Use and principle of pointer multimeter

"Multimeter" is the abbreviation of multimeter, which is an indispensable tool in our electronic production. Multimeter can measure current, voltage, resistance, and some can also measure the magnification of transistors, frequency, capacitance value, logic potential, decibel value, etc. There are many kinds of multimeters, and the most popular ones are mechanical pointer type and digital multimeter (see figure). They each have their own advantages. For electronic beginners, it is recommended to use pointer multimeter, because it is very helpful for us to be familiar with some electronic knowledge principles. Here we introduce some principles and usage methods of mechanical pointer multimeter.

Basic principle of multimeter

The basic principle of multimeter is to use a sensitive magnetoelectric DC ammeter (microammeter) as the meter head. When a small current passes through the meter head, there will be a current indication. But the meter head cannot pass large current, so some resistors must be connected in parallel and series to the meter head for shunting or voltage reduction, so as to measure the current, voltage and resistance in the circuit. The following are introduced separately.

·Principle of measuring DC current

As shown in Figure 1a, a suitable resistor (called shunt resistor) is connected in parallel to the meter head for shunting, so as to expand the current range. Changing the resistance value of the shunt resistor can change the current measurement range.

·Principle of measuring DC voltage

As shown in Figure 1b, connecting an appropriate resistor (called a multiplier resistor) in series with the meter head to reduce the voltage can expand the voltage range. Changing the resistance value of the multiplier resistor can change the voltage measurement range.

·Principle of measuring AC voltage

As shown in Figure 1c, because the meter head is a DC meter, when measuring AC, a parallel and series half-wave rectifier circuit needs to be installed to rectify the AC into DC and then pass it through the meter head. In this way, the AC voltage can be measured according to the size of the DC. The method of expanding the AC voltage range is similar to the DC voltage range.

·Principle of measuring resistance

As shown in Figure 1d, connect appropriate resistors in parallel and series to the meter head, and connect a battery in series at the same time, so that the current passes through the measured resistor. According to the size of the current, the resistance value can be measured. Changing the resistance value of the shunt resistor can change the resistance range.

Use of multimeter

The dial of the multimeter (taking the 105 type as an example) is shown in the figure on the right. The measurement items and measurement range are changed by turning the knob of the conversion switch. The mechanical zero adjustment knob is used to keep the pointer at the left zero position when it is stationary. The "Ω" zero adjustment knob is used to align the pointer with the right zero position when measuring resistance to ensure accurate measurement values.

The measurement range of the multimeter is as follows:

· DC voltage: divided into 5 levels - 0-6V; 0-30V; 0-150V; 0-300V; 0-600V. ·
AC voltage: divided into 5 levels - 0-6V; 0-30V; 0-150V; 0-300V; 0-600V
· DC current: divided into 3 levels - 0-3mA; 0-30mA; 0-300mA.
· Resistance: divided into 5 levels - R*1; R*10; R*100; R*1K; R*10K

Measuring resistance: First, short-circuit the test rods together to make the pointer deflect to the right, and then adjust the "Ω" zero adjustment knob so that the pointer points to 0. Then touch the two test rods to the two ends of the resistor (or circuit) being measured, read the pointer's reading on the ohm scale (the first line), and multiply it by the number marked on the gear, which is the resistance value of the measured resistor. For example, if the resistance is measured with the R*100 gear, and the pointer points to 80, the measured resistance value is 80*100=8K. Since the readings on the left side of the "Ω" scale are dense and difficult to read, the appropriate ohm gear should be selected when measuring. Make the pointer in the middle or right part of the scale so that the reading is clearer and more accurate. Each time you change gears, you should short-circuit the two test rods again and readjust the pointer to zero to measure accurately. [page]

Measuring DC voltage:--First estimate the magnitude of the voltage to be measured, then turn the conversion switch to the appropriate V range, connect the positive meter rod to the "+" end of the voltage to be measured, and the negative meter rod to the "-" end of the voltage to be measured. Then read the magnitude of the voltage to be measured according to the range number of the gear and the number indicated by the pointer on the scale line (the second line) marked with the DC symbol "DC-". If you use the V300 volt gear to measure, you can directly read the indicated value of 0-300. If you use the V30 volt gear to measure, just remove a "0" from the number 300 on the scale line and regard it as 30, and then regard the numbers 200, 100, etc. as 20, 10 in turn, and you can directly read the pointer indicated value. For example, if you use the V6 volt gear to measure DC voltage, the pointer points to 15, and the measured voltage is 1.5 volts.

Measuring DC current:--First estimate the magnitude of the current to be measured, then turn the conversion switch to the appropriate mA range, and then connect the multimeter in series in the circuit, as shown in the figure. At the same time, observe the scale line marked with the DC symbol "DC". If the current range is selected in the 3mA range, then the number 300 on the surface scale line should be removed by two "0"s and regarded as 3, and 200 and 100 should be regarded as 2 and 1 respectively, so that the measured current value can be read. For example, when measuring DC current with the DC 3mA range, the pointer is at 100, and the current is 1mA.

Measuring AC voltage: --The method of measuring AC voltage is similar to that of measuring DC voltage. The difference is that there is no positive or negative difference between AC, so when measuring AC, the meter rod does not need to be divided into positive and negative. The reading method is the same as the reading method of measuring DC voltage mentioned above, except that the number should be based on the position of the pointer on the scale line marked with the AC symbol "AC".

Precautions for using a multimeter

The multimeter is a relatively precise instrument. If it is used improperly, it will not only cause inaccurate measurements but also be easily damaged. However, as long as we master the use and precautions of the multimeter and use it with caution, the multimeter will be durable. When using a multimeter, the following matters should be noted:

· Do not turn the wrong gear when measuring current and voltage. If you mistakenly use the resistance gear or current gear to measure voltage, it is very easy to burn out the meter. When the multimeter is not in use, it is best to turn the gear to the highest AC voltage to avoid damage due to improper use.

· When measuring DC voltage and DC current, pay attention to the "+" and "-" polarity and do not connect them incorrectly. If you find that the pointer is reversed, you should immediately replace the meter rod to avoid damaging the pointer and the meter head.

·If you don't know the size of the voltage or current being measured, you should use the highest gear first, and then select the appropriate gear to test, so as to avoid excessive deflection of the needle and damage to the meter head. The closer the selected gear is to the measured value, the more accurate the measured value will be.

·When measuring resistance, do not touch the bare ends of the component (or the metal part of the two test rods) with your hands, so as to avoid the human body resistance being connected in parallel with the resistance being measured, making the measurement result inaccurate.

·When measuring resistance, if the two test rods are short-circuited and the "zero ohm" knob is adjusted to the maximum, the pointer still cannot reach 0. This phenomenon is usually caused by insufficient battery voltage in the meter. New batteries should be replaced for accurate measurement.

·When the multimeter is not in use, do not turn it to the resistance gear, because there is a battery inside. If you are not careful, the two test rods may touch and short-circuit, which will not only consume the battery, but may even damage the meter head in serious cases. (end)