Method for detecting input ac voltage

Abstract: A rectified ac voltage which is not smoothed, is compared with a reference voltage directly, and a pulse is generated when the instantaneous value of the rectified ac voltage exceeds the reference voltage. Based on this pulse generation, a judgement is performed as to whether the ac voltage is reaching a predetermined value or not. These operations allow the abnormality of the ac voltage to be rapidly detected with improved accuracy.

Description: The present invention relates generally to a method for detecting input ac voltage and more specifically to a method for detecting whether the crest value of the input voltage is reaching a predetermined voltage or not. Generally, apparatus such as electric devices are driven by commercial power sources with a voltage of 100 VAC and others, or driven through stabilized dc power sources which convert the ac voltage of commercial power source to a predetermined level dc voltage to be supplied. In that case, however, if the commercial power supply is interrupted or an instantaneous drop of the voltage, namely an instantaneous decrease of the crest value, occurs, various troubles may be caused in the electric device. In order to solve such a problem, there has been proposed a stabilized dc power source provided with an emergency power source such as a battery which supplies a voltage for a predetermined period of time if the power supply is interrupted or other abnormality happens. By using this power source, a predetermined voltage can be supplied for a predetermined period of time; however, if the phenomenon such as the power supply interruption continues, the supplied voltage decreases to cause malfunction, breakdown, etc. of the electric device or the like. Therefore, it is desirable that the abnormality such as the aforementioned power supply interruption is rapidly detected and notified to the following electric device and others. Apparatus, stabilized power sources, and so forth which can be utilized for both of the commercial power sources with voltages of 100 V and 200 V by automatically alternating a switch through detecting the voltage difference are also known. In this case, the level of the input voltage should be detected quickly and accurately. Consequently, a method for the detection by using a means comprising a circuit shown in Fig. 1 (a) of the accompanying drawings has been used. Namely, the voltage which is provided by the full-wave rectification of an input voltage and is supplied through a diode D1 is smoothed by a capacitor C1 connected between the terminals of an input power source, and is compared with a reference voltage E1 supplied from a reference power source 2 by a comparator 1 having hysteresis. As shown in Fig. 1 (b) of the accompanying drawings, when the voltage between the terminals of the capacitor C1 becomes lower than the reference voltage E1, an output of the comparator 1 varies, detecting an abnormal condition. In other words, when the power supply is interrupted, the charge stored in the capacitor C1 is discharged by a given amount and the terminal voltage of the capacitor C1 becomes smaller than the reference voltage E1, resulting in alternation of the output of the comparator 1. However, in accordance with the conventional method described above, since capacitor C1 is employed a large amount of current supplied by the input voltage flows into the capacitor C1, causing wave-form distortion due to the impedance of the signal source. As a result, the real crest value cannot be detected and it is difficult to determine the setting level of the reference voltage E1 serving as the reference for the abnormality detection. Also, the charge amount in the capacitor C1 depends on the condition of the signal source, namely the crest value (instantaneous value), just before the occurrence of the abnormality such as the power supply interruption. This causes a variation of the time required for the charging, resulting in fluctuation of detection time. Furthermore, the charge stored in the capacitor C1 is discharged in a single discharging operation when the power supply is interrupted. However, in case of the instantaneous drop where the voltage does not reach the predetermined value, the capacitor C1 is slightly charged while discharging and this charge-discharge operation is repeated alternately, decreasing the charge stored in the capacitor C1 gradually. Therefore, in the instantaneous drop it takes a longer time until the voltage between the terminals of capacitor C1 has decreased to a value at which the output of the comparator 1 changes, and the detection time becomes longer than that for the power supply interruption. Thus, there is also a problem in that the detection time fluctuates due to the type of the abnormal condition. Patent Abstracts of Japan Vol 6, no 168 (p-139) [1046], 2nd September 1982, & JP-A-5786766 discloses a level detecting device which detects an "object" level from the ratio of the period during which input voltage exceeds a specific value and that in which input voltage stays below a specified value. IBM Technical Disclosure Bulletin, Vol. 17, No.8, January 1975, pages 2380-2382; T.E. STAMMELY : "Line voltage threshold detector" shows a comparator which judges the level of an AC voltage. An object of the present invention is to provide a method for detecting an input ac voltage which enables the detection of the abnormality or the like of the input signal rapidly and accurately. According to the present invention there is provided a method for detecting an input ac voltage comprising the steps of: rectifying the input ac voltage; comparing said rectified ac voltage with a reference voltage directly without smoothing said ac voltage; generating a pulse when the instantaneous value of said rectified ac voltage exceeds said reference voltage; and judging whether or not said ac voltage is reaching a predetermined voltage value based on said pulse generation, characterised in that said comparing step is performed by a comparator with hysteresis, said hysteresis having a first reference voltage set in the vicinity of peak voltage of said rectified ac voltage and a second reference voltage set at positive value above ground level, and below said first reference voltage, and in that said judging step is performed by a second comparator which compares a terminal voltage of a capacitor which is charged when said pulse is not generated and is connected to ground when said pulse is generated. The ac voltage to be measured is rectified and then, without being smoothed, compared with the reference voltage. When the instantaneous value of the rectified wave-form becomes larger than the reference voltage, a pulse is generated and it is recognized that the ac voltage is normal. On the other hand, when an instantaneous drop preventing the crest value of the rectified ac voltage from reaching the reference voltage or a power supply interruption is caused, the reference voltage is always higher than the other one, so that no pulse is generated. Therefore, abnormality is detected. Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 (a) is a circuit diagram showing a conventional detecting circuit; Fig. 1 (b) is a graph showing a terminal voltage of a capacitor in the detecting circuit of Fig. 1 (a); Fig. 2 is a circuit diagram showing a preferred example of a detecting circuit for implementing a method for detecting an input ac voltage according to the present invention; Fig. 3 is a timing chart for explaining the operational principle of the invention; Fig. 4 is a circuit diagram showing a judging circuit for performing judgement on an output voltage of the detecting circuit, and Fig. 5 is a graph showing a capacitor terminal voltage input to a second comparator. As shown in Fig. 2, an input ac voltage Vin to be measured is applied to the input terminals of a bridge rectifying circuit 12 comprising four diodes D2. A first resister R1 is connected between the output terminals of the bridge rectifying circuit 12. One of the output terminals (positive) of the rectifying circuit 12 is connected to an inverting input terminal 16a of a first comparator 16 and the other terminal (negative) is directly connected to an output ground terminal. A non-inverting input terminal 16b of the comparator 16 is connected to a reference power source 20. A comparator with hysteresis is employed as the comparator 16. Therefore, once a voltage larger than a first reference voltage E1 defined by the reference power source 20 is input to the comparator 16 to change the output signal thereof from a high level to a low level, the output signal does not return to the high level instantly even if the input voltage becomes lower than the first reference voltage E1; the output of the comparator 16 remains in the low level until the input voltage decreases to a second reference voltage E2 (E2 < E1) where the signal changes to the high level. In other words, even if the input voltage reaches the reference voltage E1 and then quickly drops, a pulse with a certain width is output. Note that the term "pulse" used herein is said to be generated when the output of the comparator 16 is in the low level, with the high level as a reference. Detection of the input voltage abnormality by using the detecting circuit constituted as described above is carried out as follows. The input voltage rectified by the bridge rectifying circuit 12 is fed directly to the inverting input terminal 16a of the comparator 16 and compared with the first reference voltage E1 which is input to the non-inverting input terminal 16b of the comparator 16. If the input voltage is higher than the first reference voltage E1, the output of the comparator 16 is at the low level. By contrast, the output is at a high level if the input voltage is lower than the reference voltage. Thus, when the input voltage is normal and its crest value is constant, a sequence of cyclic pulses with the same width are output. On the other hand, if the crest value of the input voltage does not reach the first reference voltage E1 due to the instantaneous drop, the power supply interruption or the like, the voltage at the non-inverting input terminal 16b is always higher than the voltage at the inverting terminal 16a, so that the output of the comparator 16 maintains the high level without changing to the low level. This means that no pulse is output, resulting in detection of abnormality due to the power supply interruption or the like. When the crest value of the input voltage is high, the time in which the instantaneous value of the rectified input voltage reaches the first reference voltage E1 is short and it requires a longer time to drop to the second reference voltage E2 after reaching the first reference voltage and passing the peak, resulting in a long pulse width. Accordingly, if the pulse has been generated, it can be detected whether the crest value is high or low in accordance with the pulse width. The operation of the aforementioned detecting circuit is now described with reference to Fig. 3 which shows a timing chart with wave-forms at respective portions in the circuit. Firstly in Fig. 3, a wave-form of the input voltage to be measured by the detecting circuit, i.e. a wave-form at the input terminals of the bridge rectifying circuit 12 is indicated at A, a waveform input to the inverting input terminal 16a of the comparator 16 is at B, and an output wave-form of the comparator 16 is shown at C. When the input voltage is in the normal region (1), the rectified voltage having a cylic wave-form B with a constant crest value is input to the comparator 16 which outputs pulses with a predetermined pulse width W. The crest value of the input voltage then becomes high in the region (2), and pulses with larger widths are generated as described above. On the other hand, when the input voltage instantaneously drops and its crest value becomes smaller than the first reference voltage E1 in the region (3), the input voltage having the wave-form B for the comparator 16 does not exceed the first reference voltage E1 either, so that the output C of the comparator 16 maintains the high level with generating no pulse. At the time the input voltage returns to be normal again (region (4)), the comparator 16 also outputs normal pulses. Thus, the period during which no pulse is generated is a period of the instantaneous drop. Furthermore, in the region (5) where the power supply is interrupted, the output C of the comparator 16 is maintained in the high level and no pulse is generated as in the case of the instantaneous drop. If the power source is instantaneously interrupted, the output C is similar to that in the case of the instantaneous drop (no pulse is generated for a predetermined period of time, and when the normality is recovered, a sequence of pulses begin to be generated). Next, a circuit and a method for judging the output condition of the aforementioned first comparator 16 simply and accurately is described. As shown in Fig. 4, the output terminal of the first comparator 16 is connected to a non-inverting input terminal 21a of a second comparator 21 through a second resistor R2. The non-inverting input terminal 21a is also connected to the ground through a capacitor C2, and a diode D3 is connected between the terminals of the aforementioned second resister R2 so that a current flows from the second comparator 21 to the comparator 16. One of the terminals of a third resister R3 having a sufficiently smaller resistance than that of the second resister R2 is connected to the resister R2 terminal for the first comparator 16, and the other terminal of the third resister R3 is connected to a charging power source Vc. Accordingly, the power source Vc is applied to the capacitor C2 and to the non-inverting input terminal 21a through the third and the second resisters R3 and R2 connected in series. On the other hand, a second reference power source 25 is connected to an inverting input terminal 21b of the second comparator 21. The judging operation by this circuit is as follows. When the output of the first comparator 16 is at the high level, the output terminal of the first comparator 16 is isolated from the ground, so that the capacitor C2 is charged by the charging power source Vc through the third and the second resisters R3 and R2 and the voltage between the terminals of the capacitor C2 is input to the non-inverting input terminal 21a of the second comparator 21. However, so long as the input voltage is normal, the output of the first comparator 16 alters from the high to the low level after a lapse of a given time and therefore the output terminal of the first comparator 16 and the ground are shorted, allowing the charge stored in the capacitor C2 to be discharged almost instantly through the diode D3. Since the cycle (interval) for the high-low alternation of the output is constant, the maximum amount of the charge in the capacitor C2 is also constant. When the abnormal condition due to the instantaneous drop, the power supply interruption or the like happens, the output of the first comparator 16 is maintained in the high level as described above, so that the capacitor C2 continues to be charged and its terminal voltage increases continuously (see Fig. 5). Therefore, by setting a third reference voltage E3 defined by the second reference power source 25 to a value larger than the maximum terminal voltage of the capacitor C2 in the normal condition, the output of the second comparator 21 is always at the low level. If the abnormal condition due to the instantaneous drop, the power supply interruption or the like takes place, the terminal voltage of the capacitor C2 keeps on increasing to exceed the third reference voltage E3, bringing the output of the second comparator 21 to switch to the high level for notifying the abnormality of the input voltage. After the input voltage has recovered the normality, the charge stored in capacitor C2 is discharged during a period where the output of the first comparator 16 is at the low level, so that the output of the second comparator 21 returns to the low level (see the wave-form D in Fig. 3). The value of the third reference voltage E3 is required to be set relatively low in order to detect the abnormality as rapidly as possible but it should be sufficiently larger than the maximum charged voltage of the capacitor C2 in the normal condition in order to prevent malfunctioning of the circuit. Therefore, the voltage E3 should be so set as to fulfil both requirements. As set forth above, the method for detecting the input ac voltage according to the present invention enables the detection of the abnormality of the input voltage essentially without using a capacitor, so that the wave-form of an input voltage signal even from a high impedance source can also be accurately detected. Further, since the input voltage is judged through comparing the instantaneous value of the rectified voltage directly with the reference voltage, which is different from the conventional method in which the abnormality is detected after the discharge of the capacitor, the information on the abnormality or the like can be provided rapidly and within a substantially constant period of time, independently of the crest value just before the occurrence of the abnormal condition. Similarly, the abnormality can be detected within the constant period of time, independently of whether it is the instantaneous drop or the power supply interruption. In the illustrated embodiment the input voltage is applied to the inverting input terminal 16a of the comparator 16 so as to make the succeeding judging circuit output the "high" level signal during the abnormality. It is apparent, however, that the input voltage may be applied to the non-inverting input terminal 16b, depending on the circuit or device provided in the following stage. Furthermore, the present invention should not be limited to the circuit as herein described and illustrated, various circuit arrangements being possible for implementing the present invention.

Claims: A method for detecting an input ac voltage comprising the steps of: rectifying the input ac voltage; comparing said rectified ac voltage with a reference voltage directly without smoothing said ac voltage; generating a pulse when the instantaneous value of said rectified ac voltage exceeds said reference voltage; and judging whether or not said ac voltage is reaching a predetermined voltage value based on said pulse generation, characterised in that said comparing step is performed by a comparator (16) with hysteresis, said hysteresis having a first reference voltage (E1) set in the vicinity of peak value of said rectified ac voltage and a second reference voltage (E2) set at positive value above ground level and below said first reference voltage (E1), and in that said judging step is performed by a second comparator (21) which compares a terminal voltage of a capacitor (C₂) which is charged when said pulse is not generated and is connected to ground when said pulse is generated. A method for detecting an input ac voltage as claimed in claim 1, wherein said rectifying step is performed by a bridge rectifying circuit (12). A method for detecting an input ac voltage as claimed in claim 1 or 2 wherein said comparing step performed by said comparator (16) with hysteresis further comprises the steps of inputting said rectified ac voltage to one of the input terminals (16a) of said comparator (16) with hysteresis and inputting said first reference voltage (E1) to the other input terminal (16b) of said comparator (16) with hysteresis. A method for detecting an input ac voltage as claimed in any preceding claim, wherein said judging step comprises a step of comparing the said terminal voltage of said capacitor (C₂) which is charged during non-generation of said pulse, with a third reference voltage (E3) by means of said second comparator (21).

Inventors: KATO NOBORU, NAKAO FUMIAKI, SHIMIZU TOSHIHIRO, KATO, NOBORU, NAKAO, FUMIAKI, SHIMIZU, TOSHIHIRO
Applicants: FUJI ELECTROCHEMICAL CO LTD, FUJI ELECTROCHEMICAL CO.LTD.
Application year: 1988
Main classification: G01R19
IPC codes: G01R19