How to use ammeter and voltmeter

Voltmeter Principle

First of all, we need to know that there is a magnet and a wire coil in the voltmeter. When current passes through, the coil will generate a magnetic field (it seems that this content is beyond what you have learned so far. It is something you will learn in the second semester of the second year of junior high school, but you must know about electromagnets). When the coil is energized, it will rotate under the action of the magnet. This is the head part of the ammeter and voltmeter.

The current that can pass through this meter is very small, and the voltage that can be withstood by both ends is also very small (definitely much less than 1V, maybe only a few tenths of a volt or even less). In order to measure the voltage in our actual circuit, we need to connect a relatively large resistor in series with this voltmeter to make a voltmeter. In this way, even if a relatively large voltage is applied to both ends, most of the voltage acts on the large resistor we added, and the voltage on the meter will be very small.

It can be seen that the voltmeter is an instrument with a large internal resistance, which should generally be greater than several thousand ohms.

The ammeter is made based on the effect of magnetic field force on a current-carrying conductor in a magnetic field.

There is a permanent magnet inside the ammeter, which generates a magnetic field between the poles. There is a coil in the magnetic field. There is a hairspring spring at each end of the coil. Each spring is connected to a terminal of the ammeter. A rotating shaft connects the spring and the coil. There is a pointer at the front end of the rotating shaft relative to the ammeter.

When current passes through, it passes through the magnetic field along the spring and the shaft. The current cuts the magnetic lines of flux, so the coil is deflected by the force of the magnetic field, causing the shaft and the pointer to deflect.

Since the magnitude of the magnetic field force increases with the increase of current, the magnitude of the current can be observed by the degree of deflection of the pointer.

This is called a magnetoelectric ammeter, the kind we usually use in laboratories.

The ammeter is connected in series with a large resistor. When measuring, it is connected in parallel between the two points to be measured. It will not change the characteristics of the original circuit. The value displayed by the ammeter is proportional to the voltage at the measured point.

The internal resistance Ro of the ammeter is very small and can be ignored, and the external resistance R is very large. According to Ohm's law, we get:

The internal resistance of an ideal ammeter is 0; the internal resistance of an ideal voltmeter is infinite.

I = U/(R + Ro) ≈ U/R

How to use ammeter and voltmeter

1. Choose the ammeter reasonably

(1) According to the accuracy requirements of the measured value, the accuracy of the ammeter should be reasonably selected. Generally speaking, the 0.1-0.2 level magnetoelectric ammeter is suitable for standard meter and precision measurement; the 0.5-1.5 level magnetoelectric ammeter is suitable for measurement in the laboratory; the 1.0-5.0 level magnetoelectric meter is suitable for electrical equipment operation monitoring and electrical equipment maintenance in industrial and mining enterprises.

(2) Select an ammeter with a corresponding range according to the current size. A range that is too large will reduce the measurement accuracy, and a range that is too small will damage the ammeter. In order to fully utilize the accuracy of the meter, the range of the meter should be selected based on the principle of using the last 1/4 of the scale as much as possible.

(3) Reasonably select the internal resistance of the ammeter. The smaller the internal resistance of the ammeter, the better.

2. Inspection before measurement

Before measuring, check whether the ammeter pointer is aligned with the "0" scale line. If not, adjust the "zero adjuster" to return the pointer to zero.

3. Connection between ammeter and the circuit under test

(1) When measuring, the ammeter should be connected in series to the low potential side of the circuit being measured.

(2) When measuring direct current, it is necessary to pay attention to the signs of the ammeter terminals. For a single-limited current meter, the measured current should flow into the ammeter from the terminal marked with "+" and out of the ammeter from the terminal marked with "-". For a multi-limited current meter, the terminal marked with "*" is the common terminal. If other terminals are marked with the "+" symbol, the measured current should flow into the "+" terminal and out of the "*" terminal. If other terminals are marked with the "-", the connection is exactly the opposite of the above.

4. Read correctly

When reading, the pointer should be stabilized before reading, and the line of sight should be kept perpendicular to the dial as much as possible. If the dial has a reflector, the pointer and its shadow in the mirror should be kept coincident to reduce the error.

5. Maintenance methods

(1) Since the overload capacity of the magnetoelectric ammeter is very small, you must pay attention to the polarity of the connecting circuit and the selection of the limit when using it.

(2) If the pointer is found to be deflected in the reverse direction or in the forward direction and exceeds the full scale line on the scale during measurement, the power should be turned off immediately and the measurement should be stopped. The measurement can be continued after the connection is correct or an ammeter with a larger measuring range is selected.

(3) When the measurement is completed, the power supply should be turned off first, then the ammeter should be removed from the measurement circuit and placed in a dry, ventilated and cool environment. For microammeters and milliammeters with high sensitivity and accuracy, the positive and negative terminals should be connected with wires to protect the measuring mechanism of the instrument.

The use and maintenance methods of voltmeters are similar to those of ammeters. The following points should also be noted:

(1) When measuring, the voltmeter should be connected in parallel with the circuit being measured.

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

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

(4) For multi-limit voltmeters, when the limit needs to be changed, the voltmeter should be disconnected from the circuit being measured before changing the limit.