April 1967 Electronics
April 1967 edition of Electronics World featured a handful of articles
covering different types of relays and circuits for controlling them:
relays, stepping relays, mercury-wetted relays, resonant reed relays,
operate and release times, relay coil considerations, and more.
April 1967 Electronics World
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. All copyrights are hereby acknowledged.
Electronics World articles.
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
See all the available
Electronics World articles.
Roger L. Rosenberg/Systems Project Engineer, C.P. Clare & Co.
The author graduated from Drake University in 1961. He joined C.P.
Clare & Co. as an applications engineer in September of 1963.
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
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.
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 position.
Fig. 2. An assembly of reed relays on a printed-circuit board.
The 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
"Mercury can 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
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 false operation.
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.
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.
Because the 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
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 closely together.
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 applications.
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 applications.
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 milliseconds.
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 for them.
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 is needed.
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
One 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
In the following discussion, the value of
the capacitor (in microfarads) can be found from C = I2/10,
where 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).
Where 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.
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
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.