GT4508 electric meter modification and calibration experiment instrument instruction manual

I. Overview

Pointer ammeters, voltmeters, and multimeters are widely used in various electrical measurement applications, and their indications are realized by galvanometers. A simple galvanometer can only be used to measure small currents and voltages, so the galvanometer must be modified to be used in various measurement fields.

The instrument can complete the modification of ammeter, voltmeter and ohmmeter experiment by connecting, and the experiment can improve the user's ability to use the meter and use the meter.

Second, the main technical parameters

1. The pointer type is modified: the range is 1mA, the internal resistance is about 155Ω, and the accuracy is 1.5.

2, resistance box: adjustment range 0 ~ 11111.0 Ω, accuracy 0.1

3, standard ammeter: 0 ~ 2 mA, 0 ~ 20mA two range, three and a half digital display, accuracy ± 0.5%

4, standard voltmeter: 0 ~ 2V, 0 ~ 20V two ranges, three and a half digital display, accuracy ± 0.5%

5, adjustable voltage regulator source: output range 0 ~ 2V, 0 ~ 10V two ranges, stability 0.1% / min, load adjustment rate 0.1%

6, power supply: AC 220V ± 10%, 50Hz

7, dimensions: 400mm × 250mm × 130mm

Third, the instructions for use

The instrument is equipped with a pointer galvanometer, a standard voltmeter ammeter, an adjustable DC stabilized power supply, a decimal resistance box, special wires and other components, and can complete a variety of meter modification experiments without other accessories.

Figure 1 Schematic diagram of the panel

1. Regulated power supply adjustment potentiometer

2. Regulated power supply output

3. Regulated power supply indicator head

4. Standard voltmeter output

5. Standard voltmeter

6. Pointer galvanometer

7. Pointer galvanometer input

8. Standard ammeter

9. Standard ammeter input

10. RW potentiometer

11. R3 resistor

12. R1, R2 resistors

The adjustable DC voltage regulator is divided into two ranges of 2V and 10V. The voltage output is selected by the “voltage selection switch” to adjust the voltage required for the “voltage adjustment” potentiometer. The indication of the pointer voltmeter is also divided into two ranges of 2V and 10V.

The standard digital voltmeter has two ranges of 2V and 20V. The voltage range selection switch is used to select different voltage ranges, which need to be connected to the corresponding measurement terminal for measurement.

The standard digital ammeter has two ranges of 2 mA and 20 mA. Different current ranges can be selected by the “current range selection switch”, which can be connected to the corresponding measurement terminal for measurement.

Fourth, the principle introduction

1, modified large range of ammeter

Connect a suitable resistor at both ends of the meter to shunt the current in the measuring circuit so that when the meter indicates fullness, the total current of the line is the current value of the required range. At this time, the meter is converted into Large range ammeter.

2, converted into a larger range of voltmeter

When a suitable resistance is connected in series with the ammeter to make the ammeter full, the voltage on the series circuit is equal to the voltage of the required range, and the ammeter is converted into a voltmeter.

3, converted into an ohmmeter

As shown in Figure 2, a suitable resistor is connected in series with a power supply in series with the ammeter. When the RX is connected to the measured resistor, the ammeter is deflected, and different RXs will cause different ammeter deflections. After calibrating the deflection of the ammeter with a standard resistor box, it can be used to measure the resistance, and the ammeter is converted into an ohmmeter. See the experimental lectures for detailed measurement principles and wiring.

Figure 2 Schematic diagram of the ohmmeter

Five, the use of steps

1. Turn on the power switch at the back of the instrument and turn on the AC power.

2. Check the standard voltmeter and standard ammeter, which should be displayed normally. It is normal for the standard voltmeter to jump when the internal resistance is high due to high internal resistance.

3. Adjust the regulated power supply and output it normally.

4. Perform ammeter modification according to the content of the lecture and measure the unknown current with the converted ammeter.

5. Refit the voltmeter according to the content of the lecture, and measure the unknown voltage with the converted voltmeter.

6. Perform serial and parallel ohmmeter modification according to the content of the lecture, and measure the unknown resistance with the modified ohmmeter.

Sixth, maintenance and warranty

1. The instrument should be used correctly according to the experimental requirements.

2. Turn off the power switch after use. If it is not used for a long time, please dial the power plug.

3. The instrument should be stored in an environment free of corrosive substances and kept dry to prevent corrosion.

4. Under the conditions of use of the user, the warranty period of the product is 12 months. If the warranty period is exceeded, the manufacturer will still provide good service.

Experiment meter modification and calibration

Electric meters have a wide range of applications in electrical measurement, so how to understand the meter and use the meter is very important. Due to the construction, the galvanometer (header) can only measure small currents and voltages. If it is to be used to measure large currents or voltages, it must be modified to expand its range. The principle of the multimeter is to modify the micro-ampere meter to multi-range, which has been widely used in circuit measurement and fault detection.

First, the purpose of the experiment

1. Measuring internal resistance and full current of the meter

2. Master the method of changing the 1mA meter to a larger range of ammeter and voltmeter

3. Design an ohmmeter with R=1500Ω, and require E to be zeroed in the range of 1.3~1.6V.

4. Calibrate the ohmmeter with a resistor, draw a calibration curve, and measure the unknown resistance with an assembled ohmmeter based on the calibration curve.

5. Learn how to calibrate ammeters and voltmeters

Second, the experimental principle

Common magneto-electric galvanometers are mainly composed of a rotatable coil wound by a fine enameled wire placed in a permanent magnetic field, a hairspring for generating a mechanical counter-torque, a pointer for indicating, and a permanent magnet. When the current passes through the coil, the current-carrying coil generates a magnetic moment M in the magnetic field, causing the coil to rotate, thereby causing the pointer to deflect. The magnitude of the deflection angle of the coil is proportional to the magnitude of the current passed, so the current value can be directly indicated by the deflection of the pointer.

1. The maximum current allowed by the ammeter is called the range of the ammeter. It is expressed by Ig. The coil of the ammeter has a certain internal resistance, which is represented by Rg. Ig and Rg are two important parameters indicating the characteristics of the ammeter.

Common methods for measuring internal resistance Rg are:

1) The half current method is also called the median method.

The measurement principle diagram is shown in Figure 1. When the galvanometer to be tested is connected in the circuit, the current meter is fully biased, and then the hexameter is connected in parallel with the galvanometer as a shunt resistor. Changing the resistance value changes the degree of shunting, when the galvanometer indicator indicates the intermediate value, and the standard The table reading (total current intensity) remains the same, which can be achieved by adjusting the supply voltage and RW. Obviously, the shunt resistance value is equal to the internal resistance of the ammeter.

figure 1

figure 2

2) Alternative method

The measurement principle diagram is shown in Figure 2. When the galvanometer is connected to the circuit, replace it with a decimal resistor box and change the resistance value. When the voltage in the circuit is constant, and the current in the circuit (standard meter reading) remains unchanged, the resistor The resistance value of the box is the internal resistance of the galvanometer to be tested.

The alternative method is a widely used measurement method with high measurement accuracy.

2, modified into a large range of ammeter

According to the parallel law of resistance, if a resistor R2 with appropriate resistance is connected in parallel at both ends of the meter head, as shown in FIG. 3, the part of the current that the meter can not bear can be shunted through R2. The whole consisting of the meter head and the shunt resistor R2 (the portion enclosed by the dotted line in the figure) is the modified ammeter. If you need to expand the range by n times, it is not difficult to draw

R2=Rg/(n-1) 1

Figure 3 is a schematic diagram of the current meter after expansion. When measuring current with an ammeter, the ammeter should be connected in series with the circuit under test, so the ammeter should be required to have a small internal resistance. In addition, a multi-range ammeter can be fabricated by connecting shunt resistors with different resistance values ​​on the meter head.

image 3

Figure 4

3, modified into a voltmeter

Generally, the head can withstand a small voltage and cannot be used to measure a large voltage. In order to measure a large voltage, a suitable resistance RM can be connected in series to the meter head, as shown in FIG. 4, so that the portion of the voltage that cannot be tolerated on the meter head falls on the resistor RM. The whole consisting of the meter head and the series resistor RM is a voltmeter, and the series resistor RM is called an extension resistor. By selecting RMs of different sizes, you can get voltmeters of different ranges. The extended resistance value can be obtained from Figure 4:

2

The actual voltmeter principle after the extended range is shown in Figure 4.

When measuring voltage with a voltmeter, the voltmeter is always connected in parallel to the circuit under test. In order not to change the working state in the circuit due to the parallel voltmeter, the voltmeter should have a high internal resistance.

4, modified mA meter is ohmmeter

The meter used to measure the size of the resistor is called an ohmmeter. According to the different ways of zero adjustment, it can be divided into two types: series partial pressure type and parallel split type. The principle circuit is shown in Figure 5.

In the figure, E is the power supply, R3 is the current limiting resistor, RW is the "zero" potentiometer, Rx is the measured resistance, and Rg is the equivalent internal resistance of the meter. In Figure (b), RG and RW together form a shunt resistor.

Before using the ohmmeter, first adjust the “zero” point, that is, short circuit between a and b, (equivalent to RX=0), adjust the resistance of RW, so that the head pointer just deflects to fullness. It can be seen that the zero point of the ohmmeter is just at the full scale (ie, the limit) of the meter scale, which is opposite to the zero point of the ammeter and voltmeter.

In Figure (a), when the a and b terminals are connected to the measured resistance Rx, the current in the circuit is

= 3 \* GB3 3

(a) Series voltage division type (b) Parallel split type

Figure 5 ohmmeter schematic

Rg, RW, and R3 are constants for a given header and line. It can be seen that when the voltage E of the power supply terminal remains unchanged, the measured resistance and the current value have a one-to-one correspondence. That is, when different resistors are connected, the meter will have different deflection readings. The larger the Rx, the smaller the current I. Short circuit a, b both ends, that is, when Rx=0

= 4 \* GB3 4

At this time, the pointer is fully biased when Rx=Rg+RW+R3

= 5 \* GB3 5

At this time, the pointer is in the middle of the header, and the corresponding resistance is the median resistance. Obviously R = Rg + RW + R3.

When Rx = ∞ (equivalent to a, b open), I = 0, that is, the mechanical zero of the pointer at the head.

Therefore, the scale of the ohmmeter is the reverse scale, and the scale is not uniform. The larger the resistance R is, the denser the interval is. If the scale of the meter is pre-scaled to a known resistance value, the current meter can be used to directly measure the resistance.

The parallel shunt ohmmeter uses the split flow of the meter to perform zero adjustment. The specific parameters can be designed by themselves.

The end voltage of the battery will change during the use of the ohmmeter, and the internal resistance Rg of the meter and the current limiting resistor R3 are constant, so the RW is required to change with the change of E to meet the requirement of “zero”. The design uses an adjustable power supply to simulate the change of the battery voltage, and the range can be 1.3~1.6V.

Third, the experimental instrument

1. GT4508 electric meter modification and calibration experiment instrument 1 set

2, ZX21 resistance box (optional) 1 set

Fourth, the experimental content

See Appendix for the use of the GT4508 meter modification and calibration tester.

The instrument should be mechanically zeroed before the experiment.

1. Measure the internal resistance of the meter head by the median method or the substitute method, and connect according to Figure 1 or Figure 2. Rg= Ω

2. Convert a meter with a range of 1 mA into an ammeter of 5 mA range

1) Calculate the shunt resistance value according to Equation 1, first adjust the power supply to the minimum, RW to the middle position, and then wire according to Figure 3.

2) Slowly adjust the power supply, increase the voltage, and make the modified meter to the full scale (can be adjusted with the RW rheostat), then record the standard meter reading. Note: RW is used as a current limiting resistor and the resistance should not be adjusted to the minimum value. Then adjust the power supply voltage so that the reading table gradually reduces the reading to 1 point every 1 mA (1/5 of full scale); (set the standard ammeter selection switch to 20 mA range) and then adjust the power supply voltage to gradually increase the original interval. The table readings to full scale, each time the corresponding reading of the standard table is recorded in the table below.

Table 1

Retrofit meter reading (mA)

Standard meter reading (mA)

Indication error ΔI (mA)

When decreasing

When decreasing

When decreasing

1

2

3

4

5

3) The reading of the modified table is plotted on the abscissa. The average of the two readings from large to small and from small to large is the ordinate. The calibration curve of the ammeter is made on the coordinate paper, and the maximum error is based on the two tables. The value determines the accuracy level of the modification table.

4) Repeat the above steps to convert the 1 mA meter to a 10 mA meter, which can be measured every 2 mA. (optional).

5) Connect the RG and the header on the panel in series as a new header, re-measure a set of data, and compare the similarities and differences of the expansion resistors (optional).

3. Convert a meter with a range of 1 mA into a 1.5V range voltmeter

1) Calculate the resistance of the extended resistance RM according to Equation 2, and experiment with R1 and R2.

2) Connect the calibration circuit as shown in Figure 4. A digital voltmeter with a 2V range is used as a standard meter to calibrate the modified voltmeter.

3) Adjust the power supply voltage so that the conversion meter pointer points to full scale (1.5V) and write down the standard meter reading. Then gradually reduce the modified reading to 0.3 point every 0.3V, and then gradually increase to the full scale according to the original interval, each time the corresponding reading of the standard table is recorded in the following table:

4) The reading of the modified table is plotted on the abscissa. The average of the two readings from large to small and from small to large is the ordinate. The calibration curve of the voltmeter is made on the coordinate paper, and the maximum error is based on the two tables. The value determines the accuracy level of the conversion table.

Table 2

Modified table reading (V)

Standard table reading (V)

Indication error ΔU(V)

When decreasing

When decreasing

When decreasing

0.3

0.6

0.9

1.2

1.5

5) Repeat the above steps to change the 1 mA meter to a 5V meter, which can be measured every 1V. (optional).

4, modified ohmmeter and calibration surface scale

1) According to the header parameters Ig and Rg and the power supply voltage E, choose Rw to be 470Ω and R3 to be 1KΩ.

2) Connect as shown in Figure 5(a). Connect the R1 and R2 resistor boxes (in this case, the measured resistance Rx) to the a and b terminals of the ohmmeter, and adjust R1 and R2 so that R = R1 + R2 = 1500 Ω

3) Adjust the power supply E=1.5V, adjust Rw to make the modification header indication zero.

4) Take the resistance of the resistor box as a specific set of values ​​Rxi and read the corresponding number of deflections di. A dial of the modified ohmmeter is drawn using the resulting readings Rxi, di. as shown in Table 3:

Table 3 E = V, R = Ω

RxI (Ω)

R

2R

3R

4R

5R

Deflection grid number (di)

5) Connect the wiring according to Figure 5(b) and design a parallel shunt ohmmeter. What are the similarities and differences between the test and the series voltage division ohmmeter. (optional)

Five, thinking questions

1. Is there any other way to measure the internal resistance of the galvanometer? Can I use Ohm's law to make measurements? Can the bridge be used for the measurement and the current through the galvanometer does not exceed Ig?

2. Design an ohmmeter with R=1500Ω. There are two current meters with a range of 1mA. The internal resistance is 250Ω and 100Ω respectively. Which one do you think is better?

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