Principle and use of analog multimeter

"Multimeter" is the abbreviation of multimeter, which is an indispensable tool in our electronic production. Multimeters can measure current, voltage, resistance, and some can also measure transistor amplification, frequency, capacitance value, logic potential, decibel value, etc. There are many types of multimeters. The most popular ones nowadays are mechanical pointer type and digital multimeter (see picture). They each have their own advantages. For electronic beginners, it is recommended to use an analog multimeter, because it is very helpful for us to familiarize ourselves with some electronic knowledge principles. Below we introduce some principles and usage methods of mechanical pointer multimeters. The basic principle of a multimeter 

is to use a sensitive magnetoelectric DC ammeter (microampere) as the meter head. When a small current passes through the meter, there will be a current indication. However, the meter head cannot pass large current, so some resistors must be connected in parallel or series with the meter head to shunt or reduce the voltage, so as to measure the current, voltage and resistance in the circuit. They are introduced below. 

·The principle of measuring DC current 

is shown in Figure 1a. By connecting an appropriate resistor (called a shunt resistor) in parallel to the meter head to shunt the current, the current range can be expanded. Changing the resistance of the shunt resistor can change the current measurement range. 

·The principle of measuring DC voltage 

is shown in Figure 1b. By connecting an appropriate resistor (called a multiplier resistor) in series with the meter to reduce the voltage, the voltage range can be expanded. Changing the resistance of the multiplier resistor can change the voltage measurement range.

·The principle of measuring AC voltage 

is shown in Figure 1c. Because the meter is a DC meter, when measuring AC, it is necessary to install a parallel or series half-wave rectifier circuit to rectify the AC into DC and then pass it through the meter. In this way You can measure the AC voltage based on the magnitude of the DC current. The method of extending the AC voltage range is similar to the DC voltage range. 

·The principle of measuring resistance 

is shown in Figure 1d. Connect appropriate resistors in parallel and series on the meter head, and connect a battery in series at the same time, so that the current passes through the resistance to be measured. According to the size of the current, the resistance value can be measured. Changing the resistance of the shunt resistor can change the range of the resistance. Measuring resistance: - First put the meter sticks together for a short circuit to deflect the pointer to the right, and then adjust the "Ω" zero-adjustment knob so that the pointer points exactly to 0. Then touch the two meter rods to both ends of the resistance (or circuit) being measured, read the pointer's reading on the ohm scale (the first line), and then multiply it by the number on the scale to get the resistance of the resistance being measured. . For example, use the R*100 block to measure resistance, and the pointer points to 80, then the measured resistance value is 80*100=8K. Since the readings on the left side of the "Ω" scale line are dense and difficult to read accurately, the appropriate ohm level should be selected when measuring. Make the pointer in the middle or right of the scale line so that the reading is clearer and more accurate. Every time you change gears, you should short-circuit the two meter rods again and readjust the pointer to zero to achieve accurate measurement.

How to use a multimeter 

The dial of a multimeter (take model 105 as an example) is as shown on the right. Use the knob of the switch to change the measurement items and measurement range. The mechanical zero adjustment knob is used to keep the pointer at the left zero position at rest. The "Ω" zero adjustment knob is used to align the pointer with the right zero position when measuring resistance to ensure accurate measurement values.