Analysis on Working Principles of Multimeter for Measuring DC Current and DC Voltage
1. Circuit Principle for DC Current Measurement
The main component of an analog multimeter is a magnetoelectric ammeter, commonly referred to as the meter movement. However, a single meter movement can only measure currents lower than its sensitivity.
To extend the measuring‑current range, a shunt resistor shall be connected in parallel with the meter movement. Only a portion of the measured current flows through the meter movement, so the measurement range is expanded.
To guarantee specified accuracy when measuring currents of different magnitudes, ammeters are designed with multi‑range gears.
The closed‑loop tap‑type shunt circuit is the most widely adopted, as shown in the corresponding figure.
Resistors R1‑R5 are collectively called the total shunt resistor RS. For convenient adjustment and mass production, RS mostly adopts large integer‑kiloohm resistance values. A variable wire‑wound resistor R0 is connected in series with the meter movement. It provides compensation and easy adjustment when parameters of the meter movement change.
2. Circuit Principle for DC Voltage Measurement
According to Ohm's Law \(U=IR\), an ammeter with sensitivity I and internal resistance R can itself serve as a voltmeter with full‑scale range U. For example, a 100 μA ammeter with an internal resistance of 1.5 kΩ has a native voltage measurement range of 0.15 V, which is practically unusable. Its measurement range can be extended by connecting a resistor in series.
If an 8.5 kΩ resistor is connected in series, the range can be extended to 1 V, and the internal resistance of this voltmeter becomes 10 kΩ. This introduces the concept of DC‑voltage sensitivity. In this example, the voltmeter provides 10 kΩ internal resistance per volt of DC measurement, i.e. 10 kΩ/V.
With the concept of voltage sensitivity, the internal resistance of each voltmeter range can be calculated conveniently.
Meanwhile, higher DC‑voltage sensitivity means less current is drawn from the circuit under test during DC‑voltage measurement, yielding more accurate results. See Figure 2 for the DC‑voltage measurement circuit. RS in the figure denotes the shunt resistor for DC‑current ranges, while R6‑R10 are the voltage‑dropping resistors for each voltage‑measurement range.
