Measurement principle of mechanical pointer type multimeter
Basic working principle
The "multimeter" is the abbreviation for a multimeter, which is an essential tool in our production. A multimeter can measure current, voltage, resistance, and some can also measure magnification, frequency, capacity, logical potential, decibel value, etc. There are many types of multimeter, and now there are mechanical pointer type and digital multimeter. They each have their own advantages and disadvantages; For beginners in electronics, it is recommended to use a pointer type multimeter as it is helpful for us to familiarize ourselves with some electronic knowledge principles. The following mainly introduces the measurement principle of a mechanical pointer type multimeter.
The basic principle of this type of multimeter is to use a sensitive magnetoelectric DC ammeter (microampere meter) 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 through large currents, so it is necessary to shunt or reduce voltage by connecting some resistors in parallel or series on the meter head, in order to measure the current, voltage, and resistance in the circuit. Below are introductions.
1. Principle of measuring DC current.
By connecting an appropriate resistor (called a shunt resistor) in parallel on the meter head for shunt, the current range can be extended. By changing the resistance value of the shunt resistor, the current measurement range can be changed.
2. Principle of measuring DC voltage.
By connecting an appropriate resistor (called a multiplier resistor) in series on the meter head for voltage reduction, the voltage range can be extended. By changing the resistance value of the multiplier resistor, the measurement range of voltage can be changed.
3. Principle of measuring AC voltage.
Because the meter head is a DC meter, it is necessary to install a parallel or series half wave rectification circuit when measuring AC. The AC is rectified into DC and then passed through the meter head, so that the AC voltage can be measured based on the magnitude of DC current. The method of expanding the AC voltage range is similar to the DC voltage range.
4. Principle of measuring resistance.
Connect appropriate resistors in parallel or series on the meter head, and connect a battery in series to allow the current to pass through the measured resistance. Based on the magnitude of the current, the resistance value can be measured. By changing the resistance value of the shunt resistor, the range of the resistor can be changed.
Use of a multimeter
The dial of a multimeter (using the 105 model as an example) is shown in the right figure. Change the measurement items and measurement range through the knob of the conversion switch. The mechanical zero adjustment knob is used to keep the pointer stationary and in the left zero position. 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 a 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 three levels -0-3mA; 0-30mA; 0-300mA.
Resistance: divided into 5 levels - R * 1; R * 10; R * 100; R * 1K; R * 10K
Measurement of resistance: First, short circuit the meter bars together to deflect the pointer to the right, and then adjust the "Ω" zero adjustment knob to make the pointer exactly point to 0. Then, contact the two measuring rods at both ends of the measured resistance (or circuit), read the reading of the pointer on the ohm scale line (bar line), and multiply it by the number on the scale to obtain the resistance value of the measured resistance. For example, when measuring resistance in R * 100 gear and the pointer is at 80, the measured resistance value is 80 * 100=8K. Due to the dense reading on the left side of the "Ω" scale, it is difficult to accurately determine, so an appropriate ohm range should be selected for measurement. Place the pointer in the middle or right of the scale line, so that the reading is clearer and more accurate. Every time you shift gears, you should short circuit the two gauge rods again and adjust the pointer to the zero position to accurately measure.
Measure DC voltage: First estimate the magnitude of the measured voltage, then turn the conversion switch to the appropriate V range, connect the positive meter rod to the "+" terminal of the measured voltage, and the negative meter rod to the "-" terminal of the measured voltage. Then read the magnitude of the measured voltage based on the range number and the number indicated by the pointer on the scale line (second line) of the standard DC symbol "DC -". If measuring with a V300 volt range, the indicated value of 0-300 can be directly read. If using the V30 volt range for measurement, simply remove a "0" from the number 300 on the scale line and consider it as 30. Then, sequentially consider the numbers 200, 100, and so on as 20, 10 to directly read the pointer indication value. For example, when measuring DC voltage using the V6 volt range and the pointer is at 15, the measured voltage is 1.5 volts.
Measure DC current: First estimate the magnitude of the measured current, 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 at 3mA, the number 300 on the surface scale line should be removed from the two "0s" and regarded as 3. Then, 200 and 100 should be regarded as 2 and 1 in order to read the measured current value. For example, if the DC current is measured at 3mA level and the pointer is at 100, the current is 1mA.
Measuring AC voltage: The method of measuring AC voltage is similar to measuring DC voltage, but the difference is that AC does not have a positive or negative distinction, so when measuring AC, the meter rod does not need to be divided between positive and negative. The reading method is the same as the reading method for measuring DC voltage mentioned above, except that the numbers should refer to the pointer position on the scale line marked with the AC symbol "AC".
