US3296498A - Voltage sensing relay system - Google Patents

Description

Jan. 3, 1967 CHASSANOFF ET AL 3,296,498

VOLTAGE SENSING RELAY SYSTEM Filed May 14, 1963 Q\ .Hnoonocnoooonon IN V EN TORS ARNOLD l. C/mssmvorr 5RN4RD B. 0415 i jwux MM H TTORNE' Y United States Patent f This invention relates to AC. voltage sensing systems, and more particularly to a system wherein a silicon controlled rectifier. is employed to govern the actuation of a relay in response to the voltage of a power source.

In a system of this type, a silicon controlled rectifier (hereinafter referred to by its initials SCR) is connected with its current path in series with the relay coil and the ,power source. A detector circuit is connected between the power source and the gate terminal of the SCR.

. During. each half-cycle of the power source, e.g, the positive-half-cycle, the SCR is conditioned to fire, i.e., pass current, if a suflicient potential is applied to its gate. On

the other hand, during the other half of each cycle, the polarity of the SCR is reversed so that it becomes inactive in the sense that it will not fire regardless of the potential applied to its gate. .When ever the SCR fires, it energizes the relay coil and thus activates the relay which may be arranged to perform any desired function.

The dectector circuit monitors the power source, and should the voltage of the source rise above a predetermined value, the detector circuit is adapted to apply a firing potential to the SCR. As long as the voltage of the source remains above the predetermined value, the SCR will fire at or near the peak voltage during each half-cycle and discontinue firing during the other half-cycle and the initial portion of the following half-cycle, until the peak or near-peak voltage is reached once again. So that the relay does not become energized and deenergized with each half-cycle, means are often provided for maintaining the relay coil energized through the inactive periods of the SCR following the half-cycle during which the SCR fires. Consequently, if the SCR continues to fire, the relay coil will remain energized constantly. However, should the voltageof the source drop so that the SCR becomes inactive during the half-cycles, the energization of the relay coil will not be sustained and the relay will be deactivated. A disadvantage of .thistype of system stems from the fact that there is too sharp a line (zero differential) between the voltage condition that energizes the relay, and

the reduced voltage condition that deenergizesit. Thus,

if the relay is energized whenever the source voltage is at or above a predetermined value, it becomes deenergized whenever the voltage vdrops, even minutely, below this value. Attempts have heretofore been made to eliminate this so-called zero differential but they have not produced entirely satisfactory results It is an object of the present invention to provide a system of the type described wherein a differential is provided between the pick-up voltage of the relay system, i.e., the voltage of the suorce at which the SCR'will fire initially, and the drop-out voltage of the relay system, i.e., the voltage of the source 'at which the SCR will no longer have a firing potential applied to its gate, in a simple, economical and reliable manner.

It isfanother object of the invention to provide such a system which will be substantially unaffected by changes in ambient temperature, in frequency of the source, or by vibrations and shocks.

It is a further object of the invention to provide such a system wherein the pick-up voltage may .readily be adjusted.v p Y To achieve these objectives, the invention provides means for applying to the gate of the SCR, after it first 3,296,498 Patented Jan. 3, 1957 fires in response to a rise in voltage of the source above a predetermined pick-up value, an additional potential engendered by the current induced in the relay coil during the following period when the SCR is inactive. This additional potential is a make-up voltage which augments the potential applied to the gate by the detector circuit. Consequently, during the next half cycle when the SCR is conditioned to fire, it will fire even if the voltage of the power source has dropped slightly below the predetermined pick-up value. Thus, the drop-out voltage of the relay system will be a discrete amount below its pick-up voltage. The amount of the difierential between the pick-up and drop-out voltages will depend on the particular components chosen for the system.

Specifically, according to one embodiment of the invention, a diode is connected across the relay coil and arranged to pass current in the direction of the SCR gate. Current induced in the coil during inactive periods of the 7 SCR flows through the diode and back to the coil, thereby maintaining the energization of the coil. In addition, as this current flows through the diode, a voltage drop is established across the diode, and augments the potential applied to the gate by the detector circuit. Also,

special means, such as a Zener diode, is provided in the detector circuit, having a voltage drop temperature coefiicient so related to the firing voltage temperature coefficient of the SCR as to render the system as a whole substantially immune to ambient temperature variations. Use of a Zener diode is desirable also because it adds a relatively large voltage drop to the detector circuit and thus minimizes any variation in the firing voltage of the SCR. Furthermore, it is preferred to include a potentiometer between the power source and the detector circuit to permit the pick-up voltage to be selectively varied.

Other objects and advantages of the invention will be apparent from the following description in which reference is made to the accompanying circuit diagram.

The circuit chosen to illustrate the present invention is supplied with power by means of a transformer, the primary 10 of which is connected to the power source to be monitored. The primary may, if desired, be provided with a series of taps, at different points on the winding, corresponding to the normal voltage values of several different types of sources. In this way, regardless of the normal voltage value of the'particular source to be monitored, connection of the source betweenthe approriate taps on the primary will result in a uniform voltage at the seconary 11, i.e., the normal secondary voltage will always be the same although the primary is connected to sources having different normal voltages.

Connected in series with the secondary 11 is a diode 12, the current path of a silicon controlled rectifier (SCR) 13, and the coil 14 of a relay. Both the diode12 and SCR 13 are arranged to pass current in the direction of the coil 14. During one-half of each cycle of the power source, the polarity of the SCR will be such that it is in condition to fire, and if a sufficient potential is applied to its gate 15 during this half-cycle it will fire. During the other half of each cycle of the power source, thepolarity of the SCR is reversed and therefore it cannot fire regardless of what potential is applied to its gate. For the sake of convenience, the half-cycle during which the SCR is conditioned to fire will be called the positive half-cycle, and the other half-cycle will be referred to as the negative half-cycle. It is understood, however, that the SCR may if desired be conditioned to fire during either half-cycle.

It will be seen, therefore, that when a potential sufficient to fire the SCR is applied to its gate 15 during a positive half-cycle,'a' circuit is completed from the transformer secondary 11, through the diode 12, the SCR 13,

and the relay coil 14, back to the secondary 11, whereby the relay coil is energized.

A detector circuit is provided between the transformer secondary and the gate 15 of the SCR for the purpose of applying to the gate a potential representative of the voltage value of the power source. This circuit could be connected directly to the secondary 11, but preferably it is connected through a potentiometer 16 so that the pick-up voltage of the system may be selectively varied. One end of the potentiometer resistance is connected to a tap 17 on the secondary near one terminal of the latter, and the other end of the resistance is connected to the other terminal of the secondary, preferably through a resistor 20. The resistor 211 permits finer resolution on the potentiometer 16 when setting pick-up values. Advantageously, the potentiometer and resistor 20 have low resistance values so as to minimize the loading efiects of the gate circuit.

The detector circuit includes a diode 21, arranged to pass current in the direction of the gate 15, a Zener diode 22, arranged to pass current rearwardly in the direction of the gate 15, and resistors 23 and 24, the gate 15 being connected between these two resistors. The side of the resistor 24 opposite the connection to the gate is connected to a point between the SCR 13 and the relay coil 14. Due to the rectifying action of the diode 21, current flows through the detector circuit during only one-half of each cycle of the power source. Inasmuch as the SCR is conditioned to fire during positive half-cycles of the power source, the diode is arranged to pass current during positive half-cycles. During each positive half-cycle, the current flows from the tap 17 on the transformer secondary to the tap of the potentiometer 16, then through the diode 21, the Zener diode 22, the resistors 23 and 24, and the relay coil 14, back to the secondary 11. The resistance values of the resistors 23 and 24 are such that the resistance value of the coil 14 is negligible compared to them. Consequently, before the SCR fires there is almost no voltage drop across the relay coil.

The resistance values of the detector circuit components are so chosen that when the voltage of the power' source is at a normal value slightly below a predetermined pick-up value for the detection system, determined by the setting of the potentiometer 16, the voltage drop across the resistor 24 is slightly below the firing potential of the SCR. During this condition, current flows through the detector circuit, as described above, but the relay remains deactuated. Now, if the voltage of the source increases even slightly, the voltage drop across the resistor 24 increases and, at the peak voltage of the positive halfcycle, a firing potential is applied to the SCR gate 15. As a result, the SCR begins to pass current and continues to do so until the end of the positive half-cycle, representing a total conducting period of the SCR of about one-quarter of a cycle. Upon the firing of the SCR, the circuit including the diode 12 and the SCR 13 acts as a short circuit with respect to the detector circuit and causes the full voltage of the secondary 11 to be applied to the coil 14, whereupon the coil is energized and the relay actuated.

If the detection system included only the components thus far described, the coil 14 would become deenergized during each cycle of the power source even if the peak voltage of the positive half-cycle were above the predetermined pick-up voltage. This is due to the fact that the SCR 13 discontinues firing from about the beginning of the negative half-cycle to a point at or near the peak voltage of the positive half-cycle. Furthermore, as soon as the voltage of the power source dropped even slightly below the predetermined pick-up value, the SCR Would no longer fire and hence the coil 14 would not be energized, i.e., there would be zero differential between the pick-up and drop-out voltages of the relay system.

To avoid these undesirable effects, this invention provides a circuit including a diode 25 connected across the relay coil. The diode 25 is arranged to pass current in the direction of the resistor 24 and hence in the direction of the gate 15 of the SCR. It will be noted that the diode 25 circuit does not provide an alternative path, with respect to the coil 14, for currents flowing in the detector circuit or for currents flowing through the diode 12 and SCR 13, due to the direction in which the diode 25 faces. These currents must flow through the coil 14.

When the peak voltage of the positive half-cycle of the power source rises to the predetermined pick-up voltage, the SCR fires and the relay coil 14 is energized. At the end of the positive half-cycle, the SCR becomes inactive, and current from the transformer secondary discontinues flowing through the coil 14. At this point, the collapsing magnetic field around the coil 14 induces a current in the coil which flows through the diode 25 and back to the coil 14 thus maintaining the coil energized. This induced current is capable of maintaining the relay coil 14 energized for at least three-quarters of a cycle. If during the following cycle the peak positive voltage is at or above the pick-up value, full secondary current will flow through the coil 14 once again and keep it energized for another cycle. Therefore, it will be seen that the diode 25 prevents the coil 14 from becoming deenergized the moment the negative half-cycle begins, and in fact makes it possible for the coil to remain energized until the peak of the next positive half-cycle.

The diode 25 performs another most important function. As the current induced in the coil 14 flows through this diode, a voltage drop is engendered across the diode. This voltage drop represents a relatively small potential, referred to hereinafter as the make-up potential which is applied to the gate 15 of the SCR and augments the potential applied to the gate by the detector circuit. Consequently, should the source voltage drop slightly below the predetermined pick-up value, the combined potentials applied to the gate 15 by the detector circuit and the diode 25 circuit will nevertheless add up to a value equal to or greater than the firing potential of the SCR and, hence, the SCR will fire. Note that this is so even though the source voltage has dropped below the pick-up value. Thus, it will be seen that the invention provides a diiferential greater than zero between the pick-up and drop-out values of the relay, i.e., once the SCR fires as a result of the peak positive voltage rising to a predetermined pick-up voltage, the SCR will continue to fire as long as the peak positive voltage remains above a predetermined drop-out value less than the pick-up value. The amount of the differential will depend upon the particular components chosen, for the system.

For the purpose of aiding the present description, illustrative values will now be given to the components of the system described above. It is understood, however, that the invention is not limited to the particular values chosen for illustration. Assume the following values: potentiometer 16 resistor-60 ohms, 5 watts resistor 20-60 ohms, 5 wattsdiode 21--.75 amp., piv.

Zener diode 2215 volts, /1 watts resistor 23-470 ohms, /2 watt resistor 24-200 ohms, /2 watt diode 25-1.6 amp., 100 piv.

SCR 1350 PRV, 1.6 amp.

coil 146 watts, 11 ohms Further assume that the tap 17 is so located, and potent1ometer 16 so set that there is a peak voltage drop across the detector circuit of approximately 17 volts whenthe power source reaches its highest normal voltage, say volts R.M.S. At this voltage of the power source, the preferred voltage at point 17 is 15.6 volts R.M.S., and the R.M.S. voltage across winding 11 is 27 volts. This 17 voltage drop will be divided as follows: about a half volt drop across diode 21, about a 15 volt drop across the Zener diode 2 2, slightly more than a 1 volt drop across the resistor 23, and slightly less than a half volt drop across the resistor 24. The voltage drop across the resistor 24 is not quite sufficient to fire the SCR which must have a half volt applied to its gate to cause it to fire.

Now, should the voltage of the source rise above 135 volts, the voltage drop across the detector circuit will rise above 17 volts; more particularly, the voltage drop across the resistor 24 will reach about a half volt, and the SCR will fire. At the end of the positive half-cycle, the SCR becomes inactive, and the current induced in the coil 14, as described above, engenders about a half volt drop across the diode 25. This drop augments the voltage drop across the resistor 24 and their sum is applied to the gate 15 of the SCR during the next positive half-cycle. Thus, if the peak voltage of the source during the next positive halfcycle is the same as during the former positive half-cycle which caused the SCR to fire initially, the potential applied to the gate 15 will be about a half volt greater than it was when the SCR first fired. Thus, it will be seen that after the initial firing of the SCR, 5% more voltage is applied to the gate 15 than was applied when it fired initially. Consequently, the voltage of the source, which is the input to the transformer primary 10, must drop 5% below the predetermined pick-up value (slightly over 135 volts) before the SCR discontinues firing during each positive half-cycle. By means of the system just described, therefore, a 5% differential has been provided between the pick-up and drop-out values of the relay.

The Zener diode 22 serves two important functions both of which relate to the fact that the firing voltage of the SCR varies slightly with changes in ambient temperature. First, it introduces a relatively large and constant voltage drop into the detector circuit. As is known, the reverse voltage drop across a Zener diode remains constant at the Zener voltage (in the above example the Zener voltage is 15 volts), as long as the ambient temperature remains constant and the voltage drop across the circuit as a whole is at least the Zener voltage, regardless of variations in the voltage applied to the circuit. If no Zener diode were employed in the above example, a 30% change in the voltage necessary to fire the SCR would result in a 30% error in the operation of the system, since the relay would not pick up until the voltage of the power source rose 30% above the predetermined pick-up value, i.e., to effect a 30% increase in the voltage drop across the resistor 24, the voltage of the source would have to rise 30%. On the other hand, with the Zener diode 22 in the circuit, a required increase of 30% in the drop across the resistor 24, which in the present example would be .15 volt, this being 30% of a half volt, requires only a .15 v./ 17 v., or-

0.9%, increase in the voltage of the source. Thus, employment of the Zener diode renders the error introduced by variations of SCR firing voltage with temperature much less significant than it would be if no Zener diode were used.

The second function performed by the Zener diode 22 is to actually compensate for variations of the SCR gate firing voltage with ambient temperature changes. Gate firing voltage of a silicon controlled rectifier decreases with increases in temperature. On the other hand, the Zener voltage of a Zener diode, i.e., the voltage which must be applied to it before it starts passing currents rearwardly, increases with increases in temperature. Therefore, by judicious selection of components, the system can be provided with nearly a zero temperature coefficient. In other words, the error which would normally be introduced by the SCR as a result of ambient temperature changes is almost perfectly compensated for by the Zener diode.

The diode 12 and the resistors 26 and 27 are not necessary elements of the system described, during normal operation. Rather, they are in the nature of a protection device for the SCR 13 to prevent destructive breakdown of the SCR should an excessive reverse voltage be applied to it. The diode 12 has a relatively large reverse voltage drop, and the resistors 26 and 27, whose resistances are much less than the reverse resistances of the SCR and diode 12, insure that any reverse voltage applied to the SCR and diode 12 is properly divided in proportion to their rated reverse voltage drops. For example, the SCR will break down upon application to it of a reverse voltage of 50 volts, and the diode 12 is rated at volts in the reverse direction, the resistors 26 and 27 are so chosen that the resistance of the former is double that of the latter. Consequently, up to volts in the reverse direction can be applied to the SCR and diode 12 without causing destructive breakdown of the SCR. The resistance values of the resistors 26 and 27 are too high to pass sufficient current to energize the relay coil 14.

In View of the above description, it will be seen that this invention provides a voltage sensing relay system wherein a differential is provided between pick-up and drop-out voltages in a uniquely simple and practical manner. Furthermore, the pick-up and drop-out voltages remain stable despite extreme variations in temperature. What is more, the present system is substantially independent of the frequency of the power source. In addition, the system is unaffected by shock and vibration since pick-up and drop-out points are electronically controlled and do not depend upon physical adjustments of the relay.

The invention has been shown and described in preferred form only, and by way of example, and many variations may be made in the invention which will still be comprised within its spirit. It is understood, therefore, that the invention is not limited to any specific form or embodiment except insofar as such limitations are included in the appended claim.

What is claimed is:

A voltage sensing system for use with a source of alternating current power, comprising a silicon controlled rectifier and a coil connected in series with said power source, said coil being connected to the cathode of said rectifier, a junction between said coil and cathode, a detector circuit connected between said power source and said junction, the gate terminal of said rectifier being con nected to said detector circuit so that when said source rises above a predetermined value a firing potential will be applied to the gate terminal, and a diode connected in parallel with said coil and arranged to pass current in the direction of said junction, whereby upon deenergization of said coil after said rectifier discontinues firing the current induced in said coil and flowing through said diode produces a voltage drop which is added to the voltage drop in said detector circuit produced by said power source.

References Cited by the Examiner UNITED STATES PATENTS 2,939,064 5/1960 Momberg et a1.

3,045,148 7/ 1962 McNulty.

3,103,618 9/1963 Slater.

3,258,655 6/1966 Pinckaers 317148.5

OTHER REFERENCES A Survey of Some Circuit Applications of the Silicon Controlled Switch and Silicon Controlled Rectifier, Applications and Circuit Design Notes, Solid State Products, Inc., Bulletin D420-02-12-59, December 1959, pages 8, 9 and 27.

MILTON O. HIRSHFIELD, Primary Examiner.

SAMUEL BERNSTEIN, Examiner.

L. T. HIX, Assistant Examiner.

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