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Relay types and Working

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Relays are electrically actuated switches. The three basic kinds of relays include

Mechanical relays, reed relays, and solid-state relays. For a typical mechanical relay, a current sent through a coil magnet acts to pull a flexible, spring-loaded conductive

plate from one switch contact to another. Reed relays consist of a pair of reeds (thin, flexible metal strips) that spring together whenever a current is sent through an encapsulating wire coil. A solid-state relay is a device that can be made to switch states by applying external voltages across n-type and p-type semiconductive junctions

(see Chap. 4). In general, mechanical relays are designed for high currents (typically 2 to 15 A) and relatively slow switching (typically 10 to 100 ms). Reed relays are designed for moderate currents (typically 500 mA to 1 A) and moderately fast switching (0.2 to 2 ms). Solid-state relays, on the other hand, come with a wide range of current ratings (a few microamps for low-powered packages up to 100 A for high-powerpackages) and have extremely fast switching speeds (typically 1 to100 ns). Some limitations of both reed relays and solid-state

relays include limited switching arrangements (type of switch section) and a tendency to become damaged

by surges in power.



The voltage used to activate a given relay may be either dc or ac. For, example,

when an ac current is fed through a mechanical relay with an ac coil, the flexible-metal

conductive plate is pulled toward one switch contact and is held in place as

long as the current is applied, regardless of the alternating current. If a dc coil is supplied

by an alternating current, its metal plate will flip back and forth as the polarity

of the applied current changes.

Mechanical relays also come with a latching feature that gives them a kind of

memory. When one control pulse is applied to a latching relay, its switch closes. Even

when the control pulse is removed, the switch remains in the closed state. To open the

switch, a separate control pulse must be applied.


Specific Kinds of Relays

Subminiature Relays

Typical mechanical relays are designed for switching relatively

large currents. They come with either dc or ac coils.


relays typically come with excitation-voltage


of 6, 12, and 24 V dc, with coil resistances (coil ohms) of

about 40, 160, and 650 Ù, respectively. AC-


relays typically

come with excitation-voltage

ratings of 110 and 240 V ac,

with coil resistances of about 3400 and 13600 Ù, respectively.

Switching speeds range from about 10 to 100 ms, and current

ratings range from about 2 to 15 A.


Miniature Relays

Miniature relays are similar to subminiature relays, but they

are designed

for greater sensitivity and lower-level


They are almost exclusively actuated by dc voltages but may

be designed to switch ac currents. They come with excitation

voltages of 5, 6, 9, and 12, and 24 V dc, with coil resistances

from 50 to 3000 Ù.


Reed Relays

Two thin metal strips, or reeds, act as movable contacts. The reeds

are placed in a glass-encapsulated

container that is surrounded

by a coil magnet. When current is sent through the outer coil, the

reeds are forced together, thus closing the switch. The low mass

of the reeds allows

for quick switching, typically around 0.2 to

2 ms. These relays come with dry or sometimes mercury-wetted

contacts. They are dc-actuated

and are designed to switch lower-level

currents, and come with excitation voltages of 5, 6, 12, and

24 V dc, with coil resistances around 250 to 2000 Ù. Leads are

made for PCB mounting.


Solid-State Relays


Solid-State Relays or SSRs are sealed modules designed to be

used in the same way as electromechanical relays, but switching

using opto-isolators and power transistors or Triacs. As such they

are not really basic components, but modules.

They are usually divided in two varieties, AC and DC. An AC

device usually uses an opto-isolator with zero-switching detector

and a Triac to switch the load as the voltage is close to 0 V

in the cycle, but a DC device uses a MOSFET or IGBT transistor

(see Chap. 4) to switch the load.

Using an opto-isolator has the dual advantage of only requiring

a couple of mA to switch the relay on, but also isolates the control

side of the relay from the switching side.



 Few Notes about Relays

To make a relay change states, the voltage across the leads of its magnetic coil should

be at least within }25 percent of the relay’s specified control-voltage

rating. Too

much voltage may damage or destroy the magnetic coil, whereas too little voltage

may not be enough to “trip” the relay or may cause the relay to act erratically (flip

back and forth).

The coil of a relay acts as an inductor. Now, inductors do not like sudden

changes in current. If the flow of current through a coil is suddenly interrupted,

say, a switch is opened, the coil will respond by producing a sudden, very large

voltage across its leads, causing a large surge of current through it. Physically

speaking, this phenomenon is a result of a collapsing magnetic field within the

coil as the current is terminated abruptly. [Mathematically, this can be understood

by noticing how a large change in current (dI/dt) affects the voltage across

a coil (V = LdI/dt).] Surges in current that result from inductive behavior can create

menacing voltage spikes (as high as 1000 V) that can have some nasty effects

on neighboring devices within the circuit (e.g., switches may get zapped, transistors

may get zapped, individuals touching switches may get zapped, etc.). Not

only are these spikes damaging to neighboring devices, they are also damaging

to the relay’s switch contacts (contacts will suffer a “hard hit” from the flexible-metal

conductive plate when a spike occurs in the coil).

The trick to getting rid of spikes is to use what are called transient suppressors.

You can buy these devices in prepackaged form, or you can make them yourself.

The following are a few simple, homemade transient suppressors that can be used

with relay coils or any other kind of coil (e.g., transformer coils). Notably, the switch

incorporated within the networks below is only one of a number of devices that may

interrupt the current flow through a coil. In fact, a circuit may not contain a switch at

all but may contain other devices (e.g., transistors, thyristors, etc.) that may have the

same current-interrupting


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