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Electronic Relays

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Date: 11/10/87 Number: 51
Related Program Areas:
Medical Devices Radiological Health


This ITG is intended to introduce the investigator to the electronic relay. Since this is only an introduction, a discussion of the simpler relays (EMR's, dry-reeds, mercury-wetted, and SSR's) will be given; as an indepth discussion of the more complicated models may destroy the primary focus of this ITG. Included will be explanations of relay theory, environmental effects, design, and failure.


Relays are electrically controlled devices that open or close electrical contacts to effect the operation of other devices in the same or another electric circuit. This opening and closing of the relay contacts is not an instantaneous action; as a tiny amount of time, nearly 0.5 to 50 microseconds, is required in order for action to take place. The relay's most basic components are its coil, armature, and contacts. When the relay is put into some given circuit, the current from that circuit induces a magnetic field in the relay coil. The magnetic field in the coil then affects the armature in such a fashion that it causes the contacts to make or break the part of the circuit to which the relay output terminals are connected.

The relay performs by means of a series of sequential events involving both energization and deenergization. Beginning when the relay is off, if the voltage or current is increased, then the relay will start to move through its inactive (non-pickup) region where no switching takes place. Next, as the current or voltage is still increasing, the relay enters a region where it is both inactive and active (non-pickup and pickup). Here, the relay uncontrollably cuts on and off and is said to be experiencing "bounce." Then the relay reaches the active (pick-up) region and begins to fully operate. The relay is now energized and is in the "operated state." Once the voltage or current begins to continually decrease, the relay starts to move back through its active region. The relay is now trying to hold its present state (contacts open or closed). Then the relay approaches the region where it is both holding action and inactive (dropout). This state of relay operation is the parallel to the active/inactive mode when current or voltage is increased. Finally, the relay reaches the inactive region and becomes inoperable. The relay is now deenergized and is in the "restored state." Although the process of energizing and deenergizing is descriptively long, it must be restated that the actual process is faster than the blink of an eye.


Since the primary purpose of the relay is to "make" (closing of the contacts) or "break" (opening of the contacts) circuits, a discussion of the relay contacts is necessary.

The contacts of the relay must be large enough so that no deterioration from destructive melting occurs; yet they must not be too large or else the current density will fall below a critical level and hinder successful operation. The best contacting occurs when there is sufficient electrical pressure (voltage) and current, along with sufficient mechanical pressure on the contacts to cause fusing of the contact surfaces on each operation.

Contacts can be damaged on both closure and opening. Contact closure damage is usually due to current surges, because contact forces at this instant are light, permitting contact sliding and bouncing to take place. This is not good because the load current is often many times greater than the steady-state value at this instant. A microscopic weld or "bridge" will often form at the point of contact closure. In DC circuits this bridge usually ruptures asymmetrically at the next contact opening, resulting in metal transfer. In AC circuits, there is usually a net loss of contact material, and the metal vapor that condenses in the vicinity of the actual contact area is normally black and is mistaken for carbon.

Contact damage due to opening comes in two forms; DC and AC. In the DC case, transients are more than certain to exist upon contact opening. When the circuit to a DC inductive load is opened, most of the energy stored in the load must be dissipated as arcing at the contacts unless some other means of energy absorption is provided. Some of the load energy is dissipated as heat in the load resistance, in eddy-current losses in its magnetic circuit, and in the distributed capacitance of the coil winding. AC loads are treated differently because a stable arc be terminated when the current passes through zero and reverses at the end of the first half-cycle following contact separation. Under moderate arcing conditions, contact life may be greatly increased by shunting the load with a resistor-capacitor-diode combination whose time constant is equal to that of the load.

Several cautions should be observed in order to insure successful operation of the relay contacts. Relays operating near sensitive circuits may cause trouble in electronic equipment from arcs generated as the contacts function. Some type of suppression must be applied as electrical protection and interference protection. Another thing to be aware of is the transient voltages developed when the contacts open the load circuit. These voltages may exceed the dielectric withstanding voltage between the contacts and another part of the relay. In some circuits these voltages may be high enough to cause breakdown of another circuit component. These transients often cause interference in adjacent or associated circuits. Elimination of high voltage transients greatly improve system reliability, as well as speed of response and consistency. As a final caution, careful attention should be paid to contact protection. Proper protection can increase life expectancy as much as three orders of magnitude.


Listed below are some common relay specifications that the investigator should know.

Contact Bounce - This consists of the uncontrolled opening and closing of contacts due to forces within the relay.

Contact Chatter - This is the uncontrolled opening and closing of contacts due to external forces (e.g., shock/vibration).

Contact Rating - This is given as the electrical load on the contacts in terms of closing surge current, steady-state voltage and current, and induced breaking voltage.

Coil Winding Polarity - Unless conventional types of relays designed for low-voltage (50 volts) circuits are used in short-lived equipment, it is best to connect negative potential to the outer coil terminal(s). The relay can then be controlled by switching grounded positive potential to the inside terminals of the relay winding(s). This minimizes electrolysis and adds years of life to relay coils.

Contact Spring Polarity - The same potential should be connected to all movable springs. This lessens the chance of accidental short circuiting, which can destroy relay contacts in an unguarded instant.

Life - An electromagnetic/electromechanical relay's (EMR) cyclic life may vary from less than one million operations to hundreds of millions. Some special relays are capable of many billion operations. The static life of EMR's is limited by physical or chemical deterioration of their components. Other possible limitations are coil deterioration and galvanic action between certain dissimilar metals. The design, materials, and manufacturing processes of the relay are the ultimate factors that determine static life.

The cyclic life of solid state relays (SSR's) is insignificant since they are purely static devices. Their static life is limited by physical or chemical changes affecting the intended function of their junctions. The maximum junction temperature for SSR's limits the power dissipated. This internally dissipated power is caused by the forward voltage drop across the device and by the requirements of the device drive (the relay power source). Above-rated voltage transients can destroy or cause a device to go into an unwanted condition. The environmental surroundings, design, application, and fabrication of the SSR determine static life.


Relays come in a variety of types and classifications. As stated earlier, the only types to be discussed are EMR's, dry-reeds, mercury- wetted, and SSR's. Relays are classified by input, output, duty rating, usage, and overall performance.


General-purpose - These relays are such that their design, construction, operational characteristics, and ratings are adaptable to a wide range of uses. They are usually constructed to have clapper-type armature, leaf springs, button contacts and an L-or U-shaped heelpiece (Figure 1) (image size 4KB) Their operation consists of a coil pulling directly on an armature and movable contacts attached to the armature. General-purpose relays come in three duty ranges; light (two amperes of current or less), medium (two to ten amperes), and heavy or power type (15 or more amperes). They have a life expectancy of 100,000 operations for their contacts and 10 million operations overall. General-purpose relays find their most popular use in air-conditioning and heating, household electrical appliances, control of low-wattage motors, lighting controls, and elevator controls.

Power-type Relays - These look similar to general-purpose relays, only they are larger and more rugged (Figure 2) (image size 4KB). Their contacts are suited for heavy currents and highly inductive loads. Power-type relays are characterized by a contact current rating of 20-25 amperes, the ability to best handle contact loads, and easy repair. They are of little use in situations where varying positions and shock or vibration are involved. Power relays are specifically used for electric-motor control.

Telephone-type Relays - Their construction consists of an armature with an end-mounted coil and spring pickup contacts mounted parallel to the long axis of the relay coil (Figure 3) (image size 7KB). Telephone-type relays are most used in business machines, communications systems, computer input/output devices, electronic data processing, laboratory test instruments, machine- tool control logic, and production test equipment.

Resonant Reed Relays - These relays are designed to respond to a given frequency of coil input current. Their operation involves an electro-magnetic coil that, when energized, drives a vibrating reed with a contact at its end. When the coil input frequency corresponds to the resonant frequency of the reed, the reed will vibrate and cause its contact to touch a stationary contact, thereby closing a circuit once each electric cycle. At other frequencies the reed does not respond. Unfortunately, their contacts do not close with a firm positive closure and they sometimes demonstrate undesired frequency drift due to temperature extremes, tampering, shock, or vibration. Resonant reed relays are used for applications where response to frequency is only desired, such as communications, selective signaling, data transmission, and telemetry.

Crystal Can Relays - This type of relay came about when environmental conditions began to dictate that relays be hermetically sealed, light-weight, and shock and vibration-resistant (Figure 4) (image size 5KB). With crystal can relays, relatively small contacts with fairly light pressures can be operated. Also, contact ratings must be restricted to light loads. These relays are small and adaptable to printed-circuit (PC) boards and solid state circuitry. Their problem is that their inside mechanisms are inaccessible during use for inspection of remaining life and they are expensive.

Time-Delay Relays (TDR's) - TDR's basically consist of a synchronous motor used for accurate long-time delay in opening and closing contact. The most popular TDR's use a conventional relay plus some required hybrid circuitry, plus an enclosure used to combine all these elements into a unit (Figure 5) (image size 4KB). Adjustable timing is done by altering pot setting by means of a knob that can be turned externally, or slotted shaft for screwdriver setting. With TDR's, all kinds of timing functions can behandled; such as operate-time delay, release-time delay, generation of a delay interval with reset, sequence timing with repetition, pulse generation, and interval timing. The TDR's only defect is that its repeat accuracy is poor.

"Permissire Make" Relays - In this type of relay, contact switching takes place when the energized coil provides sufficient force to overpower a pretensioned spring that held the contacts in an unoperated or normal position. When the biasing force is overcome by sufficient armature pull, owing to coil energization, switching of the contacts takes place. When the coil is deenergized, the contact springs return to their unoperated position because the biasing force of the restoring spring is now unopposed.

Latch-in Relays - These relays have contacts that lock in either the energized or deenergized position until reset either manually or electrically.

Differential Relays - These function when the voltage, current, or power difference between its multiple windings reaches a predetermined value.

Stepping Relays - Stepping relays operate by having their contacts stepped to successive positions as the coil is energized in pulses. They may be stepped in either direction.


Dry-reed relays are different from EMR's in that they require no armature. They generate a flux that acts directly on the contacts without employing any linkage. They are constructed so that two normally separated, electrically conducting, and magnetic flux conducting elements in a sealed glass envelope provide a portion of the main flux path of the coil so that when the coil is energized, these elements are attached to each other to form a closed contact (Figure 6) (image size 7KB). Dry-reeds find most of their use in business machines, communication systems, computer input/output devices, electronic data processing, lab test instruments, and production test equipment.


Mercury-Wetted Contact Relays - In these relays, the electrical contacting is mercury to mercury. The contacting faces are renewed by capillary action drawing a film of mercury over the surfaces of the constant switching members as the movable contact member is moved from one transfer position to the other. The mercury film is drawn-up from a pool at the bottom of the capsule, between the stationary members to provide bridging. No solid metal to solid metal contacting takes place; so the contacts are actually renewed on each operation. With mercury-wetted contact relays, wide ranges of signal and power levels can be reliably switched without having the nature of the load affect either contact life or performance. One very important detail about these relays is that they must be mounted right-side-up with an axis tilt less than 20 -30 from the vertical. If the relay is inverted, the contacts will be flooded from the mercury pool and may not perform properly for some time. Also, since mercury is a primary part of this relay's operation; low temperatures below - 38.8 C are a problem because mercury solidifies at this temperature. Mercury-wetted contact relays are ideal for pulsing highly inductive electro-magnets, such as rotary stepping switches. They find their most common use in air-conditioning and heating, business machines, communications, computer input/output devices, electric power control, electronic data processing, lab test instruments, and production test equipment. Figure 7 (image size 7KB) shows a typical mercury- wetted contact relay.

Heavy-Duty Power-Type Mercury Contact Relays - These relays were developed from the necessity of ridding contact erosion from relays handling heavy power loads. The continuous contact-renewal properties of mercury accomplishes this task. The conduction in power-type mercury contacts is through a pool of mercury, and the two principal means for this process are: (1) The tilted mercury tube (which causes the terminals to be bridged when the tube containing the mercury is in one position and non-bridged or open in the other position) and the mercury-displacement technique. Here, a plunger is pulled down into the mercury pool so that a bridge of conducting mercury extends from one terminal to the other; thus closing a circuit over a dam that otherwise isolates one terminal from the other. (2) When the coil is deenergized, the plunger floats back up, the mercury returns to refill the pool, and the circuit is opened. Figure 8 (image size 5KB) shows a typical model.


Solid state relays are completely different from the three previously mentioned types because they have no moving parts (Figure 9) (image size 4KB). An SSR is basically a semiconductor switching device with input terminals isolated from the output switch path. The output switch may be an FET (for low-level switching) or a trial (for AC power switching, as is the case for most of today's SSR's), and the input is usually a low-level DC signal in the 3-32 Hz range. The SSR consists of a control, which is equivalent to a coil, and a controlled output, the equivalent of contacts.


For some relays any environment will do. The relay chosen should just accommodate the proposed environment and should not be overengineered. Some environments include extremes of temperature and radiation/contamination, especially as encountered in airborne service and space applications.

General Environments - In the case of EMR's, commercial atmospheres are well tolerated in either an enclosed or unenclosed condition. Extreme problems of atmosphere, particles, and moisture may require hermetic sealing. For SSR's, packaging and small mass make then immune to most environments, especially shock and vibration.

Temperature - For EMR's, the ability to withstand heat is limited by the type of insulating materials used. Greater than maximum allowed temperatures, if sustained, will produce a faster deterioration and decomposition of most insulating materials. EMR designs are available that can operate in maximum ambients of 125 C. The SSR's ability to withstand heat is limited by junction temperature considerations. Above-rated, elevated-ambient temperature surges usually have sufficient inertia to cause irreversible changes in the relay if it is operating near maximum capacity. Many SSR's can operate in temperatures of 125 C or more, but their gate sensitivity and gain fall off below - 20 C.

Contamination - In EMR's, contamination of contacts is of most concern. Results may vary from slightly increased contact resistance to an electrically open condition. The relay coils are susceptible to certain contaminants which will chemically deteriorate the coil and results in electrical breakdown and shorting. In SSR's, contamination is mostly encountered in semiconductor pellets, resulting in a decrease in blocking voltage and an increase in leakage current results.

Design Consideration

Design considerations for relays are not too complicated, but they are vital nonetheless. Considerations of primary concern to the designer include the relay contacts (as well as the attached armature and springs), the type of input (AC/DC), and the load that will be attached to the relay.

The dynamic characteristics of the armature and contact assembly are primarily determined by the mass of the armature and depend upon the magnet design and flux linkage. Contact and restoring-force springs are attached or linked to the armature to achieve the desired make and/or break characteristics. Primary characteristics for these springs are modules of elasticity, fatigue strength, conductivity, and corrosion resistance.

When choosing between AC or DC input relays, most designers prefer the performance received from DC input relays. Although AC relays offer economic advantages, DC relays are most often employed because:

  1. DC relays have longer life. The contacts of AC relays flatten prematurely due to wear from AC vibrations during their closing & opening.
  2. DC relays have greater sensitivity. Since they don't chatter, lighter energizing forces may be used than is the case for AC.
  3. DC coils have lower heat loss and can be made smaller.
  4. DC relays, especially if heavily loaded, can carry a wider voltage range then AC.
  5. Timing is impossible when operating conventional relays with AC.

If the only requirement is that the relay simply shall operate when a switch to it is closed and release when that switch is opened, then it doesn't matter whether the relays are powered by AC or DC.

In terms of loads, the investigator should be aware of what is expected from the relay when connected to such devices. For communications equipment, the relay is expected to have long life, reliability, freedom from too frequent maintenance, and favorable environments. Telephone-types are ideal for these applications. In computer I/O devices the relay must meet heavy-duty requirements and have maximum life expectancy with utmost reliability. Quick disconnect mounting, so that the unit in need of attention can be instantly replaced, is also a requirement. Environment is no problem. With electric power control, long life, reliability, and freedom from frequent maintenance are the primary requirement. Environment serves as no threat to these relays. For electronic data processing, the only things to be consider are contacts that accommodate a high variation in contact load and environment. In lab test instruments, maximum reliability with good lite and freedom from frequent maintenance are a must. Environment is of no consequence. For production test equipment, a high order of insulation and resistance dielectric withstanding voltage with low contact resistance are requirement.


Most EMR failures are easily detected because of visual evidence of failure. Failures usually occur in the contacts. Contact failure comes in the form of film formation, wear erosion, gap erosion, surface contamination, and cold welding. Film formation is the effect of organic and inorganic corrosion, causing excessive resistance, particularly at dry-crack conditions. Wear erosion results from particles in the contact area which can cause bridging between small contact gaps. Surface contamination results when dirt and dust particles on the contact surface prevent the achievement of low resistance between the contacts, and may actually cause an open crack. Cold-welding is the self-adhering of clean contacts in a dry environment. Some symptoms of contact failure are high contact resistance, mechanical failure, coil opening or shorting, and contact sticking, transferring or welding. Contact sticking and high contact resistance may be intermittent and perceived as misses instead of failures.

In SSR's, there is usually no visual evidence of failure unless it is heat discoloration. SSR failures are characterized by permanent shorts, the inability to block voltage, or leakage current reaching failure proportions. General failure factors related to SSR's are: exceeding of maximum voltage ratings; thermomechanical fatigue caused by cyclic temperature surges; chemical reactions, such as channeling; and physical changes, such as crystallization of materials. SSR failures are quickened with prolonged temperature increases. Since visual detection is so difficult, SSR failure detection can become quite involved depending on the knowledge, experience, and equipment required.


  1. Chute, George Mr., Electronics in Industry. New York: McGraw-Hill Book Company, 1971.
  2. Fink, Donald G., ed., Electronics Engineers Handbook. New York: McGraw-Hill Book Company, 1975.
  3. Fink, Donald G., ed., Standard Handbook for Electronical Engineers. New York: McGraw-Hill Book Company, 1960.
  4. Harper, Charles A., ed., Handbook of Components for Electronics. New York: McGraw-Hill Book Company, 1977.

Figures 1to 3 are on the same page. Figure 4 to 9 are on the same page.

Figure 1. General purpose relay.

Figure 1. General purpose relay

Figure 2. Power-type relay.

Figure 2. Power-type relay.

Figure 3. telephon-type relay.

Figure 3. telephon-type relay.

Figure 4. Crystal can relay.

Figure 4. Crystal can relay.

Figure 5. Knob adjustable TDR

Figure 5. Knob adjustable TDR

Figure 6. Schematic and actual pictures of dry-reeds

Figure 6. Schematic and actual pictures of dry-reeds

Figure 7. Actual and Schematic pritures of mercury-wetted relatys

Figure 7. Actual and Schematic pritures of mercury-wetted relatys

Figure 8. Heavy-duty mercutry relay

Figure 8. Heavy-duty mercutry relay

Figure 9. Typical SSR

Figure 9. Typical SSR

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