CN103293362B - DC ammeter and multimeter and apply the measuring method of this reometer or multimeter - Google Patents

Abstract

The invention discloses a DC ammeter, a multimeter and a measuring method using the ammeter or the multimeter, and relates to the technical field of electricity measuring instruments, including an MCU as a main controller, and a program-controlled voltage source for signal transmission with the MCU respectively module and a current detection module, the MCU controls the voltage signal of the program-controlled voltage source module to output changes according to the signal transmitted by the current detection module, and the voltage signal gradually increases. The internal resistance of the DC multimeter is equivalent to 0; when measuring voltage, the internal resistance of the DC multimeter can be equivalent to infinity. It can be seen that the present invention solves the technical problem of inaccurate current and voltage measurement in the prior art. The invention can accurately measure the current or voltage in the circuit under any circumstances.

Description

DC ammeter and multimeter and measurement method using the ammeter or multimeter

technical field

The invention relates to the technical field of electricity measuring instruments, in particular to a DC ammeter, a multimeter and a measuring method using the ammeter or the multimeter.

Background technique

Electricity measurement includes the measurement of current, voltage, power and electric energy, among which current and voltage are the two most basic electric quantities in electrician measurement, and other electric quantity and non-electric quantity measurements can be converted into current or voltage for measurement. At present, when measuring current or voltage, an ammeter, a voltmeter or a multimeter are generally used for measurement.

When existing equipment measures the current in a certain circuit, the measuring instrument needs to be connected in series to the measuring line. Since the internal resistance of the meter is not 0, it is equivalent to connecting a resistor in series in the measuring circuit. This resistance is very small and generally does not affect the accuracy of the measurement. However, in some special cases, such as in the circuit to be measured When the difference between the resistance value of the meter and the internal resistance of the meter is small, the measured results will deviate. As shown in Figure 1, when the meter is not connected in series to the circuit, the loop current is 1.5A; when the meter is connected in series to the circuit to measure the current, the measured current is about 1.43A, which deviates from the accurate current by 70mA, and the internal resistance of the meter Affects the loop current, resulting in erroneous measurement results.

When the existing equipment measures the voltage between two points in the circuit, the instrument needs to be connected in parallel between the two points. Since the internal resistance of the instrument is not infinite, this is equivalent to connecting a finite resistance in parallel between the two measurement points. This resistance also consumes the current of the circuit to be measured. Although this current is very small, it generally does not affect the accuracy of the measurement. , the measured results will be biased. As shown in Figure 2, when the meter is not connected in parallel to the two measuring points of the circuit, the voltage between the two points is 2V; when the meter is used to measure the voltage between the two measuring points, the measured value is 1.5V. This is because the internal resistance of the meter needs to consume current, resulting in wrong measurement results.

Contents of the invention

The first technical problem to be solved by the present invention is to provide a DC ammeter. When the DC ammeter is used for current measurement, its equivalent internal resistance is 0, which does not affect the circuit to be measured and can accurately measure the circuit current.

Based on a general inventive concept, the second technical problem to be solved by the present invention is to provide a method for measuring DC current using the above-mentioned DC ammeter, which can accurately measure the circuit current.

Based on a general inventive concept, the third technical problem to be solved by the present invention is to provide a DC multimeter whose equivalent internal resistance is 0 when measuring current; and whose equivalent internal resistance is zero when measuring voltage. Infinity can accurately measure the current and voltage in the circuit.

Based on a general inventive concept, the fourth technical problem to be solved by the present invention is to provide a method for measuring DC current using the above-mentioned DC multimeter, which can accurately measure the circuit current.

Based on a general inventive concept, the fifth technical problem to be solved by the present invention is to provide a method for measuring DC voltage using the above-mentioned DC multimeter, which can accurately measure the circuit voltage.

For solving above-mentioned first technical problem, technical scheme of the present invention is:

A DC ammeter, including a first signal input terminal and a second signal input terminal for connecting with a measurement point of a circuit to be measured, and also includes: a main controller MCU, electrically connected with a programmable voltage source module and a current detection module, It is used to control the output voltage signal of the programmable voltage source module according to the signal transmitted by the current detection module; the programmable voltage source module is used to output a changed voltage signal according to the signal of the MCU, and the programmable voltage source module A resistor R31 is connected in series to one output end of the first signal input end, and the other output end of the programmable voltage source module is electrically connected to the second signal input end; the current detection module , for transmitting the detected current signal to the MCU, one end of the current detection module is electrically connected to the first signal input end, and the other end of the current detection module is electrically connected to the second signal input end; A power supply module, configured to supply power to the DC ammeter, and the power supply module is electrically connected to the MCU.

Wherein, the DC ammeter further includes a display module for displaying the current value, and the display module is electrically connected to the MCU.

Wherein, the program-controlled voltage source module is a program-controlled voltage source device or a program-controlled voltage source circuit.

For solving above-mentioned second technical problem, technical scheme of the present invention is:

A method for measuring DC current using the DC ammeter above, comprising the following steps: S1, connecting the DC ammeter in series to the circuit to be measured; S2, detecting the current signal by the current detection module, and converting the detected current signal transmitted to the MCU; S3, the MCU judges the current direction according to the current signal transmitted by the current detection module, and then controls the programmable voltage source module to output a voltage U1 that is the same as the current direction, and the voltage U1 Gradually increase until the current detected by the current detection module is 0; S4, the MCU calculates the current value of the circuit to be measured according to the formula: I1=U1/(R31+r1), and displays the current value I1 through a display module , wherein: I1=the current value of the circuit to be measured, U1=the voltage value output by the program-controlled voltage source module when the current detected by the current detection module is 0, r1=the voltage value of the wire on the circuit of the program-controlled voltage source module equivalent resistance value.

For solving above-mentioned 3rd technical problem, technical scheme of the present invention is:

A DC multimeter, including a first signal input terminal and a second signal input terminal connected to a measurement point of a circuit to be measured, and also includes: a main controller MCU, electrically connected with a programmable voltage source module and a current detection module, It is used to control the output voltage signal of the programmable voltage source module according to the signal transmitted by the current detection module; the programmable voltage source module is used to output a changed voltage signal according to the signal of the MCU, and the programmable voltage source module One output end of the variable resistor R41 is connected in series with the first signal input end, and the other output end of the programmable voltage source module is electrically connected to the second signal input end; The current detection module is used to transmit the detected current signal to the MCU, one end of the current detection module is connected in series with a variable resistor R42, and the current detection module and the variable resistor R42 are connected in series with the described A series circuit composed of a program-controlled voltage source module and a variable resistor R41 is connected in parallel; the power module is used to supply power to the DC multimeter, and the power module is electrically connected to the MCU.

Wherein, the DC multimeter further includes a display module for displaying a current value or a voltage value, and the display module is electrically connected to the MCU.

Wherein, the program-controlled voltage source module is a program-controlled voltage source device or a program-controlled voltage source circuit.

Wherein, the variable resistor R41 and the variable resistor R42 are both sliding resistors or thermistors.

For solving above-mentioned 4th technical problem, technical scheme of the present invention is:

A method for applying the above-mentioned DC multimeter to measure DC current, comprising the following steps: S1, adjusting the resistance value of the variable resistor R42 to 0Ω, and adjusting the resistance value of the variable resistor R41 to be greater than 0Ω; S2, adjusting The DC multimeter is connected in series to the circuit to be measured; S3, the current detection module detects the current signal, and transmits the detected current signal to the MCU; S4, the MCU transmits the current signal according to the current detection module The current signal determines the direction of the current, and then controls the programmable voltage source module to output a voltage U2 with the same direction as the current, and the voltage U2 gradually increases until the current detected by the current detection module is 0; S5, MCU Calculate the current value of the circuit to be measured according to the formula: I2=U2/(R41+r2), and display the current value I2 through a display module, where: I2=the current value of the circuit to be measured, U2=detected by the current detection module The voltage value output by the program-controlled voltage source module when the received current is 0, r2=the equivalent resistance value of the wire on the circuit of the program-controlled voltage source module.

In order to solve the above-mentioned fifth technical problem, the technical solution of the present invention is:

A method for measuring DC voltage using the above-mentioned DC multimeter, comprising the following steps: S1, adjusting the resistance value of the variable resistor R41 to 0Ω, and adjusting the resistance value of the variable resistor R42 to be greater than 0Ω; S2, Connect the DC multimeter in parallel to the circuit to be measured; S3, the current detection module detects the current signal, and transmits the detected current signal to the MCU; S4, the MCU transmits it according to the current detection module The current signal judges the voltage direction, and then controls the programmable voltage source module to output a voltage U3 with the same direction as the voltage, and the voltage U3 gradually increases until the current detected by the current detection module is just equal to U3/( R42+r3), wherein: U3=the voltage value output by the program-controlled voltage source module, r3=the sum of the equivalent resistance value of the wire on the line of the program-controlled voltage source module and the internal resistance of the current detection module; S5, The voltage value of the circuit to be measured is the voltage value U3 output by the programmable voltage source module when the current is exactly equal to U3/(R42+r3), and the MCU controls a display module to display the voltage value U3.

After adopting above-mentioned technical scheme, the beneficial effect of the present invention is:

Since the DC ammeter of the present invention includes a first signal input terminal and a second signal input terminal for being connected to the measuring point of the circuit to be measured, it also includes: an MCU electrically connected with a programmable voltage source module and a current detection module; a programmable voltage source One output end of the module is connected in series with a resistor R31, the resistor R31 is connected in series with the first signal input end, the other output end of the programmable voltage source module is electrically connected to the second signal input end; one end of the current detection module is electrically connected to the first signal input end As for the input terminal, the other terminal of the current detection module is electrically connected to the second signal input terminal. When performing current measurement, connect the DC ammeter in series to the circuit, the MCU can judge the direction of the current according to the signal transmitted by the current detection module, and then control the programmable voltage source module to output a voltage with the same direction as the current, and the voltage will gradually increase. large until the current measured by the current detection module is 0. At this time, the potential difference between the two ends of the DC ammeter is 0, and the equivalent circuit can be equivalent to short-circuiting the DC ammeter, that is, the equivalent internal resistance of the DC ammeter is 0, and if the equivalent internal resistance is 0, the current of the circuit to be measured will not be consumed, that is It will not affect the circuit to be measured, so that the current of the circuit to be measured can be accurately measured.

Since the DC multimeter of the present invention includes a first signal input terminal and a second signal input terminal connected to the measuring point of the circuit to be measured, it also includes: an MCU electrically connected with a programmable voltage source module and a current detection module; a programmable voltage source One output end of the module is connected in series with a variable resistor R41, the variable resistor R41 is connected in series with the first signal input end, the other output end of the programmable voltage source module is electrically connected with the second signal input end; one end of the current detection module is connected in series with The variable resistor R42, the current detection module is connected in series with the variable resistor R42, and then connected in parallel with the series circuit composed of the program-controlled voltage source module and the variable resistor R41; the power supply module is used to supply power to the DC multimeter. When measuring the current, adjust the variable resistor R42 to 0Ω, then connect the DC multimeter in series to the circuit, the MCU can judge the direction of the current according to the signal transmitted by the current detection module, and then control the program-controlled voltage source module to output a The voltage with the same current direction, this voltage gradually increases until the current measured by the current detection module is 0. At this time, the potential difference between the two ends of the DC ammeter is 0, and the equivalent circuit can be equivalent to short-circuiting the DC ammeter, that is, the equivalent internal resistance of the DC ammeter is 0, and if the equivalent internal resistance is 0, the current of the circuit to be measured will not be consumed, that is It will not affect the circuit to be measured, so that the current value of the circuit to be measured can be accurately measured. When measuring the voltage, adjust the variable resistor R41 to 0Ω, connect the DC multimeter in parallel to the circuit to be measured, the MCU can judge the direction of the voltage according to the signal transmitted by the current detection module, and then control the output direction of the programmable voltage source module to be the same The voltage U, the voltage U gradually increases until the current detected by the current detection module is exactly equal to U/(R42+r), r is the sum of the internal resistance of the current detection module and the equivalent resistance of the wire on the program-controlled voltage source line, At this time, the multimeter does not provide current to the circuit to be measured, nor consumes the current of the circuit to be measured, that is, the equivalent internal resistance of the multimeter is infinite, and does not affect the circuit to be measured, so that the voltage value of the circuit to be measured can be accurately measured.

To sum up, the DC ammeter and DC multimeter of the present invention solve the technical problem of inaccurate measurement of current and voltage by electricity measuring instruments in the prior art. The DC ammeter and the DC multimeter of the present invention will not have any influence on the circuit to be measured when measuring current or voltage, and can accurately measure the current value or voltage value of the circuit to be measured.

Description of drawings

Figure 1 is a schematic diagram of DC current measurement;

Figure 2 is a schematic diagram of DC voltage measurement;

Fig. 3 is the structural block diagram of DC ammeter of the present invention;

Fig. 4 is the structural block diagram of DC multimeter of the present invention;

Fig. 5 is the equivalent structural block diagram when DC multimeter of the present invention measures electric current;

Fig. 6 is an equivalent structural block diagram of the DC multimeter of the present invention when measuring voltage.

Detailed ways

Below in conjunction with accompanying drawing and embodiment, further elaborate the present invention.

Embodiment one:

As shown in Figure 3, a kind of DC ammeter comprises shell, is provided with the first signal input end 300 and the second signal input end 301 that are used to be connected with the measurement point of circuit to be measured on the shell, first signal input end 300 and The second signal input terminal 301 can be connected with a measuring test lead or a measuring wire clip. A main controller MCU is arranged inside the casing, and the MCU is electrically connected with a program-controlled voltage source module and a current detection module A. One output terminal of the program-controlled voltage source module is connected in series with a resistor R31 , and the resistor R31 is connected in series with the first signal input terminal 300 , and the other output terminal of the program-controlled voltage source module is electrically connected to the second signal input terminal 301 . One terminal of the current detection module A is electrically connected to the first signal input terminal 300 , and the other terminal of the current detection module A is electrically connected to the second signal input terminal 301 . The MCU is also electrically connected with a power supply module for supplying power to the DC ammeter, and the power supply can be a dry battery or a rechargeable battery. The MCU is also electrically connected with a display module, and the display module is used to display the measured current value.

The program-controlled voltage source module can use a complete program-controlled voltage source device, which only receives the enable and voltage direction control signals from the MCU, and controls the output step voltage by itself; it can also only use a program-controlled voltage source circuit, not only the enable and voltage direction are controlled by the MCU , and the output step voltage is also controlled by the MCU. The higher the accuracy of the programmable voltage source module, the better, and the smaller the step voltage, the more accurate the measured value.

Theoretically, the larger the resistance value of resistor R31 is, the more accurate the test result will be, but the power consumption of the DC ammeter will be higher, so the resistor R31 with a relatively large resistance value is selected according to the range of the DC ammeter.

The operating principle of the DC ammeter of the present invention is as follows:

When performing current measurement, the DC ammeter is connected in series to the test point of the circuit to be measured, and the MCU judges the direction of the current according to the current signal transmitted by the current detection module A, and then controls the programmable voltage source module to output a voltage with the same direction as the current. The voltage U1, the voltage U1 changes from small to large until the current measured by the current detection module A is 0. At this time, the potential difference between the two signal input terminals of the DC ammeter is 0, and the equivalent circuit can be equivalent to directly short-circuiting the DC ammeter, that is, the equivalent internal resistance of the DC ammeter is 0, and if the equivalent internal resistance is 0, it will not consume Measuring the current of the circuit does not affect the circuit to be measured, so that the current of the circuit to be measured can be accurately measured.

The measurement error of the DC ammeter mainly comes from the resistance of the common line of the program-controlled voltage source module line and the current detection module line. In order to improve the accuracy of the test results, the contact of the two lines can be extended to the signal input end of the DC ammeter.

A method for measuring DC current by using the above-mentioned DC ammeter, comprising the following steps:

S1. Connect the DC ammeter in series to the circuit to be measured;

S2. The current detection module A detects the current signal, and transmits the detected current signal to the MCU;

S3. The MCU judges the current direction according to the current signal transmitted by the current detection module A, and then controls the programmable voltage source module to output the voltage U1 that is the same as the current direction, and the voltage U1 gradually increases until the current detected by the current detection module A is 0 , at this time, the potential difference between the two signal input terminals of the DC ammeter is 0, and the current of the circuit to be measured is equal to the current of the circuit of the program-controlled voltage source module, that is, the current of the circuit to be measured I1=U1/(R31+r1), where: U1 is the voltage value output by the program-controlled voltage source module when the current detected by the current detection module A is 0, r1 is the equivalent resistance value of the wire on the circuit of the program-controlled voltage source module;

S4. The MCU calculates the current value of the circuit to be measured according to the formula: I1=U1/(R31+r1), and displays the current value I1 through the display module.

This DC current measurement method can accurately measure the circuit current.

Embodiment two:

As shown in Figure 4, a kind of DC multimeter, comprises shell, is provided with the first signal input end 400 and the second signal input end 401 that are used to be connected with the measuring point of circuit to be measured on the shell, first signal input end 400 and The second signal input terminal 401 can be connected with a measuring test lead or a measuring wire clip. A main controller MCU is arranged inside the casing, and the MCU is electrically connected with a program-controlled voltage source module and a current detection module A. One output terminal of the program-controlled voltage source module is connected in series with a variable resistor R41 , and the variable resistor R41 is connected in series with the first signal input terminal 400 , and the other output terminal of the program-controlled voltage source module is electrically connected with the second signal input terminal 401 . One end of the current detection module A is connected in series with a variable resistor R42, and after the current detection module A is connected in series with the variable resistor R42, it is connected in parallel with the series circuit composed of the program-controlled voltage source module and the variable resistor R41. The MCU is also electrically connected with a power supply module for supplying power to the DC multimeter, and the power supply can be a dry battery or a rechargeable battery. The MCU is also electrically connected with a display module, and the display module is used to display the measured current value or voltage value.

Both the variable resistor R41 and the variable resistor R42 are sliding resistors or thermistors. If both the variable resistor R41 and the variable resistor R42 are sliding resistors, two adjustment knobs for adjusting the resistance values of the variable resistor R41 and the variable resistor R42 need to be provided on the casing of the DC multimeter. If both the variable resistor R41 and the variable resistor R42 are thermistors, the resistance values of the variable resistor R41 and the variable resistor R42 are adjusted by the control of the MCU.

The program-controlled voltage source module can use a complete program-controlled voltage source device, which only receives the enable and voltage direction control signals from the MCU, and controls the output step voltage by itself; it can also only use a program-controlled voltage source circuit, not only the enable and voltage direction are controlled by the MCU , and the output step voltage is also controlled by the MCU.

The DC multimeter can be used to measure DC current or DC voltage, and its working principle is as follows:

When used to measure current, the variable resistor R42 can be adjusted to 0Ω first, and the variable resistor R41 can be adjusted to be greater than 0Ω. At this time, the equivalent structure of the multimeter is shown in Figure 5: one output terminal of the programmable voltage source module is connected in series. The variable resistor R41 is connected in series with the first signal input terminal 400, the other output terminal of the programmable voltage source module is electrically connected to the second signal input terminal 401, and one end of the current detection module A is electrically connected to the first signal input terminal 400 , the other end of the current detection module A is electrically connected to the second signal input end 401 . At this time, the working principle of the DC multimeter is the same as that of the DC ammeter in Embodiment 1, so it will not be repeated here.

When measuring the current, the variable resistor R42 may not be adjusted to 0Ω, no matter what the resistance value of the variable resistor R42 is, as long as the current value measured by the current detection module A is 0, the internal resistance of the multimeter is equivalent to 0 , it will not affect the circuit to be measured, and the current value of the circuit to be measured can also be accurately measured.

When used to measure voltage, it is necessary to adjust the variable resistor R41 to 0Ω first, and the variable resistor R42 to be greater than 0Ω. At this time, the equivalent structure of the multimeter is shown in Figure 6. The two output terminals of the programmable voltage source module are respectively connected to Connect the first signal input terminal 400 and the second signal input terminal 401; the variable resistor R42 is connected in series with the current detection module A and then connected in parallel with the programmable voltage source module. At this time, the working principle of the DC multimeter is as follows: connect the DC multimeter in parallel to the circuit to be tested, and the MCU judges the direction of the voltage according to the current signal transmitted by the current detection module A, and then controls the program-controlled voltage source module to output a voltage with the same direction as the voltage. The voltage U3, the voltage U3 changes from small to large, until the current measured by the current detection module A is exactly equal to U3/(R42+r3), r3 is the internal resistance of the current detection module A and the equivalent of the wire on the program-controlled voltage source line and the resistance. At this time, the multimeter does not provide current to the circuit to be measured, nor consumes the current of the circuit to be measured, that is, the equivalent internal resistance of the multimeter is infinite, and does not affect the circuit to be measured, so that the voltage value of the circuit to be measured can be accurately measured. When the programmable voltage source module outputs different voltages, the current detected by the current detection module A is always equal to U3/(R42+r3), and the display module shows that the measured voltage is 0V.

This embodiment only introduces the use of the DC multimeter of the present invention for measuring current and voltage, because the technical problem mainly solved by the present invention is that the existing multimeter measures current and voltage inaccurately, so only these two measurement functions are described in detail , does not mean that the DC multimeter of the present invention cannot have other electric quantity or non-electric quantity measurement functions. If other measurement functions are added on the basis of the present invention, such as the measurement of power, resistance value or capacitance value, it still falls into the DC multimeter of the present invention. Multimeter range of protection.

A method of applying the above-mentioned DC multimeter to measure DC current, comprising the following steps:

S1. Adjust the resistance value of the variable resistor R42 to 0Ω, and the variable resistor R41 to be greater than 0Ω;

S2. Connect the DC multimeter in series to the circuit to be measured;

S3. The current detection module A detects the current signal, and transmits the detected current signal to the MCU;

S4. The MCU judges the current direction according to the current signal transmitted by the current detection module A, and then controls the programmable voltage source module to output the voltage U2 that is the same as the current direction, and the voltage U2 gradually increases until the current detected by the current detection module A is 0 , at this time, the potential difference between the two signal input terminals of the DC ammeter is 0, and the current of the circuit to be measured is equal to the current of the circuit of the program-controlled voltage source module, that is, the current of the circuit to be measured I2=U2/(R41+r2), where: U2 is the voltage value output by the program-controlled voltage source module when the current detected by the current detection module A is 0, r2 is the equivalent resistance value of the wire on the circuit of the program-controlled voltage source module;

S5. The MCU calculates the current value of the circuit to be measured according to the formula: I2=U2/(R41+r2), and displays the current value I2 through the display module.

This DC current measurement method can accurately measure the circuit current.

A method for measuring DC voltage using the above-mentioned DC multimeter, comprising the following steps:

S1. Adjust the resistance value of the variable resistor R41 to 0Ω, and the variable resistor R42 to be greater than 0Ω;

S2. Connect the DC multimeter in parallel to the circuit to be measured;

S3. The current detection module A detects the current signal, and transmits the detected current signal to the MCU;

S4. The MCU judges the voltage direction according to the current signal transmitted by the current detection module A, and then controls the program-controlled voltage source module to output the voltage U3 with the same direction as the measured voltage. The voltage U3 changes from small to large until the current detection module A detects The current is exactly equal to U3/(R42+r3), wherein: U3 is the voltage value output by the program-controlled voltage source module, and r3 is the sum of the equivalent resistance value of the wire on the line of the program-controlled voltage source module and the internal resistance of the current detection module A;

S5. The voltage value of the circuit to be measured is the voltage value U3 output by the program-controlled voltage source module, and the MCU controls the display module to display the voltage value U3.

This DC voltage measurement method can accurately measure the circuit voltage.

The present invention is not limited to the above-mentioned specific implementation manners, and various transformations made by those skilled in the art starting from the above-mentioned concept without creative work all fall within the protection scope of the present invention.

Claims (6)

1. direct current multimeter, comprising the first signal input part for being connected with the measurement point of circuit to be measured and secondary signal input end, it is characterized in that, also comprise:

Master controller MCU, is electrically connected with program-controlled voltage source module and current detection module, and the signal for transmitting according to described current detection module controls described program-controlled voltage source module output voltage signal;

Described program-controlled voltage source module, for the voltage signal of the signal exporting change according to described MCU, an output terminal of described program-controlled voltage source module is serially connected with variable resistor R41, described variable resistor R41 is connected in series described first signal input part, and another output terminal of described program-controlled voltage source module is electrically connected described secondary signal input end;

Described current detection module, described MCU is given for the current signal transfer that will detect, one end of described current detection module is serially connected with variable resistor R42, in parallel with the series circuit that described program-controlled voltage source module and variable resistor R41 form after described current detection module and variable resistor R42 contact;

Power module, for powering to described direct current multimeter, described power module is electrically connected described MCU.

2. direct current multimeter according to claim 1, is characterized in that, described direct current multimeter also comprises the display module for showing current value or magnitude of voltage, and described display module is electrically connected described MCU.

3. direct current multimeter according to claim 2, is characterized in that, described program-controlled voltage source module is program-controlled voltage source device or program-controlled voltage source circuit.

4. direct current multimeter according to claim 3, is characterized in that, described variable resistor R41 and described variable resistor R42 is swept resistance or thermistor.

5. application rights requires that the direct current multimeter described in 1 carries out the method for DC current measurement, it is characterized in that, comprises the following steps:

S1, the resistance of described variable resistor R42 is adjusted to 0 Ω, the resistance of described variable resistor R41 is adjusted to and is greater than 0 Ω;

S2, described direct current multimeter is connected in series in circuit to be measured;

S3, described current detection module sensed current signal, and give described MCU by the current signal transfer that detects;

The current signal that S4, described MCU transmit according to described current detection module judges direction of current, then control described program-controlled voltage source module and export the voltage U 2 identical with described direction of current, described voltage U 2 increases gradually, until the electric current that described current detection module detects is 0;

S5, MCU are according to formula: I2=U2/ (R41+r2) calculates the current value of circuit to be measured, and by a display module display current value I2, wherein:

The current value of I2=circuit to be measured,

The magnitude of voltage that described program-controlled voltage source module when the electric current that current detection module described in U2=detects is 0 exports,

The equivalent resistance of program-controlled voltage source module circuitry upper conductor described in r2=.

6. application rights requires that the direct current multimeter described in 1 carries out the method for DC voltage measurement, it is characterized in that, comprises the following steps:

S1, the resistance of described variable resistor R41 is adjusted to 0 Ω, the resistance of described variable resistor R42 is adjusted to and is greater than 0 Ω;

S2, described direct current multimeter is parallel in circuit to be measured;

S3, described current detection module sensed current signal, and give described MCU by the current signal transfer that detects;

The current signal that S4, described MCU transmit according to described current detection module judges voltage direction, then control described program-controlled voltage source module and export the voltage U 3 identical with described voltage direction, described voltage U 3 increases gradually, until the electric current that described current detection module detects equals U3/ (R42+r3) just, wherein:

The magnitude of voltage that program-controlled voltage source module described in U3=exports,

The equivalent resistance of program-controlled voltage source module circuitry upper conductor described in r3=and the internal resistance sum of described current detection module;

The magnitude of voltage of S5, circuit to be measured is the magnitude of voltage U3 that described program-controlled voltage source module when electric current equals U3/ (R42+r3) just exports, and MCU controls a display module display magnitude of voltage U3.