June 1961 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
Even back in 1961 when this
"How Relays Work" article appeared in Radio-Electronics magazine, there
was a large variety of electromechanical relay types. Many of those have been supplemented
or replaced by solid state equivalents, but there are still some applications where
only good old hard physical contacts can do the job for both practical and economic
reasons. Very high temperatures and systems with no electronic interface are examples.
One very common example of the latter is the contacts that a centrifugal switch
uses to take the
starter capacitor of single-phase induction motor in and out of the circuit.
The main diagram shows a clapper relay, a meter relay, an induction relay, a thermal
relay, reed relay, and a piezoelectric relay, among others. In the days before everything
being controlled by solid state devices, amazing electromechanical contraptions
were designed to perform actions according to mathematical equations describing
physics principles. Gun turrets on ships, airplanes, and tanks are significant examples
of target tracking and motion compensation used in fire-control systems which incorporated
How Relays Work: Part I - A close look at clapper, solenoid, induction, thermal,
stepping and reed type relays
Relay Operation Fig. 1 through Fig. 11
By Tom Jaski
The electronic technician who plans to go into industrial electronics should
put relays high on his list of must-know subjects. In industry, all control was
handled by relays before electronics came along. Even now the majority of industrial
control circuits use relays in many forms. With electronic controls, many relays
have to be used, and often the technician meets strange-looking devices that don't
appear to be relays but are.
Let's see how relays began. In 1824, an Englishman named Sturgeon invented the
electromagnet. In 1829, Joseph Henry, professor of mathematics and natural science
at Albany Academy, added an armature which carried contacts, and thus the relay
was born. In dc circuits, relay design has changed, mostly because of the necessity
for mass production and adaptability. But Henry's principle has never been improved
upon very much.
In 1886, alternating-current transmission was inaugurated. Where relays were
needed, modified dc units served. Then development of special ac relays began, and
there were so many contributions in that period that most contributors will probably
not get credit for their work. In 1901, Westinghouse introduced the induction relay.
These are very common, and are very important in our giant power distribution and
Only in the last few years have new principles been applied to relay service.
These principles were made available by materials technology in the electronics
industry. One such development is the Mullenbach Capaswitch, which uses piezoelectric
characteristics of artificial materials to obtain mechanical motion. But in contrast
to switching by nonmoving devices, such as magnetic amplifiers, vacuum tubes and
transistors, relays as discussed here still depend on mechanical motion for their
Classes of Relays
Relays can be classified by the method used to translate electrical power into
mechanical motion. Thus for dc there are the clapper (the most common) and the solenoid
type. Clapper relays are most familiar in the form of telephone relays, small control
relays used in model control and similar applications. Fig. 1 shows the basic construction
of a clapper relay. Another clapper relay, not as common, is the horseshoe type
used in pipe-organ construction. This one is reminiscent of the early Henry relays,
except for refinements in construction and materials.
Solenoids are a special class of relays.
G-E induction relay.
The solenoid relay, in which a movable core is used to open or close contacts,
is used where we must take advantage of the greater travel and force available in
them, and where the linear motion is particularly suitable for control of the relay
characteristics. For example, it is very simple to make a time-delay relay from
a solenoid type by adding a dashpot - a cylinder with a piston. The amount of force
it requires for motion depends on what the piston moves. Sometimes this is air,
which is allowed to leak out through a small hole controlled by a needle valve.
With a steady force on the piston it takes time to move it, and a time delay is
obtained. Initially, the air in the cylinder can be compressed somewhat, and the
time characteristic of an air dash pot is. not linear. To make it nearly so, a noncompressible
resisting medium such as oil must be used.
Ac relays also appear in the basic clapper and solenoid types, but their magnetic
structure must be laminated, as in Figs. 2 and 3. The laminations prevent the creation
of a great deal of heat in the iron due to eddy currents. A shading coil prevents
armature chattering. It is a single turn of copper around a part of the armature
or core. This single turn acts like a shorted secondary on a transformer. The current
in it is approximately 90° out of phase from the current in the coil. When the
magnetic field of the main coil goes through zero, the shaded portion of the core
still has some magnetic field, keeping the armature in place against the spring
force trying to remove it.
The next most prevalent ac relay (although not used in communications) is the
induction type. Many of them appear in industry where they protect large generators
and motors, compare the current in the three lines of a 3-phase system, and determine
whether it is flowing in the proper direction. Practically all types of induction
relays incorporate some time delay, often inversely proportional to the current.
This is one of the reasons for their adoption in the first place; their operation
depends on a motor action which takes some time. Fig. 4 shows the principle of the
induction relay. It is akin to the watt-hour meter that measures power consumption
in every home.
It consists of a disc (usually aluminum) mounted on a rotating
shaft. The disc rotates between the poles of a shaded electromagnet and a permanent
magnet. Eddy currents induced in the disc by the main portion of the electromagnet
pole are in phase with the currents in this coil, but out of phase with those in
the shaded pole piece. As a result, the eddy currents generate a magnetic field
out of phase with the shaded pole field, which "pushes" against one side of this
field and causes the disc to rotate. The permanent magnet slows down the disc. The
moving disc section between the permanent-magnet poles has currents induced in it
in such a way that the magnetic field from these currents opposes the motion that
created them. This is an application of Lenz's law. The spring retains the disc
until the current is strong enough to move it, and returns it to its starting position
after the current subsides.
The unit shown in Fig. 4 is a very elementary induction relay with a somewhat
peculiar time-current curve. To get the much used inverse time-current characteristic,
a complicated magnet structure is designed for the driving electromagnet, with several
coils above and below the disc. The explanation of this type of relay action is
too complicated for a brief discussion and falls outside the scope of this article.
Early Weston meter relay.
Panel of six meter relays.
Clapper relays come in assorted sizes and shapes.
Special relays come in many forms. Most familiar, perhaps, is the thermal relay
using a bi-metal strip to operate contacts. These are quite frequently employed
in radio transmitters to delay the application of B-plus voltage until filaments
are heated (Fig. 5).
Another special relay, used where sensitivity is of paramount importance, is
the so-called meter relay (Fig. 6). It consists of a D'Arsonval type meter movement,
with a pointer. The pointer carries very small contacts that close a circuit when
the pointer is deflected a preset amount. Sensitivity is adjusted by setting the
"fixed" contacts in a particular position. They can be moved with a small screwdriver
or a knob on the instrument. In modern versions of the meter relay, the contacts
are no longer on the pointer but on a special yoke and the pointer acts only as
A recent type of relay is the piezoelectric. It uses a barium titanate slab which
deforms when voltage is applied to electrodes on its surface. The distortion is
used to close contacts (Fig. 7).
Ratchet or stepping relays are special applications of the clapper type. Through
a pawl-and-ratchet arrangement the armature moves a shaft that carries a cam which
opens and closes contacts in sequence. Stepping switches are an extension of this
idea. In these units the armature also rotates a shaft through a pawl and ratchet,
but the shaft carries wipers which make contact with a large number of stationary
contacts successively. Figs. 8 and 9 show two kinds of stepping switches. The one
in Fig. 8 rotates continuously, stepping one notch each time the relay coil is energized.
The switch in Fig. 9 steps a certain number of times and then is returned by the
release armature, which lifts the pawl, allowing a spring to return the wipers to
their starting position.
An unusual relay is the rotary solenoid type shown in Fig. 10. Here the core
is drawn into the solenoid. Attached to the core is a disc, which is held by a spring.
The disc rides on ball bearings that ride in inclined grooves on the frame. As the
core is attracted deeper into the coil, a force is exerted on the disc, pulling
it down toward the solenoid coil. The only way it can get closer is by rolling the
ball bearings down the inclined grooves, thus rotating the disc. The shaft of the
rotary solenoid can carry contacts, cams or the rotating wiper section of a wafer
Last, there are frequency-sensitive reed relays. In these units the armature
is a tightly clamped "tongue" similar to the little tongues in a harmonica (Fig.
11). The tongue has a natural resonant frequency. If the coil carries ac of this
frequency, the tongue vibrates. The end of the tongue or reed carries a contact
which closes many times per second when the reed vibrates. A slow-opening auxiliary
relay is kept closed by this periodic contact until the reed stops vibrating. Obviously,
one coil can carry many reeds, all responding to different frequencies.
This basically is the entire array of relays available. There are many special
versions of these basic types, and some unusual devices which could also technically
be called relays (some of them pneumatic-electric), but in principle they are all
the same. To Be Continued
Posted July 31, 2023