April 1967 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
This April 1967 edition of Electronics World featured a handful
of articles covering different types of relays and circuits
for controlling them:
time-delay relays, stepping relays, mercury-wetted relays,
resonant reed relays,
operate and release times, relay coil considerations, and
Even with the advent of transistor switching, there are still
many uses in modern circuits for electromechanical relays, so
this material should prove useful.
Here are links to the other relay articles:
Operate and Release Times of Relays
Finding Relay Operate and Release Times
Arc, Surge, and Noise Suppression
By Roger L. Rosenberg/Systems Project Engineer, C.P. Clare &
The author graduated from Drake University in 1961. He joined
C.P. Clare & Co. as an applications engineer in September
Long electrical life resulting from precious-metal contacts
sealed in inert atmosphere and absence of wearing mechanical
parts are the most important advantages of this increasingly
popular relay type.
Within the last ten years the
reed relay has become recognized as a reliable, low-cost switching
device. The apparent simplicity of the reed relay probably influenced
some designers to use it, but its more subtle features have
increased its popularity. Long electrical life resulting from
precious metal contacts sealed in an inert atmosphere and absence
of wearing mechanical parts head the list. The relatively high
speed and varied package configurations give it advantages over
conventional relays. The price of a reed relay begins in the
$2 to $7 bracket, varying with the quantity, the number of contacts,
the coil size, and the manner of packaging. Its low cost, high
circuit isolation, and insensitivity to noise make it an ideal
replacement for electronic switches in many applications.
The heart of the reed relay is the reed switch. Discussion
of reed relays must begin with the switch since the former can
be no better than the switch it contains. To meet the demand,
reed-relay manufacturers have had to automate their production.
Automation of reed-switch manufacturing has required
much tighter control of all components and a better understanding
of what is required to make consistently good switches. As designers
found new applications, manufacturers had to develop both new
design and new processes. The pressurized reed switch and mercury-wetted
reed switch, along with varied contact material, have resulted.
The simplicity of the switch belies the sophisticated technology
required to manufacture switches with consistent electrical
and mechanical properties. A description of the reed switch
and its operating parameters will help illustrate the need
for this control and technology.
The Reed Switch
The basic reed switch is a normally open contact. It consists
of two ferromagnetic reeds, each of which is sealed in an end
of a glass tube. The reeds are positioned so that heir free
ends are overlapping (typically 1/16 in) and are separated by
a gap (between 0.005 and 0.012 in). These reeds constitute the
magnetic circuit of the switch. When a magnetic field is introduced
to the switch, the reeds become flux carriers. The overlapping
ends assume opposite polarities and attract each other. If the
attraction is strong enough to overcome the deflection characteristics
of the reeds, they will move together and touch, making electrical
For consistent contact resistance the overlapping
ends reeds are precious-metal plated. The contact plating must
be thin and uniform so that the magnetic properties of the switch
are not adversely affected. If the plating is too thick, the
magnetic gap will be too great to insure sufficient contact
pressure and will result in a high release characteristic. The
field strength required to close the switch depends on the size
of the reeds, the effective gap (atmosphere and plating) between
them, and the amount of overlap. Small changes in any of these
parameters can significantly alter the operating characteristics
of the switch. These factors must all be controlled to insure
the consistent characteristics necessary for designing the switches
into relays. Other factors which must be controlled to obtain
consistent and reliable operations are blade alignment, contaminants
in the gas, and seal integrity.
The amount of power
required to operate a reed switch is typically 125 mW. The more
power applied, the faster the reeds will close, until the saturation
point of the reeds is reached. The maximum speed is typically
0.8 ms, but this is usually impractical in most circuit applications
because of the power requirements. Contact bounce is also increased
when the switch is driven hard, so speed should never be considered
Contact life is affected by contact bounce, the
load switched, and the repetition rate. End of contact life,
however, can only be determined by the circuit requirements.
The load which can be handled by the reed switch depends on
the contact material, the number of operations expected, and
the failure criteria.
most common contact material is plated and sintered gold. This
contact has a relatively high rating of 15 VA and a life in
excess of 20 million operations. Plated bright gold contacts
can perform well with low-level loads because of their low and
constant contact resistance. Bright gold presents a hazard,
however, in that the closed contacts may fail to release because
of a phenomenon spoken of as "particle migration" or "cold welding".
Rhodium contacts appear good on both high- and low-level loads.
Difficulties in controlling the plating can result in inconsistent
switches. Tungsten contacts are good performers in switching
high loads such as lamps and solenoids.
be used to coat one of the reeds so that it becomes a fluid
contact. It eliminates operate bounce and maintains a constant,
low contact resistance. The switch becomes position-sensitive
since the mercury is fed up the lower reed by capillary action
from a mercury pool at the bottom of the switch. (Note that
one of the reeds is dry as contrasted to the mercury-wetted
contact relay, discussed elsewhere, in which all contacts are
wetted by mercury.-Editor)
While the basic switch is
a normally open contact, other forms are available. The normally
closed contact is made by affixing a permanent magnet of sufficient
strength to close the switch. The operating flux must oppose
the magnet sufficiently so that switch will open and remain
open as long as the operating flux is present. "Break" and "make"
can be accomplished by combining a normally open and a normally
closed switch in the operating coil. The normally closed contact
can be adjusted by magnet biasing so that break-before-make
operation can be achieved. Break-before-make is also available
in a single capsule. The break contact is accomplished by magnet
or spring biasing the armature or swing contact to one of the
stationary contacts. The most commonly used reed switches are:
Standard Dry Reed Switches. The standard reed switch,
Fig. 1 (top), is approximately 3 1/4 in long by 7/32 in in diameter.
It has plated and sintered gold contacts and is rated at 15
VA resistive (250 V maximum, 1 A maximum. Contact resistance
initially is less than 50 milliohms. The standard switch will
withstand shocks of 11 milliseconds' duration to 40 G peak without
Micro Dry Reed Switch. The micro, or
miniature. reed switch, Fig. 1 (center), is approximately 15/8
in long and 0.10 in in diameter. It is rated at 10 VA resistive
(200 V d.c. maximum, 750 mA maximum). Initial contact resistance
is typically 100 milliohms. The micro reed can withstand shocks
of 11 ms duration to 50 G peak without false operation.
Mercury-Wetted Reed Switch. The mercury-wetted reed switch,
Fig. 1 (bottom), is approximately the same size as the standard
reed switch. It is rated at 50 VA resistive (200 V maximum,
2 A maximum). It is position-sensitive and must be mounted within
30° of vertical.
Fig. 1. (Top) A standard dry reed switch.
(Center) Miniature or micro dry reed switch. (Bottom) Mercury-wetted
reed switch. These mercury-wetted types of switches, because
of a pool of mercury at one end, must always be used in vertical
High-Voltage Reed Switch. The high-voltage reed switch has the
same dimensions and contact rating as the standard reed switch.
It is pressurized to achieve the high stand-off rating of 1500
V r.m.s. Special reed switches with standoff voltages to 5000
V peak are available.
The reed relay
is made by enclosing one or more reed switches in an operating
coil. The coil is usually wound on a bobbin made of nylon or
other similar material. The bobbin may also have anchors or
pins for attaching the coil leads and reed switches. The number
of capsules to be placed within the coil determines the bobbin
size. Most manufacturers limit the coil to handle 12 of the
standard reed switches. Above this size proper operation of
all switches is limited by the efficiency of the coil. Different
contact forms can be combined in the same operating coil so
that contact configurations such as 12A, 8B, or 4C or combinations
of these are possible in a 12-switch coil.
coils are wound on bobbins of a certain size, the resistance
and turns of coils offered are determined by wire sizes. For
a given bobbin, the turns and resistance will vary with each
wire size and thus the operating voltage of the relay will change
with each wire size for switches with the same operate characteristics.
For example, a one-switch bobbin wound with #29 wire has 1200
turns and a resistance of 10 ohms while the same bobbin wound
with # 42 wire has 22,200 turns and a resistance of 3750 ohms.
The coil power required to operate the relay is determined
by the number and configurations of contacts and by the operating
speed required. A typical single form A relay will require approximated
125 mW, a 5 form A relay, approximately 450 mW. Most open-type
relays will dissipate 4 watts in 25° C ambient. The maximum
dissipation of the relay will depend on the coil wire insulation
and construction materials.
No matter what package configurations
the coil and reed switches acquire, there should be no stresses
on the reed blades. Stresses can fracture the glass-to-metal
seal on the switch and result in an early failure. Another factor
which should be considered in the final package is shielding.
Shielding of the relay can improve its characteristics and eliminate
its influence on other relays or avoid its being influenced
by them. Magnetic flux of one relay might actuate an adjoining
relay. This can be significant when sensitive relays are packed
The load to be switched must be evaluated so that the
proper reed switch can be chosen. Loads such as lamps which
have a high in-rush current can reduce life. Such schemes as
having a resistance in the lamp circuit to keep it hot but dim
can add millions of operations to the reed switch. Suppressing
an inductive load also extends contact life. Contact protection
such as an RC network can be applied, but is required only when
the load exceeds the published ratings or when life expectancy
must be extended.
After the proper switch has been chosen,
the next consideration is to select the best package for application.
The reed relay may be potted in a can and fitted with an octal-type
base for chassis mounting. Other special packages are made.
The most popular means of mounting the reed relay is on printed-circuit
boards. Its low profile and contact termination adapt it very
well to such mounting. The coil bobbin is usually fitted with
terminals which make the relay easy to install and give protection
to the reed switches. A typical example of a reed relay printed-circuit
board assembly is shown in Fig. 2. This 4 1/4" x 10 7/8" board,
having five counting stages, can be mounted on 25/32" centers.
The reed switches are replaceable in this type of assembly if
a change is required. For severe environments the relay may
be potted or molded into special configurations. An epoxy-molded
assembly is available for printed-circuit board mounting. The
open-construction relay is quite adequate for most industrial
Fig. 2. An assembly of reed relays on a printed-circuit
These relays will operate over a temperature range from -65°
C to 85° C. Special assemblies are made to operate to 125° C.
The minimum breakdown voltage is typically 500 V r.m.s. 60 Hz,
and the insulation resistance is greater than 100,000 megohms.
Magnetic shields are supplied on the relays in most cases. Even
with shields, the relays should not be located close to any
device which can generate a strong magnetic field. The reed
relay, if properly chosen. does not require a close-tolerance
power source. One with a tolerance of ±10% is adequate for most
When choosing a relay for an application,
the worst-case power and temperature conditions must be considered
together with the most unfavorable coil-manufacturing tolerance.
If under worst-case conditions the available power diminishes
to approach the just-operate value of the switches, the operate
time will increase. Reed switches release in less than a millisecond
in most relay assemblies. If the coil is shunted by a diode
or by an RC network, however, the release time may reach several
High insulation resistance requires special
materials and handling during the manufacture of the relay.
Relays with insulation resistance greater than 500,000 megohms
have been made in different configurations. Higher break-down
voltage requires special assemblies.
After the relay has been chosen it must function in
the circuit. The fast operate-time of reed switches can become
a problem unless care is taken to insure that the drive pulse
is free from "grass" or discontinuities. If the discontinuities
are long enough to allow the reed switch to release, random
faults can occur. It is recommended that reed-switch counters,
shift registers, etc., be driven by a mercury-wetted contact
relay which has been buffered against possible discontinuities.
When the coil is de-energized and the reeds move apart,
they swing through their neutral position and oscillate at their
resonant frequency until all their energy is dissipated. Unless
the switches are damped, the application of a holding voltage
during this oscillating period can cause the reeds to reclose.
An "off" time sufficient to insure the settling of the reeds
is required to provide proper operation and repeatable timing.
To get the maximum number of operations from the reed
switch every opportunity to first establish the path and then
switch the load with a single heavy-duty contact should be explored.
The most reliable circuits are those which use a combination
of coil and switch logic. An example of this is the binary-coded
decimal counter in which none of the contacts switch a load;
they only perform the steering function for the count pulse.
The output contacts can also be connected so that they can be
strobed by a single contact thus insuring the same long life
The addition of RC networks can make
reed relays slow-release or slow-operate and slow-release. If
the relay which is to be slowed has several switches, staggered
operate and release can occur. The interposing of a single form
A contact relay, having the proper delay network, to drive the
multi-contact relay will solve the problem. Since the relay
which now has the delay has a higher resistance, lower capacitance
Reed relays with multiple
wound coils yield all of the basic logic functions and numerous
special devices. Reed-relay two- and four-state flip-flops can
perform all of the standard counting functions at speeds more
than adequate for most industrial applications.
of the most popular special relays is the magnetically latching
or hi-stable relay, the windings of which are connected to oppose
each other. A magnet is adjusted to a level not sufficient to
close the reed but strong enough to hold it closed. The winding
which aids the magnet is the "set" winding. The one opposing
is the "release" winding. Voltage applied to the "set" winding
causes the reed to close. When the voltage is terminated, the
magnet holds the reed closed. Voltage applied to the "reset"
coil opposes the magnet flux causing the reed to open.
Reed relays are used in a variety of industrial control devices,
in telephone switching, materials handling, and in manufacturing
automation. They provide the true isolation between input and
output of a contact device, yet perform faster than conventional
electro-mechanical relays. They permit multiple inputs, thus
enabling logic to be performed by both the coils and contacts.
Mercury-Wetted Relay Contact Protection
Contact life expectancy is based on the use of proper contact
protection, usually in the form of an RC network installed as
close as possible to the relay terminals. Three methods of applying
this protection, and means of calculating the capacitor and
resistor values, will be covered here.
In the following discussion, the value of the capacitor
(in microfarads) can be found from C = I2
I is the current in amperes immediately prior to contact opening.
The value, in ohms, of the associated resistor can be found
from R = E / (10I)α. where E is the source voltage just prior
to contact closure and α = 1 + (50/E).
is less than 70 volts, R may be three times the calculated value;
where E is greater than 70 volts, but less than 100 volts, R
may be ±50% of the calculated value; where E is greater than
100 volts but less than 150 volts, R may be ±10% of the calculated
value; and where E is greater than 150 volts, R may be ±5% of
the calculated value. In all cases, the minimum value of R is
0.5 ohm, and the minimum value of C is 0.001 µ.F.
The arc suppressor shown in Fig. A is suitable for most load
switching demanded of mercury-wetted contact relays. If desired,
the value of the capacitor may be increased as much as 10 times
to help reduce voltage transients of inductive loads.
When contact load current is 0.5 A or less, and the source voltage
is 50 volts or less (peak values for a.c. circuits), the resistor
may be eliminated as shown in Fig. B. The capacitor value must
not exceed the calculated value.
For certain extreme
loads, such as highly inductive a.c. loads at voltages above
100 V a.c., it may be desirable to place the main RC arc suppressor
(R1-C1) across the load as shown in Fig. C. This alleviates
the problem of a.c. leakage current through an RC arc suppressor
in parallel with the contacts, but may result in a condition
which exposes the contacts to voltage transients having a rate
of rise in excess of 5 V/μs maximum, due to the inductance of
the lead wires. A secondary arc suppressor (R2-C2) must then
be included across the contacts. However, a.c. leakage across
the contacts is markedly reduced since C2 need only be one-hundredth
of the calculated value.
Both resistors should
be the calculated value although the value of C1 may be increased
up to 10 times the calculated value to further reduce transients.