The use and principle of pointer multimeter
"Multimeter" is short for multimeter, which is an essential tool in our electronic production. Multimeter can measure current, voltage, resistance, some can also measure the amplification of transistors, frequency, capacitance, logic potential, decibel value. There are many kinds of multimeter, now the most popular mechanical pointer type and digital multimeter (see figure). They each have their own advantages. For electronic beginners, it is recommended to use the pointer multimeter, because it is very helpful for us to familiarise ourselves with some electronic knowledge principles. Here we introduce some mechanical pointer multimeter principle and use.
The basic principle of multimeter
The basic principle of the multimeter is to use a sensitive magneto-electric DC ammeter (microampere meter) to make the meter head. When a tiny current passes through the head, a current is indicated. However, the head cannot pass large currents, so some resistors must be connected in parallel and series on the head to shunt or step down the voltage, thus measuring the current, voltage and resistance in the circuit. They are described below.
DC Current Principle
In the meter head in parallel with an appropriate resistance (called shunt resistance) for shunt, you can extend the current range. By changing the resistance value of the shunt resistor, the current measurement range can be changed.
Principle of DC Voltage Measurement
Connecting an appropriate resistor (called doubling resistor) in series on the meter head for voltage reduction can extend the voltage range. By changing the resistance value of the multiplier resistor, the voltage measurement range can be changed.
Principle of AC Voltage Measurement
Because the meter head is a DC meter, so when measuring AC, you need to add a parallel or series half-wave rectifier circuit to rectify the AC into DC and then through the meter head, so that you can measure the AC voltage according to the size of the DC current. The method of extending the AC voltage range is similar to the DC voltage range.
Principle of Resistance Measurement
Appropriate resistors are connected in parallel and series on the meter head, and a battery is connected in series, so that the current passes through the measured resistor, and the resistance value can be measured according to the size of the current. By changing the resistance value of the shunt resistor, the range of the resistance can be changed.
Use of multimeter
The multimeter (Model 105, for example) uses the knob of the changeover switch to change the measurement item and the measurement range. The mechanical zero knob is used to keep the pointer at rest in the left zero position. The "Ω" zero knob is used to align the pointer to the right zero position when measuring resistance to ensure accurate measurement values.
The measurement range of the multimeter is as follows:
-DC voltage: 5 grades - 0-6V; 0-30V; 0-150V; 0-300V; 0-600V.
-AC voltage: 5 steps - 0-6V; 0-30V; 0-150V; 0-300V; 0-600V.
-DC current: 3 levels - 0-3mA; 0-30mA; 0-300mA.
-Resistance: 5 steps-R*1; R*10; R*100; R*1K; R*10K.
