Magnetic Reed Switches and Relays, September 1965 Electronics World

Reed switches have been in use for a long time, and are still common today. The position-sensing ones with mercury inside as the connection making and breaking medium have long been gone from the market. Mercury is on the boogeyman list of items that shall not be used in any form, regardless of how inaccessible or how small the amount happens to be. Mercury is an excellent choice for the job because it provides a reliable contact without excessive arcing to spoil the contacts, and the angle of make/break is highly repeatable (especially as compared to a bimetal springs with hard contacts). Mercury switches were the de facto standard in wall switches for lighting for decades. Not only do I remember my elementary school teacher passing beads of mercury around for us kids to experience the properties of (heavy for its size, liquid metal at room temperature, why it's called "quicksilver," etc.), but I also remember breaking apart light switches to get the mercury out of them just to play with. Drop the bead on the floor and it "explodes" into many tiny balls, which magically recombine when you touch them together. I'd guess the danger of such a small exposure to mercury people were exposed to back in the day pales in comparison to all the toxic ingredients in food (processing and preservatives), off-the-shelf common chemicals (i.e., RoundUp™, bug poisons, etc.), the plethora of "recreational" drugs, and government-mandated virus injections a large portion of society avails itself of today. This 1965 Electronics World magazine article has, among other good information, an interesting table comparing data on typical conventional relays, reed relays, and transistor switching circuits with approximately the some amount of load-handling capabilities.

Characteristics of these fairly new components make them suitable for a variety of electronic applications. Though not as fast as transistor switches, they have lower contact resistance, higher open-circuit resistance, will switch higher voltages, and have less capacitance. Practical uses include: tach pickups, proximity switches, d.c. choppers.

Not too long ago, a new component made its first appearance in electrical and electronic equipment. As with most really basic inventions, the operation of the magnetic reed switch is strikingly simple. Rapid acceptance throughout the industry shows that it filled a definite need for bridging the gap between the ordinary electromechanical contact or or relay and the solid-state switch.

A survey of conventional relays shows that only specially designed types will operate faster than four milliseconds, will exhibit low contact bounce and contact capacitance, will work in a wide range of environment, and have long life expectancy. The reed relay makes all these features possible, and for a price that is well below that of the highly specialized electro-mechanical relay.

The reed switch has definite advantages over the transistor switch: high ratio of open-contact to closed-contact resistance, (typically 10¹⁰ to 1), high breakdown voltage (300 volts at 60 cps and more), and contact capacitance of less than 1 pf. These properties make the reed switch well suited for use in high-impedance circuits, to handle higher voltages than a transistor switch, and to switch radio-frequency signals.

Transistor-switching circuits with equal load capacity, while capable of operating in the microsecond range, seldom exceed "off-on" resistance ratios of 100,000 to 1; collector-to-emitter breakdown voltages of 50 volts to 60 volts; and generally have base-to-collector and collector-to-emitter capacitances of 40 pf. and 80 pf. respectively. Unless protected by breakdown diodes, transistors can be damaged quite easily by voltage transients which will not affect reed switches. Furthermore, a simple transistor switching circuit provides little insulation between the controlling and the controlled signal, both are applied to the same transistor which is not a unilateral device such as the vacuum tube. If the reed switch is actuated by a coil (reed relay), both signals are electrically insulated from each other and mutual capacitance between coil and reeds can be low.

Table 1 compares pertinent data on typical electromechanical, reed, and solid-state relays with similar load capacities. All values given in the table are approximate.

Fig. 1 - An s.p.s.t. normally open magnetic reed switch.

Table 1 - Comparative data on typical conventional relays, reed relays, and transistor switching circuits with approximately the some amount of load-handling capabilities.

Fig. 2 - Use of a reed switch in tachometer or rpm counter.

The reed switch consists of two or more metal reeds which are enclosed in a hermetically sealed glass capsule. The reeds are made from nickel-iron, a magnetically "soft" alloy that retains only a little magnetism. Fig. 1 shows a s.p.s.t., normally open reed switch. The overlapping ends of the nickel-iron reeds are the contact surfaces which are usually plated with gold or other suitable noble metal. This will promote lower contact resistance and help keep the surfaces electrically clean. A chemically inert gas which is enclosed in the capsule further contributes to the maintenance of good contact properties. As with any switch, contact arcing should be suppressed by suitable circuitry; whatever remains is confined to the glass capsule. This permits operation of the reed switch in chemically active or explosive atmospheres. It can be seen that the overlapping contact ends of the reed switch represent little mutually opposing area, hence the inherent contact capacitance is low.

Contact capsules are made in various sizes and for different current- and voltage-switching ranges, giving the circuit designer a wider range of specifications from which to choose. Reed switches can be actuated by permanent magnetic or electromagnetic fields, or by a combination of both.

When a magnet is moved close to a reed switch, more and more of its field lines tend to permeate the reeds because of their lower magnetic reluctance in comparison to air. As the magnet moves closer to the reeds, it reaches a point where the mutual attraction between the reeds begins to pull them toward each other. As the mutual distance is shrinking, the magnetic flux across the gap between the reeds increases as the square of the distance of separation. Even if the external magnet were not moved much beyond the point where mutual attraction begins to pull the reeds together, the increasing attraction between the reeds causes them to accelerate until they make contact.

The reverse action takes place when the external magnet is removed. As the distance between the magnet and the reeds is increased, the magnetic remanence tends to keep the ends of the reed together. But since the reeds consist of a magnetically "soft" alloy, the point is soon reached where the decreasing flux from the receding magnet is insufficient to hold the reeds together and they begin to separate. At this moment, the air gap reduces the remaining flux sharply, causing the reeds to recede with increasing speed.

It can be seen that this magnetic effect also helps to reduce contact bounce which tends to occur, especially at the moment of closure. As the reeds make contact, the magnetic flux and the mutual attraction increase greatly and help to counteract the tendency of the reeds to rebound. The magnetic effect, in conjunction with the relatively low mass of the reeds, permit the reed switch to follow rapid changes of the actuating magnetic field, allowing switching rates of several hundred cycles per second.

These properties can be put to use, for instance, in a pulse generator, tachometer, or revolution-counting circuit, as shown in Fig. 2. The drive shaft of the electric motor rotates a disk made from a magnetic shielding material such as Mu-metal with a cut-out along part of its periphery. The disk normally shields the contact capsule from the permanent magnet. As the disk is rotated, the magnetomotive force of the magnet actuates the contact capsule every time the cut-out permits the flux to reach the reeds. The reeds are connected in series with a battery and a suitable resistor. The pulsating current or voltage across the resistor is available for indicating or processing in external equipment, such as an integrating voltmeter for a tachometer or an electric counter.

Another application of the contact capsule would be to monitor the condition of a window or door. The capsule and actuating magnet can be easily concealed in adjacent parts of the wooden structures as shown in Fig. 3. With the window closed, the magnet energizes the reeds and thereby maintains a short-circuit across the pilot light at a guard's desk, preventing the bulb from glowing. As soon as the window or door is opened, the magnet no longer holds the reeds together, the short-circuit is removed, and the pilot light is energized. One advantage of the reed switch in this application is that the invisible magnetic flux actuates the contacts rather than a stud or contact button which might, by its presence, reveal the presence of an alarm system on the premises.

Fig. 3 - When the window is opened, the lamp is turned on.

Fig. 4 - Use of magnetic biasing to produce s.p.d.t. switch.

The magnetic reed switch can be actuated by an external permanent magnet as well as by the magnetic field of current flowing through a solenoid, or by both. When a small permanent magnet is placed in the vicinity of the reed capsule, the resulting flux permeates the reeds and acts as a magnetic bias which can be modulated by a changing magnetic field from the solenoid. With an actuating current of a given intensity, pull-in or drop-out points of the reed relay can be adjusted by varying the distance between the external magnet and the contact capsule and coil assembly. Magnetic biasing makes it possible to convert a normally open reed switch into a normally closed type. The constant magnetic field keeps the reeds closed. To open the contact, the electromagnetic flux of the solenoid must counteract the permanent flux of the mag-. et to neutralize the mutual attraction of the reeds.

Fig. 4 shows how magnetic biasing can be used to make a single-pole, double-throw switch. The small permanent magnet between the two reeds shown to the right in the diagram establishes a field with lines of force perpendicular to the main axis of the contact capsule. Depending on the strength and polarity of the actuating magnetic field, the large reed is attracted by either the upper or lower contact reed.

For certain applications, even the low contact bounce of a reed switch cannot be tolerated. This residual bounce may be eliminated by the use of mercury-wetted contacts. Fig. 5 illustrates this special form of reed switch. A small amount of pure mercury is enclosed in the contact capsule. The form of the reeds promotes capillary action and keeps the contact ends of the reeds covered with a thin film of mercury.

Mercury has a very high surface tension (which accounts for its tendency to form small globules when spilled on a flat surface). When an object is pressed against a mercury surface, the mercury recedes until the pressure of the object exceeds the surface tension of the liquid metal. At this point the object penetrates suddenly and rapidly and the mercury flows up on the object, seeking to cover as much of it as possible. When the object is withdrawn, the opposite action takes place. A mercury filament is formed which is suddenly broken when the surface tension is exceeded. The same surface tension is responsible for the capillary action which maintains a constant film at the contact surface that is fed by means of the mercury pool at the bottom of the capsule.

One disadvantage of the mercury-wetted contact is the need to prevent the mercury from flowing across the contact gap. To accomplish this, the capsule's longitudinal axis must be maintained in a perpendicular or near-perpendicular position, with the mercury pool at its lowest point. This might preclude the use of the switch in certain types of aircraft or for missiles whose attitude and acceleration in flight may frequently nullify the action of gravity.

The contact capsule can be actuated by a permanent magnet (to form a proximity switch) or by current flowing through a solenoid to provide the magnetic flux. In the latter case the arrangement is called a reed relay. Conventional relays require magnetic forces on the order of ounces, whereas reed relays can be actuated by considerably less than one ounce. Consequently, reed relays require driving power from as little as 20 mw. for miniature relays to approximately 500 mw. or more for multiple contact relays where several contact capsules are actuated by the same relay coil. In a conventional relay, the driving member is usually spring loaded and mechanically linked with the contact member. It can be seen from Fig. 6 that the driving member and contact member in a reed relay are one and the same and that the natural elasticity of the cantilever construction makes additional spring loading unnecessary.

Table 2 - Characteristics and performance data on a number of typical magnetic reed switches and relays.

Typical magnetic reed switches. (A) S.p.s.t. standard, 15 va., 500 v., Gordos Corp. (B) S.p.s.t. miniature, 4 va., 300 v., Gordos Corp. (C) S.p.s.t. miniature mercury-wetted magnetic reed switch inserted into test coil, 3 va., 100 v. Hamlin, Inc.

Fig. 5 - By means of capillary action, the film of mercury from the pool at the bottom of the capsule covers the contact surfaces. This is done to prevent contact bounce. The switch must be kept in a vertical position, however, in order to keep the mercury from bridging the contact gap.

Fig. 6 - Cross-sectional view of a typical magnetic reed relay.

Typical reed relays. (A MIL-type sealed s.p.d.t., C.P. Clare & Co. (B) Printed-circuit s.p.s.t., normally open, and s.p.s.t., normally closed, Wheelock Signals Inc. (C) Printed-circuit encapsulated MIL-type, s.p.s.t., manufactured by General Reed Co.

Physical dimensions, position, and magnetic properties of the reeds of a contact capsule are closely controlled during manufacture. The fabrication of capsules is largely automatic. This results in remarkable uniformity among one type, as far as pull-in, drop-out, and contact resistances are concerned. Pull-in and drop-out points are described in terms of magnetomotive force F which is measured in ampere-turns (NI).

A typical contact capsule requires 60 ± 10 ampere-turns for closure and opens at a flux corresponding to 22±10 ampere-turns. The difference between pull-in and drop-out results from mechanical and magnetic properties of the reeds, mainly from the hysteresis of the nickel-iron alloy. Once the working gap of the reed switch is closed, reluctance of the magnetic circuit is reduced and less magnetomotive force is required to maintain the flux needed to keep the reeds together. Contact resistance is on the order of 20-50 milliohms. Operating time is composed of field development time and reed motion time, and can be reduced by increasing the magnetomotive force, or electrical power, if a coil is used to activate the reeds. However, there is a minimum switching time for a given contact capsule, and any further increase in coil power generally results in an increase in contact bounce only.

To a great extent, the life expectancy of reed relays depends, among other factors, on the magnitude and type of the switching load. If used in so-called "dry circuits," i.e., with no or very low current loads (on the order of 10 ma. at less than 12 volts), billions of operations can be expected. Manufacturers' test reports show that a typical contact capsule can handle 20 million switching cycles of a 15-watt non-inductive load. Table 2 indicates the wide range of performance characteristics of contact capsules and reed relays. The photographs show typical reed relays and reed switch capsules.

If more than one contact is to be actuated by the same signal, the required number of contact capsules may be inserted in a common driving coil. In this manner, single-pole, single-throw and single-pole, double-throw capsules may be combined into one relay. Power requirements will increase accordingly because more area must be permeated by the magnetic flux. A minor problem is that, unless the capsules are specially matched, not all capsules will be actuated at exactly the same time. This particular characteristic is due to pull-in tolerances among individual reeds.

Table 2 shows that most reed relays will operate within one millisecond and release even faster. Contact bounce is in the neighborhood of 10% of the actuating time. These properties indicate that reed relays may be used as inexpensive d.c. choppers.

Choppers have been around for a long time, the best known is the car radio vibrator, which was widely used before the advent of transistorized car radios. Precision choppers are used in commercial and military equipment, such as voltage comparators and test instruments, where the cost of the precision chopper is of secondary importance. The attractive feature of a chopper is that it converts a d.c. voltage which can be measured quite accurately by a good meter movement, into a rectangular waveform with corresponding peak voltage. The waveform can be handled by a.c.-coupled amplifiers and also conveniently displayed on an oscilloscope.

The main advantage of an electromechanical chopper over a transistorized square-wave generator lies in the ease of calibration and the near constant voltage output which is almost independent of operating temperatures. The base-to-emitter and collector-to-base voltage drop of transistors vary considerably with temperature, and inexpensive transistors of the grown-junction type have objectionable base charge characteristics which preclude saturated operation if a perfect square wave is to be obtained.

The low cost of a reed relay makes it very attractive for use as a d.c. chopper. Switching rates of several hundred cycles per second are possible, and the high resistance differential between closed and open condition, together with a lack of temperature problems, offer considerable advantage over a transistorized switching circuit.

The development of the magnetic reed switch filled a definite need for a component that can supplement the solid-state switch in certain specific areas. Its tolerance to voltage transients, simple and reliable construction, together with its low cost, are added advantages which leads to various applications. The imaginative reader will think of many other uses for this extremely versatile new component.

Automatic Electric Co.
Sub., General Telephone & Electronics Northlake, Illinois

Douglas-Randall, Inc.
Sub., Walter Kidde & Co., Inc.
6 Pawcatuck Ave., Westerley, R.I.

Radio Corporation of America
Electron Tube Division
Harrison, N.J.
R.B.M. Controls Div.

Magnetic Reed Switches and RelaysSeptember 1965 Electronics World

Magnetic Reed Switches and Relays
September 1965 Electronics World