How to use a multimeter_How to use a multimeter diagram

A multimeter can measure a variety of electrical quantities. Although its accuracy is not high, it is easy to use and carry, and is particularly suitable for checking circuits and repairing electrical equipment. There are two types of multimeters: magnetic and digital.

1. Magnetoelectric multimeter

The magnetoelectric multimeter consists of a magnetoelectric microammeter, several shunts and voltage multipliers, semiconductor diodes and switching switches, and can be used to measure DC current, DC voltage, AC voltage and resistance. Figure 1 shows the panel diagram of the commonly used MF-30 multimeter. The various measurement circuits are described as follows:

(1) Measurement of DC current

The principle circuit diagram for measuring DC current is shown in Figure 2. The measured current enters and exits from the "+" and "-" terminals. RA1~RA5 are the resistances of the shunts, and they are connected to the microammeter to form a closed circuit. Changing the position of the switch changes the resistance of the shunt, and then changes the current range. For example, when it is set to 50mA, the shunt resistance is RA1+RA2, and the others are connected in series with the microammeter. The larger the range, the smaller the shunt resistance. R in the figure is the DC adjustment potentiometer.

(2) Measurement of DC voltage

The principle circuit for measuring DC voltage is shown in Figure 3. The voltage to be measured is added to the "+" and "-" terminals. RV1, RV2, ... are the voltage multiplier resistors. The larger the range, the larger the voltage multiplier resistor.

Figure 2 Principle circuit for measuring DC current	Figure 3 Principle circuit for measuring DC voltage

The higher the internal resistance of the voltmeter, the smaller the current drawn from the measured circuit, and the smaller the impact on the measured circuit. We use the sensitivity of the meter, that is, the total internal resistance of the meter divided by the voltmeter range to express this characteristic. The total internal resistance of the MF-30 multimeter at the DC voltage 25V range is 500KΩ, so the sensitivity of this range is .

(3) Measurement of AC voltage

The principle circuit for measuring AC voltage is shown in Figure 8.11. The magnetoelectric meter can only measure DC. If AC is to be measured, it must be equipped with a rectifier element, namely the semiconductor diodes D1 and D2 in the figure. The diode value allows current to pass in one direction, but not in the opposite direction. The measured AC voltage is also added to the "+" and "-" terminals. In the half-positive cycle, suppose the current flows in from the "+" terminal, through the diode D1, and some current flows out through the microammeter. In the negative half cycle, the current flows directly from the "+" terminal through D2. It can be seen that the current passing through the microammeter is the half-wave current, and the reading should be the average value of the current. For this reason, there is an AC adjustment potentiometer (the 600Ω resistor in the figure) in the meter to change the dial scale; therefore, the indicated reading is converted into the effective value of the sinusoidal voltage. As for the change of the range, it is the same as when measuring DC voltage. , ,... are the resistances of the voltage multiplier.

The sensitivity of the AC voltage range of a multimeter is usually lower than that of the DC voltage range. The sensitivity of the AC voltage range of the MF-30 multimeter is 5kΩ/V.

Generally, multimeters are only suitable for measuring AC voltages with a frequency of 45 to 100 Hz.

(4) Resistance measurement

The principle circuit for measuring resistance is shown in Figure 4. When measuring resistance, the battery should be connected, and the resistance to be measured is also connected to the "+" and "-" terminals. The smaller the resistance to be measured, the greater the current, and therefore the greater the deflection angle of the pointer. Before measuring, the "+" and "-" terminals should be short-circuited to see if the pointer deflects to the maximum and points to zero (the rightmost position of the scale). Otherwise, the zero ohm adjustment potentiometer (1.7KΩ resistor in the figure) should be turned for calibration.

Figure 4 Principle circuit for measuring AC voltage	Figure 5 Principle circuit for measuring resistance

When using a multimeter, pay attention to the position and range of the switch. Never measure resistance on a live circuit. After use, turn the switch to the high voltage position.

In addition, it can be seen from Figure 5 that the "+" terminal on the panel is connected to the negative pole of the battery, while the "-" terminal is connected to the positive pole of the battery.

2. Digital multimeter

Today, we take the DT-830 digital multimeter as an example to illustrate its measurement design and application method.

1. Measurement plan

(1) The DC voltage is divided into five levels: 200mV, 2V, 20V, 200V, +1000V. The input resistance is +10M .

(2) The AC voltage is divided into five levels: 200mV, 2V, 20V, 200V, 750V. The input impedance is 10M . The frequency is planned to be 40~500Hz.

(3) The DC current is divided into five levels: 200 , 2mA, 20mA, 200mA, and 10A.

(4) The AC current is divided into five levels: 200 , 2mA, 20mA, 200mA, and 10A.

(5) Resistance is divided into six levels: 200 , 2k , 20k , 200k , 2M , 20M .

In addition, the conductive function of semiconductor diodes can be checked, the current expansion coefficient hFE of transistors can be measured, and the on-off of circuits can be checked.

2. Panel explanation

Figure 6 shows the panel diagram of the DT-830 digital multimeter.

(1) Display: Displays four digits. The highest digit can only display 1 or no digit, which is considered half a digit, so it is called three and a half digits (3 digits). The maximum indication value is 1999 or -1999. When the measured value exceeds the maximum indication value, "1" or "-1" is displayed.

(2) Power switch: Turn the power switch to the "ON" position when in use; turn it to the "OFF" position after use.

(3) Switch: used to select function and range. Select the corresponding function position according to the quantity being measured (voltage, current, resistance, etc.); select the appropriate range according to the size being measured.

(4) Input socket: Insert the black test pen into the "COM" socket. There are three ways to insert the red test pen: insert it into the "V.Ω" socket when measuring voltage and resistance; insert it into the "mA" socket when measuring current less than 200mA; insert it into the 10A socket when measuring current greater than 200mA.

The sampling time of the DT-830 digital multimeter is 0.4 s and the power supply is DC 9V.