Principle and use of 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 (analog type) and digital multimeter. They each have their own advantages.

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Below we introduce some principles and usage methods of mechanical pointer multimeters.

The basic principle of mechanical pointer multimeter

The basic principle of a 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. However, the meter head cannot pass large currents, so some resistors must be connected in parallel or in series with 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, which can extend the current range. By changing the resistance value of the shunt resistor, the current measurement range can be changed.
· Principle of measuring DC voltage.
As shown in Figure 1b, a suitable resistor (called multiplier resistor) is connected in series with the meter head for voltage reduction, which can extend the voltage range. By changing the resistance value of the multiplier resistor, the voltage measurement range can be changed.
 

· Principle of measuring AC voltage.
As shown in Figure 1c, because the meter head is a DC meter, when measuring AC, it is necessary to install a parallel and series half-wave rectifier circuit 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. By changing the resistance value of the shunt resistor, the resistance range can be changed.

Note: Pointer type voltmeter and ammeter do not require batteries, but ohmmeter needs to be installed with batteries.

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) to be measured, read the pointer on the ohm scale (the first line), and then 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, the pointer points to 80, and 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 clear and accurate. Each time you change gears, you should short-circuit the two test rods again and readjust the pointer to zero position to measure accurately.

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 based on the range number and the number indicated by the pointer on the scale line (the second line) marked with the DC symbol "DC-".

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 in 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 above-mentioned DC voltage reading method, except that the number should be based on the position of the pointer on the scale line marked with the AC symbol "AC".

Note: When measuring DC voltage and DC current, pay attention to the "+" and "-" polarities and do not connect them incorrectly.

The Basic Principles of Digital Multimeter

 

The core of a digital meter is its A/D converter, which converts the measured analog signal into a digital signal for display on the LCD screen.

A digital meter has three conversion circuits: I/V conversion circuit (current conversion voltage circuit), R/V conversion circuit (resistance conversion voltage circuit), and C/V conversion circuit (capacitance conversion voltage circuit). In other words, no matter what signal is measured, it must always be converted into a DC voltage signal for A/D processing and display.

Measure resistance and convert the measured resistance value into a DC voltage signal for A/D processing and display.

Measure current and convert the measured current value into a DC voltage signal for A/D processing and display.

Measure capacitance and convert the measured capacitance into a DC voltage signal for A/D processing and display.

The digital ammeter is actually a voltmeter. It measures current by measuring the voltage on the internal sampling resistor. The sampling resistor is connected in series in the circuit you want to measure, and its resistance value varies according to the gear. So when measuring current, the sampling resistor will inevitably cause a voltage drop. The maximum voltage drop when FLUKE 17B measures current = 100*4000uA = 0.4V.
The schematic diagram of the digital multimeter is as follows

Note: Digital multimeters, whether measuring voltage, current or resistance, must be equipped with batteries, because the AD works and the LCD displays, and power must be supplied. Comparison between pointer multimeters and digital multimeters Pointer multimeters and digital multimeters each have their own advantages and disadvantages. 1. The pointer multimeter is an average value type instrument, which has an intuitive and vivid reading indication. (Generally, the reading value is closely related to the pointer swing angle, so it is very intuitive). 2. The digital multimeter is an instantaneous sampling type instrument. It uses 0.3 seconds to take a sample to display the measurement result. Sometimes the sampling results are only very similar, not exactly the same, which is not as convenient as the pointer type for reading the results. 3. Pointer multimeters generally do not have an amplifier inside, so the internal resistance is small. For example, the DC voltage sensitivity of the MF-10 type is 100 kilo-ohms/volt. The DC voltage sensitivity of the MF-500 type is 20 kilo-ohms/volt. 4. Since the digital multimeter uses an op amp circuit inside, the internal resistance can be made very large, often 1M ohms or more. (That is, a higher sensitivity can be obtained). This makes the impact on the measured circuit smaller and the measurement accuracy higher. 5. The pointer multimeter has a simple internal structure, so the cost is low, the functions are few, the maintenance is simple, and the over-current and over-voltage capabilities are strong. 6. The digital multimeter uses a variety of oscillation, amplification, frequency division protection and other circuits, so it has more functions. For example, it can measure temperature, frequency (in a lower range), capacitance, inductance, and act as a signal generator, etc.