WO1996039633A1

WO1996039633A1 - Method and apparatus for driving a digital multimeter having a function of a wattmeter

Info

Publication number: WO1996039633A1
Application number: PCT/KR1996/000083
Authority: WO
Inventors: Woo Kyoung Kim
Original assignee: Metex Corporation
Priority date: 1995-06-05
Filing date: 1996-06-05
Publication date: 1996-12-12

Abstract

This invention is to provide a driving apparatus and a method for a digital multimeter that contains functions of a wattmeter and a multi-display. According to the invention, the present multimeter comprising: an adapter provided with two input terminals (A, B) connected to a power supply, connecting parts (F, G) to be connected with a load which is in need to measure and three output terminals (C, D, E) which will be respectively inserted to outside terminals (X, Y, Z) of the present digital multimeter; a common terminal part (22) connected with the input terminal (Y) and output terminal (D) of the adapter so to ground thereof; a voltage attenuation driving part (21) to attenuate input voltage from the input terminal (X) which is connected with the output terminal (C) of the adapter; an A/D converting part (24) that converts input AC voltage from the voltage attenuation driving part (21) to DC voltage; and A/D converting part (31) to digitalize the analog DC voltage output from the A/D converting part (24); an amplifying part (26) for amplifying the AC voltage output from the voltage attenuation driving part (21); a current-voltage converting part (23) which may convert current inputted from the input terminal (Z) which is connected with the output terminal (E) of the adapter to voltage within the limit of 20A against voltage; an AC-DC converting part (25) that converts the AC current waveform output from the current-voltage converting part (23) to RMS direct current; an A/D converting part (28) to digitalize the analog voltage output from the A/D converting part (25); an amplifying part (27) to amplify the AC signal output from the current-voltage converting part (23); a microprocessor (29) to control the measurement of power factor on the basis of the respective measured factors; and a display driving part (30) that shows respective measured values simultaneously under the control of the microprocessor (29).

Description

METHOD AND APPARATUS FOR DRIVING A DIGITAL MULTIMETER HAVING A FUNCTION OF A WATTMETER

FIELD OF THE INVENTION This invention relates to a digital multimeter consisting of sale chips, especially, a method and a apparatus for driving a digital multimeter which may achieve a function of a wattmeter.

BACKGROUND OF THE INVENTION

Generally, conventional digital multimeter measures current, voltage, resistance, characteristic of diode, and frequency.

The Korean application, filed on April 28,1996 by the same applicant of the present invention and patented as a

Korean Patent NO 65675, discloses a digital multimeter having a bar flashing function which showing a measured value by flickering of bars.

This conventional multimeter can not achieve a function of a wattmeter measuring electric power and displaying the measured value upon a certain load.

Accordingly, in addition to the conventional digital multimeter, it has been required a wattmeter to measure electric power. Therefore, it causes great cost for purchasing respective measuring instruments and decreases operation efficiency.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a digital multimeter accomplishing a function of a wattmeter. Furthermore, the present invention is to provide a method and an apparatus for driving a digital multimeter having multi-display function which shows respective measured values simultaneously. According to the present invention, above objects are achieved by a driving apparatus comprising: an adapter provided two input terminals connected to a power supply, connecting parts to be connected with a load which is in need to measure and three output terminals which will be respectively inserted to outside terminals of the present digital multimeter; a common terminal part connected with the input terminal and output terminal of the adapter so to ground thereof; a voltage attenuation driving part to attenuate input voltage from the input terminal which is connected with the output terminal of the adapter; an amplifying part for waveform of AC voltage which is inputted from the voltage attenuation driving part; a voltage selecting IC to which the amplified AC voltage is inputted; a current-voltage converting part which may convert current(inputted from the input terminal which is connected with the output terminal of the adapter to voltage within the limit of 20A against voltage; an amplifying part for alternating current which is inputted from the current-voltage converting part; a current selecting IC to which the amplified AC current is inputted; an RMS(root mean square)-DC converting part which converts the input AC current or the AC voltage to a direct current or a direct voltage; an A/D converting part for converting the analog direct voltage or the analog direct current which is inputted from the RMS-DC converting part; a microprocessor for controlling the whole circuit and; a display driving part which shows the respective measured values simultaneously. Furthermore, the present invention is also to provide a driving method for the digital multimeter consisting of following steps: initialize a system by selecting a measuring mode to clear RAM, check the type of sale chip and start a timer; read only voltage data through the voltage selecting IC for one second, and read only current data through the current selecting IC for another one second; determine the current state whether it is high or low so to count(COUNTl = COUNTl + 1) when it is high, or to re- read the current data when it is in low state; determine the current state of counted value(COUNTl) whether it is high or low so to re-count(COUNTl = COUNTl + 1) when it is high, or to determine the voltage state when the current state is in low; count as follows according to the voltage state,

COUNT2 = COUNT2 + 1 or COUNT3 = COUNT3 + 1; determine the voltage state of the counted value(COUNT2) so to continue to count(COUNT2 = COUNTl + 1) when the voltage is high, or to count as follows when it is in low,

COUNT = COUNTl - COUNT2; determine the voltage state of the counted value(C0UNT3) so to continue to count(C0UNT3 = C0UNT3 + 1) in the case of low state, or to count(C0UNT4 = COUNTl - C0UNT3) in the case of low state; calculate COS@ by using the counted values in forgoing steps; compute electric power by using the measured factors in each steps and; show the respective measured factors on a display, communicate with computer and, clear the counted values.

Furthermore, the present driving apparatus for a digital multimeter having a function of a wattmeter comprising: an adapter provided two input terminals connected to a power supply, connecting parts to be connected with a load which is in need to measure and three output terminals which will be respectively inserted to outside terminals of the present digital multimeter; a common terminal part connected with the input terminal and output terminal of the adapter so to ground thereof; a voltage attenuation driving part to attenuate input voltage from the input terminal which is connected with the output terminal of the adapter; an A/D converting part that converts input AC voltage from the voltage attenuation driving part to DC voltage; an A/D converting part to digitalize the analog DC voltage output from the A/D converting part; an amplifying part for amplifying the AC voltage output from the voltage attenuation driving part. a current-voltage converting part which may convert current inputted from the input terminal which is connected with the output terminal of the adapter to voltage within the limit of 20A against voltage; an AC-DC converting part that converts the AC current waveform output from the current-voltage converting part to RMS direct current; an A/D converting part to digitalize the analog voltage output from the AC-DC converting part; an amplifying part to amplify the AC signal output from the current-voltage converting part; a microprocessor for control the measurement of power factor on the basis of the respective measured factors and; a display driving part that shows respective measured values simultaneously under the control of the microprocessor.

Another aspect of the present invention is a driving method for the digital multimeter consisting of following steps: select the mode to electric power measurement mode; measure the voltage by using an internal A/D converting part, and measure the current by using an external A/D converting part; determine the voltage state and the current state whether they are high or low respectively. check the current frequency whether it is in rising edge or in falling edge so to save 16 bit counter values to the current memory part F(A) of the microprocessor in the case of rising edge state; check the voltage frequency state whether it is in rising edge or in falling edge so to save 16 bit counter values to the voltage memory part F(V) of the microprocessor in the case of rising edge state, and stop the 16 bit counter; calculate the delay angle and; compute electric power by calculate cosine value.

The present invention contains respective analog integrated circuits(IC) to measure RMS current and RMS voltage. The analog IC measures analog current for a second and then measures analog voltage for another one second by turns, wherein the analog current and the analog voltage are inputted to the driving part of the digital multimeter through three output terminals. According to the present invention, it can be possible to compute electric power by control the program of the microprocessor, and may display the measured value on LCD

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit block diagram corresponding to a wattmeter in a digital multimeter according to a first embodiment of the present invention,

FIG. 2 is a flowchart of a microprocessor for measuring electric power according to the first embodiment,

FIG. 3 shows a waveform for measuring an AC current, FIG. 4 shows a waveform for measuring an electric power factor and an cosine angle of load AC current,

FIG. 5 shows a constitution of an adapter for measuring electric power according to load,

FIG. 6 shows a circuit block diagram corresponding to a wattmeter in a digital multimeter according to a second embodiment of the present invention. FIG. 7 is a flowchart of a microprocessor for measuring electric power according to the second embodiment,

FIG. 8 shows a waveform illustrating a time delay of voltage and current.

DETAILED DESCRIPTION OF THE EMBODIMENT

Fig. 1 shows a block corresponding to wattmeter of a first embodiment according to the present invention, and Fig. 5 shows a construction of an adapter measuring electric power according to a load.

According to the Fig. 5, a terminal(B) of an adapter is internally connected with terminals (C,F) and externally with terminals(E,G) through the outside load. Furthermore, a terminal(A) is internally connected with a terminal (D) . Wherein, general power supply is inputted to the terminal(F) of the adapter and the terminals (C,D) through the input terminal (A,B) of the adapter, and then outputted to the terminals (F,G) to measure outside load.

When the inputted general power supply and load current is inputted to the input terminals (X,Y,Z) through the output terminal(C,D,E) , AC voltage, inputted to the input terminal (X) of the digital multimeter, is attenuated through the voltage attenuation driving part. Thereafter the attenuated AC voltage applies to an amplifier(9) amplifying waveform of frequency thereof and then applies to an voltage selecting IC(4) that selects voltage by control of a microprocessor(8) . Wherein said voltage is selected just for a second by control of the microprocessor(8) and the IC(5) is not operated during the selection of voltage. The alternating current inputted from the input terminal(Z) through the output terminal (E) applies to an amplifier(10) through an current-voltage converting part(3). The input AC current is applied to the IC(5) which selects current by control of the microprocessor. The IC(5) selects current for a second by control of the microprocessor(8) and the IC(4) is not operated during the selection of current.

The output terminal (D) among output terminals(C,D,E) of the adapter is connected with a common terminal part(2) of the digital multimeter so to ground the whole circuit.

Respective RMS of the respective selected AC voltage and the alternating current are counted, and the counted values are then inputted to RMS-DC converting part(6) so that the counted values are converted in direct current values. The direct current values are applied to the A/D converting part(7), and the applied direct voltage and current are converted to digital information. The digital information is controlled by the microprocessor and then displayed through a display driving part(11).

For example, voltage is down to 0.02V(corresponding to 1/10000) by register contained in the voltage attenuation driving part(l). Wherein, said voltage is an AC voltage, and the frequency thereof is amplified to 50 ~ 60Hz through the amplifier(9,10) so to recount after selection of the current selecting IC(4,5) for a second. Respective RMS of selected voltage and current are converted by the RMS DC converting part(6) so that inputted to A/D converting part(7) as direct values. The values are digitized in the A/D converting part(7) as follows.

V_I (input voltage)/V_RF(standard voltage) x 10000, i.e.. 0.02V/1V X 10000 = 200

The digitized value is visualized through the display driving part(11) by control of the microprocessor.

It will be described about operation of the first embodiment according to the present invention with a reference of the Fig. 2 as follows:

First, turn on the digital multimeter to operate the system as a step SI;

In a step S2, select the mode for measuring a desired factor, so to initialize the system, clear the RAM, check the type of sale chips, and also to operate timer that make it possible to measure voltage and current at an interval of a second repeatedly;

In a step S3, analog switch voltage is selected by the voltage selecting IC(4) to repeatedly measure RMS of applied AC voltage at an interval of a second, wherein said voltage is delayed for 0.9 second to stabilize the system, and then read the data of measured voltage;

In a step S4, analog switch current is selected by the current selecting IC(5) so to repeatedly measure RMS of applied alternating current according to the load at an interval of a second, wherein said current is delayed for 0.9 second in order to stabilize the system, and then read the date of the measured voltage; In a step S5, current data measured in preceding step S4 is read;

In a step S6, determine the current state whether it is high or low and then return to the step S5 to reread the measured current in the case of low. On the contrary it will progress to the following step S7 when the current state is high; In a step S7, count as follows when the current state of the proceeding step S6 is high, COUNTl = COUNTl + 1;

In a step S8, the current state is determined so that it will return to the preceding step S7 when the current is in high state, or progress to the following step S9;

In a step S9, the voltage state of proceeding step S8 is determined whether it is high or low;

In a step SIO, count as follows when the voltage state of the step S9 is low, COUNT3 = COUNT3 + 1;

In a step Sll, determine the voltage state of the proceeding step SIO. According to the decision, it returns to the step SIO when the voltage is in low state, in other word, continue to count until the voltage state becomes high. On the contrary, it will progress next step when the voltage state of the step Sll is high;

In a step S12, count as follows, COUNT4= COUNTl + COUNT3; In a step S13, count as follows when the voltage state of the step S9 is high, COUNT2= COUNT2 + 1;

In a step S14, determine voltage state, and then return to the proceeding step S13 in the case of high; In a step S15, count as follows when the voltage state of the step S14 is low,

COUNT4 = COUNT1-COUNT2;

In a step S16, calculate COS@ by following equations,

COS@ = (COUNT4/COUNT1) * 180 (1) (wherein the COUNT4=COUNTl - COUNT3 or

COUNT4=COUNTl - COUNT2) In a step S17, compute electric power on the basis of the measured factors in the forgoing steps;

In a step SI8, display the respective measured factors are displayed(for example, current, voltage, COSΘ and, time) and;

In a step S19, the measured values are displayed or communicate with the computer, and then clear the counted values. Wherein, in order to calculate the electric power by measuring the waveform of the current and voltage, return to the step S3 and select analog switch voltage.

Figs 3(a) and 3(C) shows waveform that is used to measure the alternating current.

It will be described about operation of the average value by repeatedly measuring the waveform of the applied alternating current and the load AC current at an interval of a second.

This is achieved to compute a voltage and a current by following expressions, v = v * sin(wt) 1 = 1 * sin(wt-theta) , and then compute electric power as follows, p = v * I = V * I * sin(wt) * sin(wt-theta) p = V * I * cos(theta) - v * I * cos(2wt-theta) = p(const) - p(variable), wherein, it becomes zero to integrate P with time, since P is constant without regard to time and P is determined by time factor.

Consequently, P = V * I * cos(theta; the terminal current is a following angle of the terminal voltage) . Furthermore, active power is a current factor(I * COS(theta)) that is related with the average power, and reactive power is a current factor(I * SIN(theta)) that is not related with the average power. Accordingly, apparent electric power is represented by multiply of virtual values of the terminal voltage and the terminal current, and AV is a unit thereof.

Fig.4 shows a waveform to calculate the cosine angle(difference of phase) and the power factor of the load AC current. This is achieved by generating a basic count pulse in the regular time, and calculate total period and time corresponding to phase difference between the applied current and the load current. After that convert the ratio to angle, i.e., measured in forward or backward by measuring the frequency that is converted from the phase difference. While said phase difference may be represented in negative or in positive, the corresponding cosine value to the phase difference of the power factor is always positive. Furthermore, voltage is a standard to measure the relation between the frequency and the phase of applied power supply. Wherein, 1) when the voltage goes ahead, check the time(tl) for delay in high state of voltage, i.e., the time delay(tl) of T(H) becomes COUNT4 (wherein, the total period is 2 second, H is a period in high state, L is a low state period, T(H) and T(L) are respectively half period of H and L, COUNTl is the standard voltage waveform in T(H)) during a second.

Accordingly, ANGLE = -^- x 180

T(H)

COSINE PHI = COSINE(ANGLE)

2) when the current goes ahead, check the time for delay(t2) in low state of voltage, i.e., time delay(t2) in half period of low state(TL) becomes COUNT 4 (wherein, the total period is 2 second, H is a period of high state, L is a period of low state, COUNTl is the standard voltage waveform in half period of high state, T(H) and T(L) are respectively represent half periods of H and L) . Consequently,

ANGLE = -^- x 180 T{L)

COSINE PHI = COSINE(ANGLE)

The phase difference may be measured without regard to input frequency of the standard power supply according to aforesaid measuring of phase difference.

Fig. 6 shows a block corresponding to wattmeter of a second embodiment according to the present invention.

According to the Fig. 6, when the inputted general power supply and load current is inputted to the input terminals (X,Y,Z) , AC voltage which is inputted to the input terminal(X) of the digital multimeter, is attenuated through the voltage attenuation driving part(21).

Thereafter the attenuated AC voltage applies to an amplifier(26) amplifying waveform of frequency thereof. The input AC voltage is measured by the microprocessor.

The AC current applied to the input terminal (Z) through the output terminal(E) is inputted to a amplifier(27) through a current-voltage converting part(23). The frequency waveform of the input AC current is amplified in amplifier(27) and then controlled by a microprocessor(29) .

Wherein, the output terminal (D) among the output terminal(C,D,E) of the adapter is connected with a common terminal part(22) to ground the circuit.

Respective RMS of the AC voltage and current are counted, and then inputted to AC-DC converting part(24,25) so to converted to DC value. The DC current is inputted to A/D converting part(28) and the DC voltage is inputted to A/D converting part(31). The applied DC voltage and DC current are controlled by the microprocessor(29) as a digitalized information and then displayed on the multi-LCD through a display driving part(30).

For example, as a load of 220V is applied to the present multimeter by the adapter, the voltage is down to 2.2V(corresponding to 1/1000) by register contained in the voltage attenuation driving part(21) and the output part. Wherein, the voltage attenuation driving part(21) outputs an AC voltage, and the current-voltage converting part(23) outputs AC current. The said voltage and the current is amplified to 50 ~ 60Hz through the amplifier(26,27) . Thereafter, RMS of the voltage is inputted to a A/D converting part(31) of the microprocessor(29) as a direct value through the amplifier(26) and current is inputted to A/D converting part(28) as direct value through a A/D converting part(25) . The A/D converting part(31) within the microprocessor(29) produces a digitalized information upon

(Vin/Vref) X 256 i.e., (2.2V/2.56) X 256 = 220. The measured value of the digitalized information is visualized on the display driving part in multi-display form by control of the microprocessor.

The multi-display means that a plurality measured values(i.e., AC voltage, AC current, power consumption and time delay(COSπ;power factor) of the voltage and the current are displayed in simultaneous. In more details, electric power is displayed in a first segment and AC current is displayed in a second segment. AC voltage is displayed in a third segment and power factor measured by phase difference of voltage and current is displayed in a fourth segment. Fig. 2 shows a follow chart of a program for measuring a electric power contained in the microprocessor.

Firstly, in a step SI, turn on the digital multimeter to operate the system.

In a step S2, initialize the system and clear internal memory. In a step S3, select the measuring mode to select a factor which is in need to measure.

In a step S4, measure the voltage by using an internal A/D converting part(31), and measure the current by using an external A/D converting part (28) . In a step S5, check the voltage state whether it is high or low so to check the current state in the case of high, or to indicate zero power in the case of low state.

In a step S6, check the current state whether it is high or low so to check the voltage frequency when the current is in high state, or to indicate zero power in the case of low state.

In a step S7, check the voltage frequency whether it is in rising edge state or in falling state so to operate an internal counter in the case of rising edge state, or to re-check the voltage frequency state when it is in falling edge state.

In a step S8, set up the 16 bit internal counter and start thereof.

In a step S9, check the current frequency whether it is in rising edge state or in falling edge state so to save the internal counter values till then in the current memory part F(A) of the microprocessor in the case of rising edge state. On the contrary, re-check the current frequency state when the current frequency is falling edge state.

In a step SIO, check the voltage frequency state whether it is in rising edge or in falling edge so to save the internal counter values till then in the voltage memory part F(V) of the microprocessor in the case of rising edge state. On the contrary, re-check the voltage frequency state when the voltage frequency is in falling edge. In a step Sll, calculate cosine value of the calculate angle according to the following expression,

THETA = (F(A)/F(V)) X 360, wherein the F(A) and the F(V) is obtained in the forgoing steps.

In a step S12, calculate electric power(P = V x I x COSINE Value) on the basis of the measured values i.e., voltage, current and cosine value.

In a step S13, display the measured values (for example, voltage, current, and cosine value) respectively, and communicate with another function according to a wattmeter and with a computer. In addition, re-clear the counted values. It may possible to return to the step S3 for re¬ computing the power by measuring the waveform of voltage and current.

Fig. 8 shows a waveform according to a time delay of voltage and current, which shows a proceeding for calculate cosine value in the step Sll.

The F(V) is corresponding to a total period(T) and the F(A) is corresponding to a period(Tl) or a period(T2) .

Accordingly, delay load current having a positive vale is calculated as follows, delay angle(cosine) = Tl/T x 360 ^* Proceeding load current having negative value is obtained as follows. delay angle(cosine) = T2/T1 x 360 °

Wherein, since power factor(cosine φ) is equal to the delay angle(cosine) , it is possible to calculate electric power.

Accordingly, the present invention contains a circuit having a function of wattmeter and control thereof. In addition it is provided an adapter having 3 output terminals. Furthermore, it may simply construct the driving apparatus for digital multimeter that may achieve a function of wattmeter is contained in the digital multimeter.

Claims

1. A driving apparatus for a digital multimeter having a function of a wattmeter comprising: an adapter provided two input terminals (A,B) connected to a power supply, connecting parts (F,G) to be connected with a load which is in need to measure and three output terminals (C,D,E) which will be respectively inserted to outside terminals(X,Y,Z) of the present digital multimeter; a common terminal part(2) connected with the input terminal(Y) and output terminal (D) of the adapter so to ground thereof; a voltage attenuation driving part(l) to attenuate input voltage from the input terminal (X) which is connected with the output terminal (C) of the adapter; an amplifying part(9) for waveform of AC voltage which is inputted from the voltage attenuation driving part(l); a voltage selecting IC(4) to which the amplified AC voltage is inputted; a current-voltage converting part(3) which may convert current inputted from the input terminal (Z) which is connected with the output terminal (E) of the adapter to voltage within the limit of 20A against voltage; an amplifying part(10) for input alternating current which is inputted from the current-voltage converting part(3) ; a current selecting IC(5) to which the amplified alternating current is inputted; an RMS (root mean square)-DC converting part(6) which converts the input AC current or the AC voltage to a direct current or a direct voltage; an A/D converting part(7) for converting the analog direct voltage or the analog direct current which is inputted from the RMS-DC converting part(6); a microprocessor(8) for control the whole circuit and; a display driving part(11) which is showing the respective measured values simultaneously.

2. A driving method for the digital multimeter consisting of following steps: initialize a system by selecting a measuring mode to clear RAM, check the type of sale chip and to start a timer; read only voltage data through the voltage selecting IC for one second, and read only current data through the current selecting IC for another one second; determine the current state whether it is high or low so to count(COUNTl = COUNTl + 1) when it is high, or to re¬ read the current data when it is in low state; determine the current state of counted value(COUNTl) whether it is high or low so to re-count(COUNTl = CUNTl + 1) when it is high, or to determine the voltage state when the current state is low; count as follows according to the voltage state, COUNT2 = COUNT2 + 1 or COUNT3 = COUNT3 + 1; determine the voltage state of the counted value(COUNT2) so to continue to count(COUNT2 = COUNTl + 1) when the voltage is high, or to count as follows when it is in low,

COUNT4 = COUNTl - COUNT2; determine the voltage state of the counted value(COUNT3) so to continue to count(COUNT3 = COUNT3 + 1) in the case of low state, or to count(COUNT4 = COUNTl - COUNT3) in the case of low state; calculate COS@ by using the counted values in forgoing steps; compute electric power by using the measured factors in each steps and; show the respective measured factors on a display, communicate with computer and, clear the counted values.

3. A driving apparatus for a digital multimeter having a function of a wattmeter comprising: an adapter provided two input terminals (A,B) connected to a power supply, connecting parts (F,G) to be connected with a load which is in need to measure and three output terminals(C,D,E) which will be respectively inserted to outside terminals(X,Y,Z) of the present digital multimeter; a common terminal part(22) connected with the input terminal(Y) and output terminal(D) of the adapter so to ground thereof; a voltage attenuation driving part(21) to attenuate input voltage from the input terminal (X) which is connected with the output terminal(C) of the adapter; an A/D converting part(24) that converts input AC voltage from the voltage attenuation driving part(21) to DC voltage; an A/D converting part(31) to digitalize the analog DC voltage output from the A/D converting part(24); an amplifying part(26) for amplifying the AC voltage output from the voltage attenuation driving part(21). a current-voltage converting part(23) which may convert current inputted from the input terminal (Z) which is connected with the output terminal (E) of the adapter to voltage within the limit of 20A against voltage; an AC-DC converting part(25) that converts the AC current waveform output from the current-voltage converting part(23) to RMS direct current; an A/D converting part(28) to digitalize the analog voltage output from the AC-DC converting part(25); an amplifying part(27)to amplify the AC signal output from the current-voltage converting part(23); a microprocessor(29) for control the measurement of power factor on the basis of the respective measured factors and; a display driving part(30) that shows respective measured values simultaneously under the control of the microprocessor(29) .

4. A driving method for the digital multimeter consisting of following steps: select the mode to electric power measurement mode; measure the voltage by using an internal A/D converting part, and measure the current by using an external A/D converting part; determine the voltage state and the current state whether they are high or low respectively. check the current frequency whether it is in rising edge state or in falling edge state so to save 16 bit counter values to the current memory part F(A) of the microprocessor in the case of rising edge state; check the voltage frequency state whether it is in rising edge or in falling edge so to save 16 bit counter values to the voltage memory part F(V) of the microprocessor in the case of rising edge state, and stop the 16 bit counter; calculate the delay angle and; compute electric power by calculate cosine value.