Operating Principles for Measuring Direct Current (DC) and DC Voltage Using a Multimeter
A multimeter is a commonly used electrical testing instrument, and there are many tips for using a multimeter. Today, the editor will analyze the working principle of using a multimeter to measure DC current and DC voltage.
1. First, let's take a look at the working principle of the DC current measurement circuit:
The main component of a pointer multimeter is a magneto electric ammeter, commonly referred to as the meter head. But a meter can only measure currents that are less than its sensitivity. In order to expand the range of the measured current, it is necessary to add a shunt resistor to it, so that the current flowing through the meter is a part of the measured current, thereby expanding the range. In order to obtain a certain degree of accuracy when measuring currents of different sizes, ammeters are designed with multiple ranges.
The most commonly used is the closed-circuit tap type shunt circuit, as shown in the diagram. In the figure, R1 to R5 are collectively referred to as the total shunt resistor RS. In actual products, for ease of adjustment and batch production, the total shunt resistor RS mostly uses a larger integer resistance value in kiloohms, and a variable wire winding resistor R0 is connected in series to the meter head. When the meter head parameters change, it can still be compensated and easily adjusted.
2. Working principle of DC voltage measurement circuit
According to Ohm's law U=IR, an ammeter with sensitivity I and internal resistance R is itself a voltmeter with a range of U. For example, a 100 μ A ammeter with an internal resistance of 1.5K Ω can measure a voltage range of 0.15V, which is obviously not practical. However, we can connect a resistor in series to it to expand its range.
If a resistor of 8.5 K Ω is connected in series, the range can be extended to 1V, and the internal resistance of the voltmeter is 10K Ω. This leads to the concept of DC voltage sensitivity; For this example, this voltmeter requires a 10K Ω internal resistance to measure each volt of DC voltage, which is 10K Ω/V. With the concept of voltage sensitivity, it is easy to calculate the internal resistance of each level of the voltmeter.
At the same time, the higher the sensitivity of DC voltage, the smaller the current measured when measuring DC voltage, and the more accurate the measurement results. The DC voltage measurement circuit is shown in the diagram. RS in the figure is the shunt resistor for the DC current range, and R6 to R10 are the voltage reducing resistors for each voltage measurement range.
