Introduction of Voltmeter

Overview

Voltmeter is an instrument for measuring voltage, commonly used voltmeter – voltmeter. Symbol: V, there is a permanent magnet in the sensitive galvanometer, a coil composed of wires is connected in series between the two terminals of the galvanometer, the coil is placed in the magnetic field of the permanent magnet, and is connected to the pointer of the watch through the transmission device . Most voltmeters are divided into two ranges. The voltmeter has three terminals, one negative terminal and two positive terminals. The positive terminal of the voltmeter is connected to the positive terminal of the circuit, and the negative terminal is connected to the negative terminal of the circuit. The voltmeter must be connected in parallel with the electrical appliance under test. A voltmeter is a fairly large resistor, ideally considered an open circuit. The commonly used voltmeter ranges in junior high school laboratories are 0~3V and 0~15V.

Working principle

Traditional pointer voltmeters and ammeters are based on a principle that is the magnetic effect of current. The greater the current, the greater the magnetic force generated, which shows the greater the swing of the pointer on the voltmeter. There is a magnet and a wire coil in the voltmeter. After passing the current, the coil will generate a magnetic field. After the coil is energized Deflection will occur under the action of the magnet, which is the head part of the ammeter and the voltmeter.

Since the voltmeter needs to be connected in parallel with the measured resistance, if the sensitive ammeter is directly used as a voltmeter, the current in the meter will be too large and the meter will burn out. At this time, a large resistance needs to be connected in series with the internal circuit of the voltmeter. , After this transformation, when the voltmeter is connected in parallel in the circuit, most of the voltage applied to both ends of the meter is shared by this series resistance due to the function of the resistance, so the current passing through the meter is actually very small, so the It can be used normally.

The symbol of the DC voltmeter should add a “_” under V, and the symbol of the AC voltmeter should add a wavy line “~” under V.

Application

Used to measure the voltage value across the circuit or electrical appliance.

Classification

A mechanical indicating meter for measuring DC voltage and AC voltage. Divided into DC voltmeter and AC voltmeter.

The DC type mainly adopts the measurement mechanism of the magnetoelectricity meter and the electrostatic meter.

The AC type mainly adopts the measurement mechanism of rectifier type electricity meter, electromagnetic type electricity meter, electric type electricity meter and electrostatic type electricity meter.

The digital voltmeter is an instrument that converts the measured voltage value into a digital form with an analog-to-digital converter and is expressed in digital form. If the voltage is abnormal due to reasons such as lightning, use an external noise absorbing circuit such as a power line filter or non-linear resistor.

Selection guide

The measuring mechanism of ammeter and voltmeter is basically the same, but the connection in the measuring circuit is different. Therefore, the following points should be noted when selecting and using ammeters and voltmeters.

⒈ Type selection. When the measured is DC, the DC meter should be selected, that is, the meter of the magnetoelectric system measuring mechanism. When the measured AC, should pay attention to its waveform and frequency. If it is a sine wave, it can be converted to other values ​​(such as maximum value, average value, etc.) only by measuring the effective value, and any kind of AC meter can be used; if it is a non-sine wave, it should distinguish what needs to be measured For the rms value, the instrument of the magnetic system or the ferromagnetic electric system can be selected, and the average value of the instrument of the rectifier system can be selected. The instrument of the electric system measuring mechanism is often used for the precise measurement of alternating current and voltage.

⒉ The choice of accuracy. The higher the accuracy of the instrument, the more expensive the price and the more difficult the maintenance. Moreover, if the other conditions are not matched properly, the instrument with high accuracy level may not be able to obtain accurate measurement results. Therefore, in the case of selecting a low-accuracy instrument to meet the measurement requirements, do not choose a high-accuracy instrument. Usually 0.1 and 0.2 meters are used as standard meters; 0.5 and 1.0 meters are used for laboratory measurement; instruments below 1.5 are generally used for engineering measurement.

⒊ Range selection. In order to give full play to the role of the accuracy of the instrument, it is also necessary to reasonably select the limit of the instrument according to the size of the measured value. If the selection is improper, the measurement error will be very large. Generally, the indication of the instrument to be measured is greater than 1/2~2/3 of the maximum range of the instrument, but cannot exceed its maximum range.

⒋ The choice of internal resistance. When selecting a meter, the internal resistance of the meter should also be selected according to the size of the measured impedance, otherwise it will bring about a large measurement error. Because the size of the internal resistance reflects the power consumption of the meter itself, when measuring current, an ammeter with the smallest internal resistance should be used; when measuring voltage, a voltmeter with the largest internal resistance should be used.

Maintenance

1. Strictly follow the requirements of the manual, and store and use it within the allowable range of temperature, humidity, dust, vibration, electromagnetic field and other conditions.

2. The instrument that has been stored for a long time should be checked regularly and the moisture should be removed.

3. Instruments that have been used for a long time should be subject to necessary inspection and correction according to electrical measurement requirements.

4. Do not disassemble and debug the instrument at will, otherwise its sensitivity and accuracy will be affected.

5. For instruments with batteries installed in the meter, pay attention to check the discharge of the battery, and replace them in time to avoid the overflow of battery electrolyte and corrosion of the parts. For the meter that will not be used for a long time, the battery in the meter should be removed.

Matters needing attention

(1) When measuring, the voltmeter should be connected in parallel to the circuit under test.

(2) Since the voltmeter is connected in parallel with the load, the internal resistance Rv is required to be much larger than the load resistance RL.

(3) When measuring DC, first connect the “-” button of the voltmeter to the low-potential end of the circuit under test, and then connect the “+” end button to the high-potential end of the circuit under test.

(4) For a multi-quantity voltmeter, when the quantity limit needs to be changed, the voltmeter should be disconnected from the circuit under test before changing the quantity limit.

Troubleshooting

The working principle of the digital voltmeter is more complicated, and it has many types, but the commonly used digital voltmeters (including digital multimeters) can basically be divided into time-coded DC digital voltmeters of ramp A/D converters and successive comparisons. There are two types of feedback-encoded DC digital voltmeters for A/D converters. Generally, there are the following maintenance procedures.

1. Qualitative test before revision

This is mainly through the “zero adjustment” and the “voltage calibration” of the machine after the startup is preheated to determine whether the logic function of the digital voltmeter is normal.

If the polarity of “+” and “-” can be changed during “zero adjustment”, or when the voltages of “+” and “-” are calibrated, only the displayed numbers are inaccurate, and even the voltage numbers displayed by either of the two are correct. , which indicates that the overall logic function of the digital voltmeter is normal.

Conversely, if zero adjustment is impossible or there is no voltage digital display, it indicates that the logic function of the whole machine is abnormal.

2. Measure the supply voltage

The inaccurate or unstable output voltage of various DC regulated power supplies inside the digital voltmeter, and the zener diodes (2DW7B, 2DW7C, etc.) that are used as the “reference voltage” source have no regulated output, which lead to the logic function of the digital voltmeter. One of the main causes of disorder. Therefore, when starting to repair the fault, you should first check whether the various DC voltage stabilized outputs and reference voltage sources inside the digital voltmeter are accurate and stable. If the problem is found and repaired, the fault can often be eliminated and the logic function of the digital voltmeter can be restored to normal.

3. Variable adjustable device

Semi-variable devices in the internal circuits of digital voltmeters, such as “reference voltage” source trimming rheostats, differential amplifier operating point trimming rheostats, and transistor regulated power supply voltage-regulating potentiometers, etc., because the sliding terminals of these semi-adjustable devices have poor contact, or Its wire-wound resistance is mildewed, and the display value of the digital voltmeter is often inaccurate, unstable, and cannot be measured. Sometimes a slight change in the related semi-adjustable device can often eliminate the problem of poor contact and restore the digital voltmeter to normal.

It must be pointed out that due to the parasitic oscillation of the transistor regulated power supply itself, it often causes the digital voltmeter to display an unstable failure phenomenon. Therefore, under the condition of not affecting the logic function of the whole machine, the voltage regulating potentiometer can also be slightly changed to eliminate parasitic oscillation.

4. Observe the working waveform

For the faulty digital voltmeter, use a suitable electronic oscilloscope to observe the signal waveform output by the integrator, the signal waveform output by the clock pulse generator, the working waveform of the ring step trigger circuit and the ripple voltage waveform of the regulated power supply, etc. It is very helpful for finding the fault location and analyzing the cause of the fault.

5. Study circuit principle

If no problem is found through the above maintenance procedures, it is necessary to further study the circuit principle of the digital voltmeter, that is, to understand the working principle and logical relationship of each component circuit, so as to analyze the circuit parts that may cause faults, and plan inspections A test plan for the cause of the failure.

6. Develop a test plan

A digital voltmeter is a precision electronic measuring instrument with complex circuit structure and logic functions. Therefore, on the basis of in-depth study of its working principle of the whole machine, a test plan can be drawn up according to the preliminary analysis of the possible failure causes to effectively determine the fault location and find out the damaged and variable value devices, so as to achieve the purpose of repairing the instrument.

7. Test and update the device

There are many devices used in the circuit of the digital voltmeter, among which the Zener as the reference voltage source, that is, the standard Zener diode, such as 2DW7B, 2DW7C, etc., the reference amplifier and the integrated operational amplifier in the integrator circuit, the ring step trigger The switching diodes in the circuit, as well as the integrated blocks or switching transistors in the registered bistable circuit, are often damaged and changed in value. Therefore, the device in question must be tested, and the device that cannot be tested or that has been tested but still has problems must be updated so that the fault can be quickly eliminated.