Mechanical Pointer Multimeter Measuring Theory
1. Basic working principle
"Multimeter" is the abbreviation of multimeter, and it is an indispensable tool in our production. The multimeter can measure current, voltage, resistance, and some can also measure magnification, frequency, capacity, logic potential, decibel value, etc. There are many kinds of multimeters, and now there are mechanical pointer and digital multimeters. They each have their own advantages and disadvantages; for electronic beginners, it is recommended to use a pointer multimeter, because it is very helpful for us to familiarize ourselves with some electronic knowledge principles. The following mainly introduces the measurement principle of the mechanical pointer multimeter.
The basic principle of this type of multimeter is to use a sensitive magnetoelectric DC ammeter (microampere meter) as the head. When a small current passes through the meter head, there will be a current indication. However, the meter head cannot pass a large current, so some resistors must be connected in parallel or in series on the meter head to shunt or lower the voltage, so as to measure the current, voltage and resistance in the circuit. Introduce them separately below.
1. The principle of measuring DC current.
Connect an appropriate resistor (called a shunt resistor) in parallel on the meter head for shunting, and the current range can be expanded. By changing the resistance value of the shunt resistor, the current measurement range can be changed.
2. Measuring DC voltage principle.
The voltage range can be extended by connecting an appropriate resistor (called multiplier resistor) in series on the meter head to lower the voltage. By changing the resistance value of the multiplying resistor, the measuring range of the voltage can be changed.
3. Measuring AC voltage principle.
Because the meter is a DC meter, when measuring AC, it is necessary to install a parallel and serial half-wave rectifier circuit to rectify the AC into DC and then pass through the meter, so that the AC voltage can be measured according to the magnitude of the DC. The method of extending the AC voltage range is similar to the DC voltage range.
4. Measuring resistance principle.
Connect appropriate resistors in parallel and in series on the meter head, and connect a battery in series at the same time, so that the current passes through the measured resistance, and the resistance value can be measured according to the magnitude of the current. Changing the resistance value of the shunt resistor can change the range of the resistance.
Second, the use of multimeter
The dial of the multimeter (take the 105 model as an example) is shown on the right. Change the measurement item and measurement range by the knob of the changeover switch. The mechanical zero adjustment knob is used to keep the pointer at zero position at rest. The "Ω" zero adjustment knob is used to align the pointer to the right zero position when measuring resistance, so as to ensure the accuracy of the measured value.
Measuring the resistance:--First short-circuit the watch sticks together to make the pointer deflect to the right, then adjust the "Ω" zero adjustment knob so that the pointer just points to 0. Then touch the two test rods to both ends of the measured resistance (or circuit), read the reading of the pointer on the ohm scale line (line), and multiply it by the number of the scale to get the resistance value of the measured resistance. For example, use R*100 gear to measure resistance, and the pointer points at 80, then the measured resistance value is 80*100=8K. Since the readings on the left side of the "Ω" scale line are dense, it is difficult to see accurately, so the appropriate ohm file should be selected when measuring. Make the pointer in the middle or right of the scale line, so that the reading is more clear and accurate. Every time you change gears, you should re-short the two watch rods and re-adjust the pointer to zero to measure accurately.
Measuring DC voltage: --First estimate the voltage to be measured, then turn the switch to the appropriate V range, connect the positive gauge to the "+" terminal of the measured voltage, and the negative gauge to the "-" terminal of the measured voltage . Then read the measured voltage according to the number of the range and the number pointed by the pointer on the scale line marked "DC-" (the second line). If you use the V300 volt range to measure, you can directly read the indicated value from 0-300. If you use the V30 volt range to measure, you only need to remove a "0" from the number 300 on the scale line, and regard it as 30, and then treat the numbers 200, 100, etc. as 20, 10 to read the pointer indication value directly. For example, use the V6 volt gear to measure the DC voltage, and the pointer points at 15, then the measured voltage is 1.5 volts.
Measuring DC current: --Estimate the magnitude of the measured current first, then turn the switch to the appropriate mA range, and then connect the multimeter in series with 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 at the 3mA range, at this time, the number 300 on the surface scale line should be removed from the two "0"s, and it will be regarded as 3, and then 200, 100 is regarded as 2, 1, so that the value of the measured current can be read. For example, use the DC 3mA range to measure the DC current, if the pointer is at 100, the current is 1mA.
Measuring AC voltage: - The method of measuring AC voltage is similar to measuring DC voltage, the difference is that because AC has no positive and negative points, when measuring AC, the meter stick does not need to be divided into positive and negative. The reading method is the same as the above-mentioned method for measuring DC voltage, except that the number should be read on the pointer position on the scale marked with the AC symbol "AC".
