US3678291A - Solid state relay - Google Patents

Abstract

A solid-state switching device capable of replacing mechanical relays. When the relay is conducting rated load current, a single saturated transistor carries the current with a very low voltage drop. When the load current rises above a pre-determined level, the load-carrying circuit automatically becomes a modified Darlington circuit, which is capable of carrying substantial overloads without being damaged, and with voltage drops within a desirable range. Optionally, means are provided for disabling the load circuit and, in effect, opening the relay, when a certain level of overload current is reached, thereby turning the device into a circuit-breaker.

Description

United States Patent 15] 3,678,291 Coe [451 July 18, 1972 54] SOLID STATE RELAY OTHER PUBLICATIONS [72] Inventor: Ronald J. Coe, Huntsville, Ala. Circuit Breaker" by Erdman, Jr. in IBM Tech Disclosure Assignee: SCI y Inc. Huntsville, Ala. gglltletin, Vol. 5, No. 11, April 1963, page 51, copy in 307- 22 i May 18, 1970 Current Supply" by May, Jr. in IBM Tech Disclosure Bulletin, Vol. 10, No. 7, Dec. 1967, pages 1045- l046, copy in [21] Appl. No.: 38,471 307-- 315 Y i I Primary Examiner-Stanley D. Miller, Jr. [52] U.S. Cl ..307/202, 307/254, 307/297,

, 307/315, 317/33 R, 317/60 R, 330/207 P [51] Int. Cl .....l-l02h 7/20, H03k 3/26 TR! (58] Field of Search ..307/202, 253, 254, 297, 315; I57] ABS CT 330/207 P; 323/ l, 2, l6, l7, l9-22 T; 317/33 R, 58, A solid-state switching device capable of replacing mechani- 60 R, 148, 5 R cal relays. When the relay is conducting rated load current, a single saturated transistor carries the current with a very low 56 Rd (jif d voltage drop. When the load current rises above a pre-determined level, the load-carrying circuit automatically becomes a UNITED STATES PATENTS modified Darlington circuit, which is capable of carrying substantial overloads without being damaged, and with voltage 3 i s" drops within a desirable range. Optionally, means are provided for disabling the load circuit and, in efiect, opening the 2928'009 3/1960 f "307/254 relay, when a certain level of overload current is reached, 2?) i; i gl t) thereby turning the device into a circuit-breaker.

isen erg 2,949,543 8/1960 Nordahl et'al. 307/315 X 11 Chins, 2 Drawing figures /a gimp l/VPUT {4L4 Lu 20 Z I d o 52 CONSTANT \CONSTAN 2 CURRENT CURRENT SOURCE SOURCE PATENIED Jun 8 m2 \CONSTANT CURRENT SOURCE \CONSTANT CURRENT SOURCE LEVEL DETECTOR AMPLIFYING CONSTANT CURRENT SOURCE F/GZ ' INVENTOIR P044410 J, 605 BY ATT RNEYS SOLID STATE RELAY The present invention relates to electrical control devices, and particularly to electrical relays and circuit breakers. More particularly, the present invention relates to solid-state switching circuits for replacing the mechanical relays and circuit breakers.

lt'long has been desired to provide a solid-state circuit device which will satisfactorily replace the mechanical relay. Such a device would have the advantage of having no mechanical contacts to become fouled, and would be more reliable in operation. However, it is believed that prior attempts to provide such a device have met with only limited success. There, are various reasons for the limited nature of this success. One such reason is believed to be that many prior devices burn out when excessive load currents pass through them,

although the same currents do not burn out mechanical relays. Another shortcomingofsuch prior. devices is that they are relatively large and unreliable, and they produce relatively large amounts of electrical. noise. Furthermore, some prior devices are relatively inefficient and have relatively high voltage losses across the relay when inoperation.

In accordance with the foregoing, it' ,is an object of the present invention to provide a reliable, light-weight, compact, noise-free and rugged solid-state switching 'device which is usable in place of mechanical relays. vIt is another'object to provide such a device which will not burn out when operated with rated overload current, which is relatively efficient at normal load currents, and has relatively low voltage losses across it at rated current levels. It is a further object to provide such a device withoverload circuit-breaker capabilities.

In accordance with the present invention, the foregoing objects are satisfied by the provision of a multistage semiconductor switching device which provides a very low-impedance path between a load and an electrical source. Only one stage of the device is activated when relatively low (rated) load currents flow through the device, but, when the load current increases above a pre-determined level, the second stage of the device is activated. This enables the device to conduct substantial overload currents without damage, and with voltage drops which are within desired low limits. Preferably, the device operates as a modified Darlington circuit during the overload mode, but operates as a simple saturated transistor switch at lower, rated load levels.

The foregoing objects and advantages of the present invention will be in part described in and in part apparentfrom the following description and drawings.

In the Drawings:

FIG; 1 is a schematic circuit diagram of one embodiment of the present invention; and

FIG. 2 is a schematic circuit diagram of another embodiment of the present invention.

FIG. 1 shows a solid-state relay circuit which has a pair ofinput terminals 12, and a pair ofoutput terminals 18. The function of the additional input circuitry to the left ofinput terminals 12 will be explained below. Theinput terminals 12 are connected to a conventional constantcurrent generator 14 which supplies constant current over anoutput lead 48 to a switching circuit 16 which is indicated in dashed outline. Aload 20 is connected between theoutput terminals 18. The switching circuit 16 operates upon receiving a signal for theinput terminals 12, to provide a low-impedance path between theoutput terminals 18 and theterminals 22 and 24 of a direct current power supply.

The switching circuit 16 includes twotransistors 26 and 28,

a pair ofbias resistors 32 and 44, and adiode 46. Theemitter lead 34 of the first transistor 26 is connected toterminal 22 of the power supply, and the collector lead of transistor 26 is connected to one of theload terminals 18. Thetransistors 26 and 28 are connected together to form a modified Darlington circuit. Thus, the base lead of transistor 26 is connected to theemitter lead 38 oftransistor 28, and thecollector lead 36 of transistor 26 is connected to the collector lead 40 oftransistor 28 through thediode 46. But for the presence of thediode 46, the switching circuit 16 would be an ordinary Darlington circuit. However, thediode 46 changes the operation of the circuit quite significantly.

When an input signal is received atterminals 12, the constantcurrent generator 14 is activated and supplies a constant output current overlead 48 to thefirst transistor 28. This current turns on transistor 26, and, but for the presence of thediode 46, also would turn on thetransistor 28. However, when the transistor 26 is turned on, at relatively low load current levels, the voltage on thecollector lead 36 and, hence, the cathode ofdiode 46, is relatively high; that is, it is relatively close to V, the supply-voltage. The voltage on collector lead' 40 oftransistor 28 is substantially less than the supply voltage, with the result that thediode 46 is back-biased and does not conduct current. Thus, thetransistor 28 is disabled and the below its anode voltage so'that it now conducts negative feedback current. Under'these conditions, the circuit 16 now is operating as a Darlington circuit which is capable of withstanding substantially greater overloads without burn out than would the transistor 26 alone. In this mode of operation, the transistor 26 operates in a different region of its characteristic curves than it did when thediode 46 was back-biased.

The result is that the transistor 26 still operates within its saturation range so that it still presents low impedance to the flow of the current through it. Thus, the voltage drop across the solid-state relay 10 is maintained at a consistently low level, one commensurate with the voltage drops usually experienced with mechanicel relays.

The portion of the circuit to the left of theinput terminals 12 provides conductive isolation of two further input leads 52 from the remainder of the relay circuit. This portion of the circuit, which is optional, therefore serves to replace the solenoid or coil of an ordinary mechanical relay, without sacrificing the conductive isolation provided by such a coil.

Isolation is provided by a conventional photoncoupled isolator circuit 50, which consists of a galliumarsenide lightemitting diode 56 which delivers light to a photo-diode 58. An example of one such device is the HP 4310 isolator which is sold by the Hewlett-Packard Corp. Another constantcurrent generator circuit 54 is provided to supply current to thediode 56. When input signals are received at theterminals 52, thecircuit 54 supplies current to thediode 56, which emits and shines light on thediode 58, causing it to become conductive and send a signal through theterminals 12 to the constantcurrent source 14. This causes therelay circuit 10 to operate in the manner described above. The circuit 50 thus provides conductive insulation of the input from theoutput terminals 18.

The transistor 26 should be selected so that it has current gain (beta) values which are fairly constant over a reasonably broad range of currents. That is, the current gain should be relatively steady from rated" to overloa current values. For example, one transistor which has been tested and found to be satisfactory for a rated load of 1 amp and an overload of 10 amps is the Motorola 2N 4398 high-power PNP silicon transistor. Similarly, the Motorola 2N 4918 medium-power" plastic PNP silicon transistor has been used successfully astransistor 28. The 2N 4398 transistor has a very low on" or saturation resistance at l ampere. By the use of the present invention, a similarly low saturation resistance at overload current also is provided.

Thediode 46 should have as small a forward voltage drop as possible. A diode which has been tested successfully is the UTX-2 l 0 diode which is sold by Unitrode Corporation.

In a circuit using the components specified above, and with a supply voltage V of 20 volts, a voltage drop of less than millivolt was measured across the emitter-collector path of the transistor 26 when rated load current of 1 ampere was flowing. At l amperes load current the voltage at the same location was 1.8 volts. Both of these voltage drops are substantially identical to the voltage drops across the contacts of a typical mechanical relay having the same load ratings. Moreover, the overload current of amperes was sustained for substantial periods of time without burn out and without approaching the condition of thermal run-away which would cause burnout.

The alternative embodiment shown in FIG. 2 is identical to that shown in FIG. 1 except that the input isolation circuit is omitted, andNPN transistors 60 and 62 replacePNP transistors 28 and 26, respectively (with reversed emitter-collector connections, of course.) In addition, a circuit-breaking function is provided by acircuit 68 which is a conventional amplifying level detector with a latching output.Detector circuit 68 detects the voltage drop across a very small (e.g. 0.05 ohm) resistor 66 which is connected in series with'the load. When the voltage across resistor 66 exceeds a pre-determined value, indicating the flow of a load current above a desired limit, thedetector 68 supplies a current to the base lead 70 of transistor 60 in a sense opposing the base drive current flowing in lead 70, thus turning offtransistors 60 and 62 and opening" the-.relay. The output of thedetector circuit 68 latches in its new condition until it receives a reset signal, or until the input signal is removed from the circuit. Thus, a convenient, safe relay with an integral circuit-breaker function has been provided.

The switching circuit portion of the circuit shown in FIG. 2 operates in the same manner as the circuit 16 shown in FIG. 1, except that the connections of the load to the transistors are reversed, in the manner shown in FIG. 2.

The solid-state relay of the present invention has numerous advantages. First, unlike some prior devices, it does not produce significantly large amounts of electrical noise. Furthermore, the relay has voltage drops across its output terminals which match those of ordinary relays having the same ratings. Also, the relay does not easily burn out due to sudden overload, thermal run-away or other effects. Additionally, the circuit is relatively simple and inexpensive to fabricate, it is compact, electrically efficient, relatively lightweight, and it is reliable. Other advantages have been explained above, and will be evident from the foregoing description.

The above description of the invention is intended to be illustrative and not limiting. Various changes or modifications in the embodiments described may occur to those skilled in the art and these can be made without departing from the spirit or scope of the invention.

Iclaim:

I. A relay device comprising output terminal means, semiconductor means for selectively providing a very low impedance path between said load terminal means and an electrical source, said semiconductor means comprising a current amplifier having at least two stages, means for selectively enabling said current amplifier, and means for selectively disabling at least one stage of said current amplifier in response to the flow of relatively low-level load currents to said load terminal means.

2. A relay device as in claim 1 in which said current amplifier forms a substantially saturated semiconductor switch when said one stage is disabled.

3. A relay device as in claim 1 in which said current amplifier comprises a Darlington amplifier during the flow of relatively high-level load currents to said load terminal means.

4. A relay device as in claim 1 including a constant current source for supplying bias current to said current amplifier.

5. A device as in claim 1 including circuit-breaker means for selectively disabling both of said stages in response to the flow of load currents above a pre-determined level.

6. A switching device comprising output terminal means, a Darlington amplifier circuit for providing a very low impedance path for connecting said output terminal means to an electrical source, means for enabling said amplifier circuit in response to an input si nal, and means for selectively disabling one stage of said amp rfier circuit when the flow of current to said output terminal means is below a pre-determined minimum value.

7. A device as in claim 6 in which said Darlington amplifier circuit includes a driver stage and a second stage, each stage including a transistor, the emitter-collector path of said second-stage transistor being connected between said output terminal means and a terminal for said source.

8. A device as in claim 7 in which said disabling means comprises unidirectional conduction means connected between the collector leads of said transistors and adapted to conduct only when the load current through said device rises to a predetermined value.

9. A device as in claim 6 including circuit-breaker means for selectively disabling both of said stages in response to the flow of load currents above a pre-determined level.

10. A device as in claim 6 including isolator means at the input of said device for conductively isolating said input from said output terminal means.

11. A device as in claim 10in which said isolator means comprises a light-emitting diode positioned to shine light on a photo-diode when said light-emitting diode is energized by an input signal.

Claims (11)

1. A relay device comprising output terminal means, semiconductor means for selectively providing a very low impedance path between said load terminal means and an electrical source, said semiconductor means comprising a current amplifier having at least two stages, means for selectively enabling said current amplifier, and means for selectively disabling at least one stage of said current amplifier in response to the flow of relatively low-level load currents to said load terminal means.

2. A relay device as in claim 1 in which said current amplifier forms a substantially saturated semiconductor switch when said one stage is disabled.

3. A relay device as in claim 1 in which said current amplifier comprises a Darlington amplifier during the flow of relatively high-level load currents to said load terminal means.

4. A relay device as in claim 1 including a constant current source for supplying bias current to said current amplifier.

5. A device as in claim 1 including circuit-breaker means for selectively disabling both of said stages in response to the flow of load currents above a pre-determined level.

6. A switching device comprising output terminal means, a Darlington amplifier circuit for providing a very low impedance path for connecting said output terminal means to an electrical source, means for enabling said amplifier circuit in response to an input signal, and means for selectively disabling one stage of said amplifier circuit when the flow of current to said output terminal means is below a pre-determined minimum value.

7. A device as in claim 6 in which said Darlington amplifier circuit includes a driver stage and a second stage, each stage including a transistor, the emitter-collector path of said second-stage transistor being connected between said output terminal means and a terminal for said source.

8. A device as in claim 7 in which said disabling means comprises unidirectional conduction means connected between the collector leads of said transistors and adapted to conduct only when the load current through said device rises to a pre-determined value.

9. A device as in claim 6 including circuit-breaker means for selectively disabling both of said stages in response to the flow of load currents above a pre-determined level.

10. A device as in claim 6 including isolator means at the input of said device for conductively isolating said input from said output terminal means.

11. A device as in claim 10 in which said isolator means comprises a light-emitting diode positioned to shine light on a photo-diode when said light-emitting diode is energized by an input signal.

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