GB2339295A - Instrument for measuring alternating current - Google Patents

Description

DESCRIPTION ALTERNATING CURRENT MEASURING APPARATUS

TECHNICAL FIELD:

The present inve-.tion relates to an alternating current (AC) measuring apparatus and particularly to an AC measuring apparatus usable for current measurement requiring high insulation properties as in measuring a high-voltage AC.

BACKGROUND ART:

High insulation properties are generally required when high-voltage AC is to be measured in the distribution lines of distribution facilities, the power transmission lines of power generation facilities or the high-power loads of factories. In such a case, an AC measuring device in which the current is detected after it has been converted into optical signals may be used, as disclosed in Japanese Patent Laid-Open Application No. Sho 5663264.

Fig. I I is a circuit diagram of an AC measuring device according to the prior art. As shown in Fig. 11, a conductor 52 extends through a transformer 5 1. The transformer 51 is excited by the primary AC flowing through the conductor 52.

The transformer 51 includes a secondary coil 53 which is in turn connected to a diode 54. The diode 54 is adapted to half-wave rectify the secondary current produced in the secondary coil 53 of the transformer 51 by the primary AC flowing through the conductor 52. The cathode of the diode 54 is connected to a capacitor 55 which is charged by the rectified current from the diode 54.

A Zener diode 56 is connected to the capacitor 55 and the cathode of the diode 54 and is adapted to permit the current to pass therethrough as the charged voltage in the capacitor _'_5 becomes higher than a predetermined voltage. The cathode of the Zener diode 56 is connected to a light emitting diode 59 through a coil 57 and resistance 58. The light emitting diode 56 is connected to a thyristor 60 which is switched on to permit the charge of the capacitor 55 when the charged voltage of the capacitor 55 becomes higher than the predetermined voltage and the current passes through the Zener diode 56. The gate of the thyristor 60 is connected between the anode of the Zener diode 56 and a resistance 6 1.

The operation of.the AC measuring device according to the prior art will be described below.

As the primary,.'C flows through the conductor 52 to be measured, the coil 53 around the transformer 51 generates a voltage which permits a secondary current to pass through the coil 53. The secondary current is half-wave rectified by the diode 54 and then charged into the capacitor 55. At this time, however, the capacitor 55 will not be discharged since the Zener diode 56 and thyristor 60 are in their OFF state.

As the charged voltage in the capacitor 55 gradually increases and reaches a level higher than the predetermined voltage in the Zener diode 56, the current passes through the Zener diode 56 so that the voltage will be applied to the gate of the thyristor 60 through the resistance 61. Thus, the thyristor 60 is switched on to discharge the accumulated current from the capacitor 55. The discharged current flows into the light emitting diode 59 through the coil 57 and resistance 58. As a result, the light emitting diode 59 will emit pulsed light.

As an excessive current is discharged from the capacitor 55 to the coil 57, the capacitor 55 will reversely be charged. As the backward voltage is applied to the thyristor 60, the latte-r is then switched off.

As the thyristor 60 is switched off, the capacitor 55 is newly initiated to be charged. As the charged voltage reaches the predetermined level, the light emitting diode 59 again emits light. Thereafter, the thyristor 60 is switched off.

When such steps are repeated, the light emitting diode 59 is intermittently lighted or flashed to create pulses. The frequency of the created pulses depends on the speed at which the capacitor 55 is charged. This speed is determined depending on the induced voltage in the coil 53 or the current passing through the conductor 52.

Therefore, the current passing through the conductor 52 can be measured at any remote and insulated observation point by transmitting the light signals generated by the light emitting diode 59 to the observation point through an optical-fiber cable 70 and measuring the frequency of the optical pulses.

The AC measuring device of the prior art requires no extemal operating power and can easily perform an insulation from external since the light emitting diode 59 is lighted by the secondary current produced by the transformer 5 1.

In the AC measuring device according to the prior art, however, the forward voltage Vf is generated at the light emitting diode 59 together with the saturation voltage Vs at the thyristor 60 even if the latter is switched on. Thus, the capacitor 55 will not completely be shortcircuited. Even when the thyristor 60 is switched on, therefore, a constant charge remains at the capacitor 55. This

2 provides an adverse influence in that the next charging time is reduced.

The coil 57 is used to charge the capacitor 55 in the backward direction. This is for effectively switching the thyristor 60 off and does not completely contribute to improvement of the accuracy of measurement. Thus, the amount of backward charging will also be influenced by the voltages Vs and V,-.

If the saturation voltage Vs at the thyristor 60 and the forward voltage V, at the light emitting diode 59 are both constant, the effects from these voltages can be offset together. However, this is actually difficult due to variations in the components and their temperature characteristics.

Although the charging and discharging times at the capacitor 55 depend on the capacitance thereof, -he capacitance of the capacitor 55 also greatly varies from one component to another. In addition, an influence due to variations in temperature cannot be neglected. Particularly, the capacitor 55 requires its capacitance sufficient to light the light emitting diode 59. Usually, as the capacitance in the capacitor 55 increases, variations in the components and their temperature characteristics increase.

Since the AC measuring device of the prior art used the same components to perform both the power procurement and current measurement, the irregularity from one device to anot' er is larger. The AC measuring device is easily subjected to the influence of temperature. Therefore, the AC measuring device of the prior art could not precisely measure the current.

DISCLOSURE OF INYENTI

It is therefore an object of the present invention to provide an AC measuring apparatus which can precisely measure the current without being subjected to the influence of the variations in the components and their temperature characterist-'zs by separating the power procurement and current measurement from each other.

To this end, the present invention provides an AC measuring apparatus which comprises a transformer for causing a primary alternating current at the primary side to generate a secondary current at the secondary side, rectifier means for rectifying the secondary current generated at the transformer, charge/discharge means for charging and discharging the rectified current from the rectifier means, charge/discharge control means for controlling said charge/discharge means to charge up to the voltage generated by the secondary current is a predetermined voltage and for controlling said charge/discharge means to discharge when said 3 charged voltage therein becomes higher than said predetermined voltage, a load resistance located in a circuit through which the secondary current passes, voltage frequency conversion means supplied with the current discharged from said charge/discharge means as its operating power, said voltage frequency conversion means being adapted to convert the value of voltage generated by the load resistance into the value of frequency, and transmission means supplied with the current discharged by said charge/discharge means, said transmission means being adapted to transmit measurement signals outputted from said voltage frequency conversion means to an external.

According to the present invention, the AC measuring apparatus will not be influenced by variations in the components and their temperature characteristics since the power procurement and current measurement are separated from each other to use the voltage frequency conversion means for measuring the current and to use the current discharged from the charge/discharge means only as the operating power for the voltage frequency conversion means.

Preferably, the charge/discharge means includes a Zener diode permitting a current to pass therethrough when the voltage generated by the secondary current becomes higher than a predetermined level, a thyristor for reducing the voltage generated by the secondary current to a saturation voltage when the current passes through the Zener diode, and a backf1ow preventing diode for preventing the discharged current from the charge/discharge means from flowing back to the thyristor. In this case, even if the voltage induced by the secondary current is larger, the thyristor is switched on to reduce such a voltage to the saturation voltage in the thyristor. Therefore, the power consumed by the entire processing circuit can also be reduced.

The AC measuring apparatus may further include switching means for stopping the supply of discharged current as the voltage generated by the discharged current from the charge/discharge means becomes lower than a preselected operation stopping voltage and for re-initiating the supply of discharged current as the voltage generated by the discharged current from the charge/discharge means becomes higher than the preselected operation start voltage. In such a case, it can be ensured that the current is measured since the voltage frequency conversion means can intermittently be operated even though the primary alternating current is faint.

The rectifier means may be either of full-wave or half-wave rectification type.

4 BRIEF DESCR N OF DRAWINGS Fig. I is a circuit diagram of an AC measuring apparatus constructed according to the first embodiment of the present invention.

Fig. 2 shows (A) a waveform graph of primary alternating current i I; (B) a waveform graph of secondary current '2; (C) a waveform. graph of voltage V,; and (D) a waveform graph of voltage V2- Fig. 3 shows (A) a waveform graph of voltage V3; (B) a wavefon-n graph of voltageV4; and (C) a waveform graph of voltage V,.

Fig. 4 is a circuit diagram of an AC measuring apparatus constructed according to the second embodiment of the present invention.

Fig. 5 shows (A) a waveform. graph of primary alternating current i,; (B) a waveform graph of secondary current i,; (C) a waveform graph of voltage V,; and (D) a waveforni graph of voltageV2' Fig. 6 is a circuit diagram of an AC measuring apparatus constructed according to the third embodiment of the present invention.

Fig. 7 shows (A) a waveform graph of primary alternating current i,; and (B) a waveform graph of voltage V,.

Fig. 8 is a waveform graph of voltageV3 in the first embodiment.

Fig. 9 is a circuit diagram of an AC measuring apparatus constructed according to the fourth embodiment of the present invention.

Fig. 10 shows (A) a waveform graph of voltageV6; (B) a waveform graph of voltageV2; (C) a waveform graph of voltageV3; (D) a waveform graph of voltageV4; and (E) a waveform. graph of voltage V,.

Fig. I I is a circuit diagram of an AC measuring apparatus constructed according to the prior art.

BEST MODE FOR CARRYING OUT THE INVENTIO

Some preferred embodiments of the present invention will now be described in detail with reference to the drawings. Fig. I is a circuit diagram of an AC measuring apparatus constructed according to the first embodiment of the present invention.

Referring now to Fig. 1, a transformer I includes a secondary side coil 3 connected to a rectifying circuit 4. The rectifying circuit 4 is in the form of a bridge circuit constructed by four diodes, which is designed to full-wave rectify a secondary current i, generated at the secondary side coil 3 of the transformer I by a primary alternating current i, flowing through a conductor 2. The rectifying circuit 4 is connected to a capacitor 6 which is adapted to be charged with the secondary current i, through a backflow preventing circuit 5 (forward voltage V,).

Between the rectifying circuit 4 and the anode of the diode 5 are connected in parallel a Zener diode 7 for permitting the current to pass therethrough when the charged voltage in the capacitor 6 becomes higher than a predetermined voltage and a thyristor 8 switched on when the current passes through the Zener diode 7, said thyristor 8 being adapted to reduce a voltage V, generated by the secondary current i2 to a saturation voltage Vs. The gate of the thyristor 8 is connected between the anode of the Zener diode 7 and a resistance 9.

The capacitor 6 and the cathode of the diode 5 are connected to a voltage/frequency (V/F) converting circuit 10 and an electricity/optic (E/0) converting circuit I I through a power procurement path K, for procuring the necessary power for current measurement. Preferably, the V/F converting circuit 10 may be VFC62B manufactured by U. S. Burr Brown, for example. VFC62B is a superior instrument which can realize an accuracy with a non-linearity equal to 0.004% within a range of -200C to 709C. It would extremely be difficult that such an accuracy was accomplished in a combination of separate components. The E/O converting circuit I I may be in the form of a light emitting diode.

Between the VIF converting circuit 10 and the rectifying circuit 4 is located a current measurement path K2 (which is shown by dotted line for distinguishing from the other path K,) for feeding signals for measuring the current to the V/F converting circuit 10. The current measurement path K2 is connected at one end to between the rectifying circuit 4 and a load resistance 12, the other end thereof being connected to the V/F converting circuit 10. All of the load resistance 12, resistance 9, one end of the capacitor 6 and the cathode of the thyristor 9 are grounded.

The E/O converting circuit I I is connected to an optic/electricity (O/E) converting circuit 14 through an optical-fiber cable 13, the OiE converting section 14 being adapted to receive signals from the E/O converting circuit 11. The O/E converting section 14 is connected to a frequency/voltage (FN) converting section 15.

The operation of the first embodiment will now be described. It is now assumed that the primary alternating current i, having such a waveform as shown in Fig. 2(A) (e.g., at 50 Hz or 60 Hz) passes through the conductor 2 to be measured.

6 The primary alternating current i, generates a voltage at the coil 3 wound around the transformer 1, resulting in a secondary current '2 passing therethrough (in which if the number of windings in the coil 3 is equal to N, i, is equal to 1,N). The secondary current '2 flows to and charges the capacitor 6 through the rectifying circuit 4 and diode 5.

The discharged current from the capacitor 6 then flows into the V/F converting circuit 10 and E/O converting circuit I I through the power procurement path K,. Thus, the power required to operate the V/F converting circuit 10 and E/O converting circuit I I can internally be provided.

Among the voltages induced by the secondary current '2, a voltage V, at the cathode of the Zener diode 7 has such a waveform as shown in Fig. 2(C). The constant voltage VTH in the Zener diode 7 is preset to be higher than a voltage V3 required by the post-stage circuits (or the V/F converting circuit 10 and E/O converting circuit 11) by the forward voltage Vf in the diode 5. As shown in Fig. 3(A), the voltage V3 becomes a constant voltage VD (=VTH-VI) which has been smoothened and DC- converted by the capacitor 6. For example, if the voltage required by the post-stage circuits is 5V and the forward voltage Vr in the diode 5 is 0. 7V, the voltage VTH may be set to be 5.7V. Even if the primary alternating current i, is larger and the voltage induced by the secondary current '2 is thus larger, the thyristor 8 may be switched on by a voltage exceeding the voltage VTH, the voltage V, may be reduced to the saturation voltage Vs in the thyristor 8. As a result, the power consumed by the entire processing circuit can be reduced (in which if the saturation voltage Vs in the thyristor 8 is equal to 1.5V, the power consumed by the circuit becomes about 1.5W). Consequently, the secondary current '2 Will less be influenced by heat.

Since the transformer I will not permit the direct current to pass therethrough, the secondary current '2 necessarily becomes zero after a determined time. Thus, the thyristor 8 is also necessarily switched off after the determined time. Such a cycle will be repeated (see Fig. 2(C)). For example, if the secondary current '2 is a sine wave at 50 Hz, the same cycle will be repeated at the time interval of 10 msec. From such an operation, it is understood that the capacitance of the capacitor 6 connected to the cathode side of the diode 5 may be equal to a level sufficient to provide the current consumed by the post-stage circuits only for 10 msec.

The diode 5 can prevented the charge accumulated in the capacitor 6 from flowing back to the switched-on thyristor 8 since the saturation voltage Vs in the 7 switched-on thyristor 8 is lower than the voltage V3.

On the other hand, the transformer I functions as a source of constant current unless the load resistance 12 is extremely large. Therefore, the transformer I has a function of regulating its own generating voltage to pen-nit the constant or secondary current i, to flow therethrough, independently of the forward voltage at the diode of the rectifying circuit 4 and the variations in the saturation voltage Vs of the switchedon thyristor 8 as well as the variations in temperature characteristics.

The secondary current '2accurately reflected by the current flowing through the conductor 2 as well as the number of windings N will pass through the load resistance 12. Since in the first embodiment, the rectifying circuit 4 is adapted to perform the full-wave rectification, the current '2having such a full-wave rectified waveform. as shown in Fig. 2(B) will pass through the load resistance 12.

The secondary current i, generates a voltageV2having such a waveform as shown in Fig. 2(D) by the presence of the load resistance 12 (which voltage becomes its minus polarity if the reference potential in Fig. 5 is set as in Fig. 1). This voltageV2 is then converted into a frequency of pulse train as shown in Fig. 3 (B) by the V/F converting circuit 10 located on the one end of the current measurement path K2. Measurement signals at the converted frequency are then converted into optical signals by the E/O converting circuit 11. The optical signals are transmitted to a remote and insulated point through the optical-fiber cable 13. At the remote and insulated point, the transmitted optical signals are demodulated into such a voltage V5 as shown in Fig. 3(C) by the O/E converting section 14 and F/V converting section 15. Thus, the current passing through the conductor 2 can be measured at the remote and insulated observation point.

According to the first embodiment, the thyristor 8 and Zener diode 7 are located upstream of the capacitor 6 and the load resistance 12 for measuring the current is added into the system. Therefore, a single transformer I can simultaneously perform both the current measurement and power procurement. In addition, the capacitor 6, which would be used to perform both the power procurement and current measurement according to the prior art, is only used to procure the power, but not to measure the current. Thus, the AC measuring apparatus will not be influenced by variations in the components and their temperature characteristics. Irregularity in characteristics from one product to another can be minimized to enable a high-accuracy current measurement over a widened range of temperature.

8 Since the thyristor 8 is used to procure the power, the AC measuring apparatus will not take the power not required by the post-stage circuits except the minimum necessary power. This can less heat the AC measuring apparatus and also provide a widened range of current measurement. Particularly, even if the primary alternating current i, is larger to increase the secondary current '2, the heat will less be generated in the system. This is more effective on the side of large current in the secondary current '2' The second embodiment of the present invention will now be described below. As shown in Fig. 4, the AC measuring apparatus of the second embodiment uses a diode 20 for half-wave rectifying the secondary current '2' If such a primary alternating current i, as shown in Fig. 5(A) passes through the conductor 2, the current '2having such a half-wave rectified waveform as shown in Fig. 5(B) will flow through the load resistance 12. The voltage V, at the cathode of the Zener diode 7 and the voltageV2between the coil 3 and the load resistance 12 have such waveforms as shown in Figs. 5(C) and (D), respectively.

The third embodiment of the present invention will now be described. As shown in Fig. 6, the AC measuring apparatus of the third embodiment does not use such a thyristor 8 as in the first embodiment, but may use a Zener diode 30 having a constant voltage equal to 5.7V, for example. If such a primary alternating current i, as shown in Fig. 7(A) passes through the conductor 2, therefore, the voltage V, at the cathode of the Zener diode 7 becomesVTHover a constant period, as shown in Fig. 7(B). Since the third embodiment does not use the thyristor 8 as in the first embodiment, it can be of a simplified structure having fewer parts.

The fourth embodiment of the present invention will now be described. In the AC measuring apparatus according to the first embodiment of the present invention, if the primary alternating current i, is sufficiently large, the voltageV3 has such a waveform as shown in Fig. 3(A) and the voltageVDexceedsthe operation assuring voltage for the V/F converting circuit 10 and E/O converting circuit 11. Thus, the AC measuring apparatus wil I stably be operated. This provides such an output voltageV4 as shown in Fig. 3(B). However, the voltage VDdecreases as the primary alternating current i, decreases. If the voltageV3 becomes below the operation assuring voltage for the circuitVKas shown in Fig. 8, the circuit stops its operation. In the first embodiment, therefore, the operating current has its lower limit (ilmij below which any measurement is impossible.

On the other hand, since the V/F converting circuit 10 and E/O converting circuit I I are being supplied with the operation current even though the voltage 9 becomes below the operation assuring voltageVK, the power will be wasted by the other inoperative circuit. The general property of the transformer I is to reduce the generating voltage thereof associated with the current consumed by the connecting circuit. Thus, the circuit will be inoperative when the current lower than the lower limit becomes below the operation assuring voltage.

The fourth embodiment is characterized by that if the operating voltage becomes lower than a predetermined voltage, the supply of power to the VIF converting circuit 10 and E/O converting circuit I I is positively shut off.

Fig. 9 is a circuit diagram of an AC measuring apparatus constructed according to the fourth embodiment of the present invention. This AC measuring apparatus additionally comprises a circuit operation switching section 40 added to the first embodiment of the present invention. The circuit operation switching section 40 comprises a Zener diode 41 for setting a voltage used to start the circuit operation, a PNP-type transistor 42 for switching on and off the circuit operation, a thyristor 43 for controlling the transistor 42, a Zener diode 44 for setting a voltage used to stop the circuit operation, and resistances 45 and 46.

The emitter and base of the transistor 42 are connected to the cathode of Zener diode 41 and to the anode of the thyristor 43, respectively. The collector of the transistor 42 is connected to the V/F converting circuit 10 and E/O converting circuit 11.

The gate of the thyristor 43 is connected between the Zener diode 41 and the resistance 45, with the cathode thereof being connected to the cathode of the Zener diode 44.

The operation of the fourth embodiment will now be described. If the primary alternating current i, is sufficiently large to supply a rich current to the circuit, the voltage V3 becomes VCONT by a constant voltage set by the Zener diode 7. Since a voltage VON set by the Zener diode 41 is smaller than the voltage VCONT set by the Zener diode 7, the voltage VON is always in ON state. Thus, the transistor controlling thyristor 43 is switched on to activate the transistor 42. Therefore, the V/F converting circuit 10 and E/O converting circuit I I are always energized and continuously activated.

When the primary altemating current i, decreases to a level insufficient to provide the necessary power to the post-stage processing circuits, the voltage V3 across the capacitor 6 also decreases. If this voltage V3 becomes below a voltage VOrF set by the Zener diode 44, the current passing through the diode 5 is shut off to inactivate the transistor controlling thyristor 43. Thus, the transistor 42 is also 1 0 switched off to block the current to the V/F converting circuit 10 and E/O converting circuit 11.

When the transistor 42 is switched off to block the current to the V/F converting circuit 10 and E/O converting circuit 11, the load on the transformer I increases to increase the voltage V,. This voltage V, is charged to the capacitor 6. Consequently, the voltageV3 Will gradually increase in proportion to the level of the accumulated charge.

When the voltageV3 exceeds the voltageVONset by the Zener diode 41, the transistor 42 is switched on to initiate the supply of power to the VIF converting circuit 10 and E/O converting circuit 11.

If VON>VOFF is set, this circuit will have a hysteresis. Therefore, the poststage circuits are activated for a predetermined time fromVON to VOFF. During the other time, the post-stage circuits are stopped in operation so that the capacitor 6 will be charged. At this time, a voltageV6at the collector side of the transistor 42 varies as shown in Fig. I O(A). If the voltageVOFF is set to be equal to or higher than the operation assuring voltageVKin the circuit, the post-stage processing circuits can surely be activated for the period of time fromVON to VOFF- VoltagesV21 V31 V4andV5are respectively as shown in Figs. I O(B)-(E). The voltageV2across the load resistance 12 is the same as in the first embodiment, as shown in Fig. I O(B). Since the V/F converting circuit 10 and E/O converting circuit I I are activated only for the period of time fromVON to VOFFas shown in Fig. I O(A), the output voltageV4of the V/F converting circuit 10 is provided only for the aforementioned period time, as shown in Fig. I O(D). Thus, the output voltage (or demodulating voltage)V5 to be transmitted is as shown by solid line in Fig. I O(E). Signal disappearing portions between the solid-line portions in Fig. I O(E) can presumptively be interpolated as shown by dotted line in the same figure.

As described, if the voltageVCONTset by the Zener diode 7 is selected to be larger than the voltageVON set by the Zener diode 41 which is in turn selected to be larger than the voltageVOFFset by the Zener diode 44, the AC measuring apparatus is continuously activated for a period of time for which the voltageV3 is larger than the voltageVONand intermittently operated for a descending period of time for whichVON>V3>VOrF. For a period of time for which the voltageVOFF is larger than the voltageV3, the intermittent operation is stopped. For an ascending period of time for whichVON>V3>VOFFafter the voltageV3has become below the voltageVOFF, the intermittent operation is in its inactive state (see Fig. IO(Q. 1 1 According to the fourth embodiment, even if the primary alternating

current is lower, the current can be measured. By further setting for the circuit operation stopping voltageVOFF to be higher than the circuit operation assuring voltageVK, it can be avoided that the circuit unstably operates at any voltage near the circuit operation assuring voltageVK- The present invention is not limited to the aforementioned embodiments, but may be carried out in any of various other forms without departing from the sprit and scope of the invention as defined in the appending claims. For example, the E/O converting circuit I I may be replaced by such a wireless transmitter as disclosed in U. S. Patent No. 4,384,289, since the E/O converting circuit I I is internally energized. Furthermore, the rectifying circuit 4 in the third and fourth embodiments may be replaced by a single diode for half-wave rectifying the secondary current i, INDUSTRLAL AP ICABILI The present invention can provide the following advantages superior to the prior art.

(1) The AC measuring apparatus of the present invention can perform the power procurement and current measurement independently of each other. The current measurement is carried out by the voltage frequency conversion means which is activated only by the current discharged from the charge/discharge means. Therefore, the AC measuring apparatus will not be influenced by variations in the components and their temperature characteristics. This can minimize variations in characteristics from one product to another. As a result, the AC measuring apparatus of the present invention can more accurately measure the current over a widened range of temperature.

(2) Since a single transformer can separately perform the power procurement and the current measurement, the AC measuring apparatus can be reduced in size.

(3) Since the power procurement and current measurement are separately carried out, the transmission through the optical-fiber cable may easily be replaced by any of the other wireless transmission systems.

1 2

Claims (5)

1. An AC measuring apparatus comprising: a transformer for causing a primary alternating current at the primary side to generate a secondary current at the secondary side, rectifier means for rectifying the secondary current generated at the transformer, charge/discharge means for charging and discharging the rectified current from the rectifier means, charge/discharge control means for controlling said charge/discharge means to charge up to the voltage generated by the secondary current is a predetermined voltage and for controlling said charge/discharge means to discharge when said charged voltage therein becomes higher than said predetermined voltage, a load resistance located in a circuit through which the secondary current passes, voltage frequency conversion means supplied with the current discharged from said charge/discharge means as its operating power, said voltage frequency conversion means being adapted to convert the value of voltage generated by the load resistance into the value of frequency, and transmission means supplied with the current discharged by said charge/discharge means, said transmission means being adapted to transmit measurement signals outputted from said voltage frequency conversion means to an external.

2. The AC measuring apparatus of claim 1, wherein said charge/discharge means includes a Zener diode permitting a current to pass therethrough when the voltage generated by the secondary current becomes higher than a predetermined level, a thyristor for reducing the voltage generated by the secondary current to a saturation voltage when the current passes through the Zener diode, and a backflow preventing diode for preventing the discharged current from the charge/discharge means from flowing back to the thyristor.

3. The AC measuring apparatus of claim I or 2, including switching means for stopping the supply of discharged current as the voltage generated by the discharged current from the charge/discharge means becomes lower than a 1 3 preselected operation stopping voltage, and for initiating the supply of said discharged current as the voltage generated by the discharged current from the charge/discharge means becomes higher than the preselected operation start voltage.

4. The AC measuring apparatus of any one of claim I to 3, wherein said rectifier means is a full-wave rectification type.

5. The AC measuring apparatus of any one of claim I to 3, wherein said rectifier means is a half-wave rectification type.

1 4