The digital multimeter is a versatile instrument that combines a digital voltmeter (DVM) with various conversion circuits, enabling it to measure multiple electrical parameters such as AC and DC voltage, AC and DC current, resistance, and capacitance. This makes it an essential tool for both professional and hobbyist use in electronics and electrical testing.
The following block diagram illustrates the internal structure of a digital multimeter. It consists of three main sections: the input and conversion section, the A/D converter, and the display unit. The input and conversion section includes several key components such as a current-to-voltage converter (I/V), an AC-to-DC converter (AC/DC), a resistor-to-voltage converter (R/V), and a capacitor-to-voltage converter (C/V). These converters are responsible for transforming the measured quantities into a form suitable for processing by the A/D converter. The range selector switch determines the measurement range, and the signal is then amplified or attenuated before being sent to the A/D converter.
The A/D converter circuit and the display unit are typically built using integrated circuits like the ICL7106 and an LCD screen. The ICL7106 is a popular choice due to its high accuracy and ease of integration with liquid crystal displays.
The basic measurement range of the multimeter is set at 200 mV DC, and all other measurements—such as AC voltage, current, resistance, and capacitance—are linearly converted from this base range. This allows the multimeter to accurately display the corresponding electrical values in numerical form.
**Functional Circuit and Working Principle**
**1. Resistance Measurement Circuit and Decimal Point Display Circuit**
Resistance is measured using a proportional method. The unknown resistor Rx is connected in series with a reference resistor between V+ and COM. The voltage across Rx is given by Uin = V+ * Rx / (R + Rx). Once the measurement range is selected, R is fixed, and the higher the value of Rx, the higher the Uin. As the range changes from 200 Ω to 20 MΩ, the corresponding value of R also increases. This ensures that the voltage across Rx remains within a manageable range for each setting.
The ICL7106 IC only has a liquid crystal segment driver and a back electrode drive. To display the decimal point, an operational amplifier (OP1) is used to create an inverting amplifier, forming a decimal point display circuit. This circuit sends a 50 Hz square wave pulse (50% duty cycle) to the back electrode BP of the LCD, ensuring that the effective AC voltage is zero, which helps prolong the life of the LCD. The pulse signals for each decimal point (BP2, BP20, BP200) are reversed based on the display principle of the liquid crystal, allowing the correct decimal point to illuminate.
**2. DC Voltage and AC Voltage Measurement Circuits**
For DC voltage measurement, a resistor divider network is used to scale the input voltage down to a level compatible with the A/D converter. The measured DC voltage is converted to a 0–200 mV DC signal through a voltage divider and transfer switch, then sent to the A/D converter for display. As the range changes from 200 mV to 1000 V, the corresponding resistance values decrease. This ensures that the input voltage to the ICL7106 does not exceed 200 mV, maintaining balance across all ranges.
For AC voltage measurement, the principle is similar to DC measurement, but the switch configuration changes to 8-1 through 8-4. The AC signal is rectified using a diode and converted to DC before being sent to the IN+ terminal of the ICL7106 via the G point. The output at G represents the average value of the AC voltage, which is then displayed on the LCD.
These circuits work together to provide accurate and reliable measurements, making the digital multimeter an indispensable tool in modern electronic testing and troubleshooting.
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