Agricultural and Biosystems Engineering Agricultural and Biosystems Engineering Publications

1976 Solid-State Relays for Control L. H. Soderholm United States Department of Agriculture

Carl J. Bern Iowa State University, [email protected]

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Abstract Solid-state relays (SSRs) »J offer improved reliability and performance over that of electro-mechanical relays (EMRs) in applications requiring large numbers of contact closures, interfacing to low power solid-state circuits, maximum speed and control of contact closure, and minimum production of radio-frequency interference. SSRs may not be used as a direct replacement for EMRs in many circuits, however, and a number of the important fac-tors required for satisfactory SSR application and performance are considered.

Disciplines Agriculture | Bioresource and Agricultural Engineering

Comments This article is from Transactions of the ASAE 19 (1976): 596–600, doi:10.13031/2013.36077. Posted with permission.

This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/abe_eng_pubs/455 Solid-State Relays for Control

L. H. Soderholm, Carl Bern SENIOR MEMBER MEMBER ASAE ASAE

ABSTRACT Q OLID-STATE relays (SSRs) »J offer improved reliability and performance over that of electro­ mechanical relays (EMRs) in appli­ cations requiring large numbers of contact closures, interfacing to low power solid-state circuits, maximum speed and control of contact closure, and minimum production of radio- B.C. SiPPLY frequency interference. SSRs may not be used as a direct replacement for EMRs in many circuits, however, SCOPE and a number of the important fac­ tors required for satisfactory SSR FIG. 1 Relay test circuit. application and performance are considered. MATERIALS AND METHODS Electromechanical Relays INTRODUCTION Solid-state relays have been tested For conventional applications re­ Electromechanical relays (EMRs) both in the laboratory and in con­ quiring multiple poles and for or contactors have been a primary trol applications for determination of normal environments, the EMR is method of switching and power con­ their performance and their limita­ generally the most cost effective choice. trol for many years. They have, how­ tions. The circuit shown in Fig. 1 EMRs offer a closed resistance that ever, been subject to a number of was used to evaluate speed of re­ is essentially zero (in the order of problems and limitations, particularly sponse, turn-on characteristics, and milliohms) and an extremely high in the hostile environments often performance of a number of solid- open circuit resistance that is limited found in farm applications. state relays. A storage-type oscillo­ only by the insulation used in the Although solid-state relays (SSRs) scope was used to record relay re­ relay. EMRs are insensitive to voltage offer possible solutions to many of the sponse time, and measurements were transients on their load side and can EMR disadvantages, SSRs may not be made of both turn-on and turn-off operate at high temperatures. used to directly replace EMRs in characteristics from the oscilloscope Disadvantages of EMRs, however, many circuits. The basic differ­ trace. are limited contact life, slow speed ences between SSRs and EMRs Because most controlled circuits of operation, contact bounce, electri­ must be carefully considered if sat­ encountered in farm applications cal spark hazard, and opening upon isfactory application and performance operate on ac, the four basic types of momentary low coil voltage. EMRs are to be obtained as a number of relays considered were the electro­ may also produce radio frequency authors have indicated (Arnett 1967; mechanical relay (EMR), the hybrid interference (RFI) that can inter­ Andreiev 1973; and Dowdell 1974). solid-state relay (HSSR), the totally fere with other control circuits be­ As part of a project for improving solid-state relay (SSR), and the triac cause of their unpredictable time of farmstead electrical equipment, ad­ switch as shown in Fig. 2. contact opening and closure in power vantages and limitations of solid- control circuits. state relays for control applications have been investigated. RESULTS Solid-State Relays Typical characteristics of control In control circuits involving solid- Article was submitted for publication in circuit relays as determined from both state logic or other low- August 1975; reviewed and approved for publi­ manufacturers' and experimental data level applications and in many types of cation by the Electric Power and Processing are shown in Table 1. Operational hostile environments, the SSR and Division of ASAE in March 1976. Presented as ASAE Paper No. 75-3008. characteristics of SSRs vary over a the HSSR have characteristics un­ Journal Paper No. J-8199 of the Iowa Agricul­ wide range as shown and are de­ obtainable in the normal EMR. ture and Home Economics Experiment Station, pendent on the exact type and model Solid-state relay characteristics and Ames, IA. Project 1867. of relay. The following discussion gives the intended application must be The authors are: L. H. SODERHOLM, Ag­ a number of major considerations that thoroughly understood, however, to ricultural Engineer, ARS, USDA, and CARL BERN, Assistant Professor, Agricultural En­ were found to be important in control obtain proper performance and to gineering Dept., Iowa State University, Ames. circuit relay selection and use. prevent malfunctions.

596 TRANSACTIONS of the ASAE—1976 LOAD This may be easily overcome by using some means of control contact iso­ lation as shown in Fig. 2(e). IHPHT AC c SOURCE The true SSR uses all-semicon­ ductor circuitry to trigger the out­ put switching element (Sahm III, 1974), but the HSSR uses a mechani­ ELECTROMECHANICAL RELAY cal contact such as a reed relay to (a) provide the trigger signal. Both LOAD methods of triggering can provide isolation of 1,000 V or more between L7 TRIAC AC INPUT the input and output circuits, but SOURCE the HSSR has an advantage over the SSR in its integration of noise REED transients on the input. Because the FIG. 3 Typical HSSR turn-on characteristics RELAY reed relays used in the HSSR gen­ for a 60 Hz, 120 V circuit. Time base 2ms/Divi- sion. HYBRID SOLID-STATE RELAY erally take 1 to 2 ms to operate, (b) short duration transients at the in­ on the other hand, usually accept put usually will not cause unwanted a wider range of input voltage; 3 to 32 LOAD operation. This slow response, how­ V dc or 90 to 280 V ac are not un­ ever, generally prevents the use of common. Turn-on occurs at the

SOLID-STATE TRIAC HSSRs for fast switching. minimum voltage of the SSR voltage AC INPUT ISOLATION SOURCE Measurements made of the turn- range, however, and the upper CIRCUIT fy f on characteristics of a typical HSSR value of specified voltage indicates are shown in Fig. 3. A random closure only the maximum voltage that at TT/2 in the ac cycle is indicated, they can accept without damage. as well as a typical delay in turn-on Many solid-state relays are opto- SOLID-STATE RELAY of 2 ms. (c) isolated. That is, a light-emitting diode Because the reed relay in the HSSR (LED) is optically coupled to a has definite limits on input voltage photo-sensitive semiconductor. The (similar to limits on input voltage input signal activates the LED and its for the EMR), it must be selected for emitted light is picked up by the AC specific input conditions. SSR inputs, photo-sensitive transistor or silicon- SOURCE TABLE 1. TYPICAL CHARACTERISTICS OF CONTROL CIRCUIT RELAYS.

Type of relay TRIAC SWITCH Comparison Electromechanical Hybrid Solid-state (d)

Operating frequency 1-10 Hz 1-100 Hz 1 Hz to 1 kHz Minimum load No general limitation 10=100 mA, common 10-100 mA, common current (Dry circuit types available) Contact resistance Closed 10-100 mi^ 100 m£2 to 2 ohms 100 mfi to 2 ohms Open 500 Gtt >20M£2 >20Mft Synchronous opera­ tion and zero Possible, voltage turn-on No generally no Yes Coil voltages, ac 6,12,24,48,115,230, 460 V Generally the same 3-280 V TRIAC SWITCH WITH dc 6,12,24,110,220 V as for EMRs 3-200 V ISOLATED CONTROL CONTACT Speed, turn-on 1-20 ms 1-20 ms 5 jus turn-off (e) 2-20 ms 100 jus to 8.3 ms 100 MS to 8.3 ms Input power As low as 400 mW, As low as 400 mW As low as 6 mW typically 1-3 W FIG. 2 Basic relay types. Life cycles 100,000 to 1 million 100 million Virtually unlimited (limited primarily (limited primarily Proper initial selection of SSRs was by the reed relay) by solid-state found to be the most important factor device life) in the successful application of SSRs Contact bounce Yes No No Electrical spark to control circuits. When the operat­ hazard Yes No No ing parameters specified in Table 1 RFI/EMI Yes, unless suppressed Generally yes, unless None if switched at and the application considerations with auxiliary com­ specifically switched zero crossover ponents at zero crossover or were properly followed, all SSRs tested otherwise suppressed gave very satisfactory performance. Vibration/shock Susceptible Highly resistant Highly resistant The only problem observed was Can exhibit unwanted Generally resistant to No unwanted closures the possible safety hazard presented closure unwanted closures from shock by the lack of isolation between the from shock control contact and the load circuit for Transient Not susceptible 100 V/MS 100 V/Ms the triac switch circuit of Fig. 2(d). immunity (dv/dt)

1976—TRANSACTIONS of the ASAE 597 Supply voltage, VQC ~ (Voltage drop in diode D9) - (Voltage drop in LED, Dr)

(LED current, 1^ )

(A) RESISTIVE INPUT For a typical application: 3 W or more for a power control EMR. Saturated voltage drop of transistor Measured values have been found as Qa = 0.7 V low as 0.1 mW. SIPPLY VOLTAGE LED voltage drop pVCC = 1.3 V APPLICATION CONSIDERATIONS Gain of Qx = 20 Low-cost SSRs may use a single Required current of LED SCR for control and have a rectified = 0.05 A dc output suitable only for heating or lighting loads. Even ac SSRs using back-to-back SCRs, a single SCR in a diode bridge or a triac may partly Vm-0.7-1.3 - == 400 (VIN - 2.0 V) fail and produce dc. Circuits and use 0.05/20 (I) TIANSISTIR INPIT should be planned to accept inadver­ tently produced dc. FIG. 4 Typical solid-state relay input circuits. and SSR open circuit contacts are not the same as open EMR contacts. 0 7 1 3 SSR contacts are never really open controlled rectifier. The input current .Vcc- - - - 20 (Vcc-2.0) and always have some value of leak­ required to drive the LED must be 0.05 age current through them. In the off- supplied from the control circuitry. state, an SSR presents a high im­ An important design consideration is, pedance but still may apply an ap­ therefore, the interface compatibil­ Solid-state relays, however trig­ preciable voltage across a load. When ity between the controlling device gered, use solid-state output devices SSRs are used to control unloaded and the relay input requirements, to close the circuit. Power control transformers or other high impedance particularly in relation to pull-in and devices in all the SSRs tested were loads, some provision, such as a drop-out voltages and currents. either back-to-back SCRs or a triac dummy load resistor, should be used Variation of LED light output (a bidirectional device named from for reducing the off-state voltage. also must be considered in critical the construction of TRIode AC semi­ Transient voltage spikes or over- applications. Light output and rate conductor switch). voltage conditions can turn on an of decrease in output with age vary Triacs or SCRs are capable of SSR without a trigger signal and re­ between units. Such variation may switching ac loads, but dc control sult in conduction for the remainder produce a change in turn-on param­ usually is not obtained because the of the voltage cycle. Although this eters and cause system failures, par­ load current must be reduced below may not be a destructive condition, ticularly when analog signals are the holding current of the SCR or it can result in unwanted or unex­ applied and relay hysteresis becomes triac for turn-off. pected power-on conditions. Power important. When contact chatter, arcing, or line transients usually are more Two input circuits are commonly hesitation must be avoided, or when a serious with SSRs than with EMRs. used. Both have isolation provided by load must be switched in synchroniz- Transient voltage surges of up to 10 an optical photocoupler as shown in tion with power or control timing, times the nominal line voltage are Fig. 4. The light-emitting diode (LED) SSRs offer definite superiority to often encountered on power lines, and in Fig. 4(A) with a resistive input is EMRs. In control applications where provision must be made to prevent the simplest. Resistors R1 and R2 are potentially explosive atmospheres are unwanted application of power to selected to provide the proper current encountered, the advantage of the the load when SSRs are used. through the LED for the control sig­ SSR arcless operation becomes ob­ Transient problems also may be nal used and for the desired turn-on vious. Zero-voltage turn-on of SSRs caused by the steep wave fronts and turn-off thresholds of the SSR. offers less obvious advantages but or the high frequency oscillations Operating current is supplied by can be extremely important in pre­ produced by phase-controlled the input control signal, and varia­ venting the malfunctioning of other loads or high current switching of tions in the LED over its operating life control circuits or adjacent equipment other loads by EMRs. A high rate of must be compensated for by control by eliminating power line surges and rise of forward applied voltage (dv/dt) of the input signal. The circuit of RFI due to mid-cycle turn-on can cause thyristors to switch into Fig. 4(B) uses a transistor to obtain (McLendon 1974). the "on" or low impedance forward the equivalent of single-pole, double- Generally, the power required to conducting state even though break­ throw switching. Values of R3 and R4 turn on an SSR is lower than that over voltage has not been exceeded. may be determined by the following for an EMR. Typically this may be Snubber networks consisting of a relationships: as low as 100 mW as compared with small resistor in series with a capa­ citor as shown in Fig. 5 can be placed across the SSR output terminals Input voltage, Vjjsf - (Voltage drop in QjJ - (Voltage drop in LED, D^) to prevent dv/dt triggering. The capa­ citor limits the rate of voltage rise LED current, I Di) (1/Gain Ql) across the open SSR contacts, and the

598 TRANSACTIONS of the ASAE—1976 100 LOAi ducting the heat away and requires *^x •0RNTED ON $" X 6 X 0.125 -AAAA- derating the power with temperature to ALRMINHM PANEL as indicated in the typical derating so \ \ SNUBBER curve of Fig. 6. 40 \ Care must be taken in mounting an NETWORK 20 UNMOUNTED \ \ \ SSR to be sure that it is mounted 0 2 4 0 8 1\0 12 14 to a heat sink and not to a heat LIAR CRRRERT-AMPERES source. Increased ambients caused by TYPICAL SSR DERATING CORVES heat buildup within enclosures or the FIG. 6 Typical SSR derating curve. FIG. 5 Snubber network for suppressing dv/dt mounting of a solid-state device to a triggering. surface that is above normal ambient may cause turn-off failure. In enclosed been triggered. If load current drops below a minimum value, the SSR resistor prevents ringing and limits cabinets and wall boxes, additional heat sinking is often required to use will shut off. Typical values for this the discharge current of the capacitor. SSRs at full ratings. minimum current may vary from 200 Snubber currents combine with mA for a 25-amp rated SSR to 500 Brownout conditions also may cause semiconductor leakage currents to mA or more for a 75-amp rating. problems in SSRs because of the produce minimum off-state currents When highly inductive loads are that may affect high impedance cir­ reduced voltage conditions. Load current of controlled motors will encountered, additional circuitry may cuits. The maximum leakage cur­ be required to compensate for the fact rent that the load can stand and still increase and the load on-time of heater loads may increase, causing that zero voltage and zero current do be considered off determines the not occur at the same time, thus relay and snubber network selection. increased dissipation and possible overheating of the SSR. Low voltage creating timing problems. Current The size of the snubber capacitor must limiting also may be required to keep be chosen with care because snubber also may cause problems in SSR turn- 2 on because of insufficient trigger the SSR within its I t rating or it may capacitor current usually is the major be destroyed. portion of the total leakage current. energy. Some SSR designs have been found that will not operate below 75 For pulsed applications, SSR aver­ Solid-state relays are much faster age power ratings must not be ex­ than EMRs but do not have unlimited percent of nominal line voltage. Surge current ratings are an ceeded by pulsing too often. Pulsed speed. They operate well at 60 Hz and operation may also cause thermal may perform well at 400 Hz. At important consideration in SSR ap­ plications. For short-period over- fatigue at the thyristor mounting frequencies of 1,000 Hz or greater, junction because of temperature however, the zero-voltage time window currents, thyristors behave essen­ tially like a resistance with a fixed gradients and uneven expansion. in which the thyristor can be triggered, Noise immunity is also a considera­ decreases and susceptibility to dv/dt thermal capacity and no power dis­ sipating means. Although the surge tion in applying SSRs because of turn-on increases. their sensitivity and ability to oper­ Several additional factors must currents that may be applied to SSRs are generally greater than those that ate at high speeds. If a relay is be considered in using SSRs. The specified to close at 3 V at worst case importance of each is dependent on may be applied to EMRs and have less effect on relay life, they must be and open at 1 V at worst case, and the the application, but a user of SSRs actual operating voltage is 2 V for must be aware of their possible effect. limited. Typically a 1,000 percent surge current may be safely applied to a specific unit, then only 1 V of noise The EMR is usually free from immunity is obtained as shown in Fig. thermal heat buildup problems, but an SSR for one ac cycle (16 ms) or 650 percent for up to 10 cycles, 8. devices using semiconductors are not. Most SSRs have build-in hystere­ Voltage drops across semiconductor as shown in Fig. 7. The maximum forward non­ sis to prevent chatter when driven junctions are on the order of 0.5 to 1.5 by analog marginal or noisy sig­ V. For every ampere of current, there­ recurring overcurrent for very short durations (generally 8.3 ms or less) nals. Best noise immunity is obtained fore, 0.5 to 1.5 W of heat are created by using a relay with a low dynamic that must be conducted away. Heat must also be limited to prevent thyristor damage in SSRs. This input impedance and a large hystere­ buildup must be limited so that the sis. Noise problems on input circuits junction temperature of the solid- current capability is expressed as the 2 require more care in the application of state device stays within its ratings. I t rating where I is the rms current value over the interval t. SSRs than of EMRs. Techniques such Otherwise, the relay may not turn off. as single-point grounding, push-pull This necessitates heat sinks for con- Care must also be taken in burst firing of SSRs so that half-cycle firing driver circuits, and twisted and or unidirectional grouping of the shielded input lines may be required •— to achieve the desired noise immunity. UJ •— 1000 pulsed ac power does not occur when

811 magnetic cores are involved. This s^s prevents the addition of a dc bias to M »2 60S the magnetic circuit which may cause MIST CASE CLOSE VOLTAGE 2 JZ •• 416 a. a. u. saturation and nonlinear operation. ACTIRL CLOSE VOLTAGE ac ° » tt 211 The holding current required to WORST CASE POOP OUT VOLTAGE X N°'SE IMMUNITY 3 keep a thyristor conducting may be • II 111 , ,111 , 0 ;10,00 11 an important consideration in some SRR6E CIRRERT ••RATION (MILLISECONDS) applications; SSRs that use triacs or SSI NOISE IMMUNITY FIG. 7 Surge handling capability of typical SSR SCRs require a minimum load cur­ FIG. 8 Noise immunity of an SSR in relation [one minute maximum repetition rate]. rent to stay turned on after they have to operating specifications.

1976—TRANSACTIONS of the ASAE 599 tact closures of an SSR, however, obtain proper performance and pre­ CONCLUSIONS allow operations to be performed in a vent malfunctions. Solid-state relays are most useful way that would be impossible with for applications requiring maximum EMRs. Power turn-on for as little as References reliability for many contact closures one ac cycle or for a controlled num­ 1 Andreiev, N. 1973. Power relays - solid and minimum RFI. Principal uses of ber of ac cycles becomes possible. state vs elecromechanical. Control Engineering. SSRs at present are control of heaters, The most important factor in the pp. 46-49. January. motors, or lighting where their con- use of SSRs in control circuits is 2 Arnett, W. 1967. Metallic contacts vs solid tactless switching and zero voltage to recognize that SSRs may not be state switches. Control Engineering, pp. 67-68. turn-on properties may be used to used as a direct replacement for March. 3 Dowdell, E. 1974. The solid state relay: advantage. EMRs in most circuits. Although an answer that raises questions. Electronic A given set of EMR contacts can SSRs and EMRs have some common Products, pp. 47-50. July. switch dc, ac, or control signals, but performance characteristics, each has 4 McLendon, B. D. 1974. Selection of solid an SSR generally may be used for only capabilities that the other cannot state relays. ASAE Paper No. 74-3544, ASAE, St. Joseph, MI 49085. a single task and many can only switch match. Both knowledge of solid- 5 Sahm III, W. 1974. Solid state relays ac power into relatively low impedance state relay characteristics and the in­ aren't all alike. Electronic Products, pp. 51-57. loads. The fast and controlled con­ tended application are essential to July.

Cubicabical Thermal Expansion for Peanuts (Continued from page 595) sented at the Southwest Region ASAE Meeting, Carolina. H. Hopper. 1944. Analysis of peanut kernels Fountainhead State Park, Oklahoma April 3-4, 12 Shackelford, P. S. 1974. Skin removal with relation to U.S. Standards for Farmers' 1975. from Spanish peanuts by heating to moderate Stock Peanuts. Oil and Soap. 21 (8):239-247. 11 Service, J. 1972. A user's guide to the sta­ temperatures. Unpublished Ph.D. thesis, 14 Woodruff, J. G. 1973. Peanuts - Produc­ tistical analysis system. Student Supply Stores, Oklahoma State University. tion, Processing, Products. The AVI Publishing North Carolina State University, Raleigh, North 13 Stansbury, M. F., J. D. Guthrie, and T. Co., Inc. Westport, CT

600 TRANSACTIONS of the ASAE—1976