed position, which means that the input gate is not excited, the output contact is close. When the input gate is excited then the output contact is open.
·         change-over (CO): also called Form C. There is a common terminal connected on the normally close contact when the input gate is not excited and on the normally open contact when the input gate is excited.
·         make-before-break: when there is a switching all the contacts of the relay are close for a time Tclose.
·         break-before-make: when there is a switching all the contacts of the relay are open for a time Topen.
·         reed: generally this type of contact is normally open and has an high resistance versus the atmospheric corrosion. It is contained inside a vacuum or inert gas filled glass tube and the contact is closed by means of the magnetic field generated from a coil around the glass tube.
·         mercury wetted: it is a particular type of reed contact where the two terminals of the output gate are connected to each other by means of liquid mercury.
·         forced-guided: this type of contacts are linked all together by means of a mechanical link. In this way when the coil is energized all the contacts are moved together and if one or more of them is blocked then all the other can't move.
A third distinction is based on the electrical characteristics of the input gate:
·         AC: the input signal must be an alternate voltage.
·         DC: the input signal must be a direct voltage.
·         Polarized: the input signal must be a direct voltage and there are specific terminals for positive and negative voltage.
·         Snubbered: the input coil is equipped with a snubber circuit for the extra-current and the over-voltage.
A fourth distinction is based on the functionality:
·         General purpose: this is a simple relay with a large number of contacts of the type change-over.
·         Contactor: is a relay able to interrupt high current and high voltage.
·         Latching: this relay has two coils. When the first one is energized the relay remains in the on-state also if the feed is removed to the first coil. To switch off the relay needs to be energized on the second coil. This relay is a simple flip-flop.
·         Impulse: open and close its contacts after a pulse on the input gate.
·         Stepping: at every pulse on the input gate the contacts change their configuration in cycle way.
·         Protection: there are a lot of sub-classes of protection relays. Generally they are very complex and disconnect an electrical circuit when a fault is detected.
·         Timer: in their simplest form, this relay open and/or close its contacts with a delay respect to the energizing/relaxing of the coil. There are several types more complicated that execute a temporized sequence after the energizing/relaxing of the coil.
·         Monitoring: this type of relay collects a large number of sub-classes. In general a monitoring relay is a relay that accepts on the input gate an analog variable and open or close its contacts if the analog variable hits a threshold. Typically, in addition to the threshold, there is an hysteresis and a delay settable by the user.

Types Of Relay

Latching relay

A latching relay has two relaxed states (bistable). These are also called 'keep' relays. When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remnant core. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage.

Reed relay

A reed relay has a set of contacts inside a vacuum or inert gas filled glass tube, which protects the contacts against atmospheric corrosion. The contacts are closed by a magnetic field generated when current passes through a coil around the glass tube. Reed relays are capable of faster switching speeds than conventional relays. See also reed switch.

Mercury-wetted relay          

·         A mercury-wetted relay is a form of reed relay in which the contacts are wetted with mercury. Such relays are used to switch low-voltage signals (one volt or less) because of its low contact resistance, or for high-speed counting and timing applications where the mercury eliminated contact bounce. Mercury wetted relays are position-sensitive and must be mounted vertically to work properly. Because of the toxicity and expense of liquid mercury, these relays are rarely specified for new equipment. See also mercury switch.

Polarized relay

A Polarized Relay placed the armature between the poles of a permanent magnet to increase sensitivity. Polarized relays were used in middle 20th Century telephone exchanges to detect faint pulses and correct telegraphic distortion. The poles were on screws, so a technician could first adjust them for maximum sensitivity and then apply a bias spring to set the critical current that would operate the relay.

Machine tool relay

A machine tool relay is a type standardized for industrial control of machine tools, transfer machines, and other sequential control. They are characterized by a large number of contacts (sometimes extendable in the field), which are easily converted from normally open to normally closed status, easily replaceable coils, and a form factor that allows compactly installing many relays in a control panel. Although such relays once
were the backbone of automation in such industries as automobile assembly, the programmable logic controller mostly displaced the machine tool relay from sequential control applications.

Contactor relay

A contactor is a very heavy-duty relay used for switching electric motors and lighting loads. With high current, the contacts are made with pure silver. The unavoidable arcing causes the contacts to oxidize and silver oxide is still a good conductor. Such devices are often used for motor starters. A motor starter is a contactor with an overload protection devices attached. The overload sensing devices are a form of heat operated relay where a coil heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts. These auxiliary contacts are in series with the coil. If the overload senses excess current in the load, the coil is de-energized. Contactor relays can be extremely loud to operate, making them unfit for use where noise is a chief concern.

Solid state contactor relay

A solid state contactor is a very heavy-duty solid state relay, including the necessary heat sink, used for switching electric heaters, small electric motors and lighting loads; where frequent on/off cycles are required. There are no moving parts to wear out and there is no contact bounce due to vibration. They are activated by AC control signals or DC control signals from Programmable logic controller (PLCs), PCs, Transistor-transistor logic (TTL) sources, or other microprocessor controls.

Buchholz relay

A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled transformers, which will alarm on slow accumulation of gas or shut down the transformer if gas is produced rapidly in the transformer oil.

Forced-guided contacts relay

A forced-guided contacts relay has relay contacts that are mechanically linked together, so that when the relay coil is energized or de-energized, all of the linked contacts move together. If one set of contacts in the relay becomes immobilized, no other contact of the same relay will be able to move. The function of forced-guided contacts is to enable the safety circuit to check the status of the relay. Forced-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked contacts", or "safety relays".

Solid-state relay

A solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability. With early SSR's, the tradeoff came from the fact that every transistor has a small voltage drop across it. This collective voltage drop limited the amount of current a given SSR could handle. As transistors improved, higher current SSR's, able to handle 100 to 1,200 amps, have become commercially available.

Overload protection relay

One type of motor overload protection relay is operated by a heating element in series with the motor. The heat generated by the motor current operates a bi-metal strip or melts solder, releasing a spring to operate contacts. Where the overload relay is exposed to the same environment as the motor, a useful though crude compensation for motor ambient temperature is provided.


When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.
If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a spike of voltage and might cause damage to circuit components. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.
By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation, a light-emitting diode (LED) is used with a phototransistor.


A DPDT AC coil relay with "ice cube" packaging
Relays are used:
  • to control a high-voltage circuit with a low-voltage signal, as in some types of modems,
  • to control a high-current circuit with a low-current signal, as in the starter solenoid of an automobile,
  • to detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays),
  • to isolate the controlling circuit from the controlled circuit when the two are at different potentials, for example when controlling a mains-powered device from a low-voltage switch. The latter is often applied to control office lighting as the low voltage wires are easily installed in partitions, which may be often moved as needs change. They may also be controlled by room occupancy detectors in an effort to conserve energy,
  • to perform time delay functions. Relays can be modified to delay opening or delay closing a set of contacts. A very short (a fraction of a second) delay would use a copper disk between the armature and moving blade assembly. Current flowing in the disk maintains magnetic field for a short time, lengthening release time. For a slightly longer (up to a minute) delay, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape slowly. Increasing or decreasing the flow rate can vary the time period. For longer time periods, a mechanical clockwork timer is installed.

Relay application considerations

A large relay with two coils and many sets of contacts, used in an old telephone switching system.
Selection of an appropriate relay for a particular application requires evaluation of many different factors:

  • Number and type of contacts - normally open, normally closed, changeover (double-throw)
  • In the case of changeover, there are two types. This style of relay can be manufactured two different ways. "Make before Break" and "Break before Make". The old style telephone switch required Make-before-break so that the connection didn't get dropped while dialing the number. The railroad still uses them to control railroad crossings.
  • Rating of contacts - small relays switch a few amperes, large contactors are rated for up to 3000 amperes, alternating or direct current
  • Voltage rating of contacts - typical control relays rated 300 VAC or 600 VAC, automotive types to 50 VDC, special high-voltage relays to about 15,000 V
  • Coil voltage - machine-tool relays usually 24 VAC or 120 VAC, relays for switchgear may have 125 V or 250 VDC coils, "sensitive" relays operate on a few milliamperes
  • Package/enclosure - open, touch-safe, double-voltage for isolation between circuits, explosion proof, outdoor, oil-splashresistant
  • Mounting - sockets, plug board, rail mount, panel mount, through-panel mount, enclosure for mounting on walls or equipment
  • Switching time - where high speed is required
  • "Dry" contacts - when switching very low level signals, special contact materials may be needed such as gold-plated contacts
  • Contact protection - suppress arcing in very inductive circuits
  • Coil protection - suppress the surge voltage produced when switching the coil current
  • Isolation between coil circuit and contacts
  • Aerospace or radiation-resistant testing, special quality assurance
  • Expected mechanical loads due to acceleration - some relays used in aerospace applications are designed to function in shock loads of 50 g or more
  • 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.  
The switching contact of an electromechanical relay may be either normally open (NO) or normally closed (NC). The contacts are held in their normal or resting positions by springs or some gravity-actuated mechanism. The movable arm on a relay is called the armature. The armature is magnetic and has contacts which open or close with other contacts when the relay energized. The pickup current of a relay is the minimum amount of relay coil current which will energize the relay. The holding current is the minimum relay coil current required to keep a relay energized. The main advantage of a relay over an ordinary mechanical switch is that it can control a high power load a long distance away with very low I²R power losses in the wire conductors. The relay is energized using a low power source. The switching contacts are used to turn on and off the high power load.
In spite of electronic progresses (as programmable devices), relays are still used in applications where ruggedness, simplicity, long life and high reliability are important factors (for instance in safety applications). General purpose industrial control relays are available in ice cube, card relay, and solid state relay styles for use in a wide range of applications. All electrical relays, including the solid state relay, feature LED indicators for easy troubleshooting. These solid state relays have a high resistance to shock and vibration and have immunity to moisture and dirt.


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