The digital multimeter is a versatile instrument that combines the functionality of a digital voltmeter (DVM) with various conversion circuits, enabling it to measure a wide range of electrical parameters such as AC and DC voltage, AC and DC current, resistance, and capacitance. This makes it an essential tool for both professionals and hobbyists in electronics and electrical engineering.
The internal structure of the digital multimeter can be divided into three main sections: the input and conversion section, the A/D converter section, and the display section. The input and conversion part includes several key components like the current-to-voltage converter (I/V), AC-to-DC converter (AC/DC), resistor-to-voltage converter (R/V), and capacitor-to-voltage converter (C/V). These converters are responsible for transforming the measured quantity into a suitable voltage signal that can be processed by the A/D converter. Depending on the selected measurement range, a range switch determines the appropriate scaling factor, and the signal is either amplified or attenuated before being sent to the A/D converter.
The A/D converter and the display unit are typically built using integrated circuits such as the ICL7106, which converts the analog signal into a digital format, and an LCD screen for displaying the results. The ICL7106 serves as the core of the meter, operating on a basic DC range of 200 mV. It allows for linear transformation of DC voltage and current measurements, while AC voltage, current, resistance, and capacitance are displayed through their respective conversion circuits.
**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, Uin, is calculated as Uin = V+ * Rx / (R + Rx). Once the measurement range is set, the value of R is fixed, and the larger the Rx, the higher the Uin. As the range changes from 200 Ω to 20 MΩ, the corresponding R value also increases. This ensures that the voltage across Rx remains within a balanced range, preventing overloading of the system.
To display the decimal point on the LCD, an operational amplifier (OP1) is used to create an inverting amplifier circuit. This generates a 50 Hz square wave with a 50% duty cycle, which is applied to the back electrode of the LCD (BP point). This helps maintain a low effective AC voltage, prolonging the life of the display. The pulse signals for each decimal point (BP2, BP20, BP200) are reversed according to the LCD’s display principle, ensuring the correct decimal point lights up at the right time.
**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 range compatible with the A/D converter. The input voltage is converted into a 0–200 mV DC signal through a voltage divider and transfer switch, then sent to the ICL7106 for processing and display. When the range is changed from 200 mV to 1000 V, the corresponding resistor values are adjusted accordingly. This ensures that the input voltage to the ICL7106 never exceeds 200 mV, maintaining balance across all ranges. However, it's important to note that the 1000 V range should not be used for voltages above 1001 V due to safety concerns.
For AC voltage measurement, the process is similar to DC measurement, but the input signal is first rectified using a diode after passing through the AC/DC conversion circuit. The output is then sent to the IN+ terminal of the ICL7106 via the G point. The resulting signal represents the average value of the AC voltage, which is then displayed on the LCD. This method ensures accurate representation of the RMS value of the AC signal.
The primary purpose of any fuse cutouts is to provide protection to the lines of your system and the various apparatus on those lines such as transformers and capacitor banks. Fuse cutouts provide reliable protection from low-level overloads that just melt the fuse link, intermediate faults, and very high faults, through maximum interrupting capacity.
Fuse Cutout,Fuse Cutout Amperes,Porcelain Fuse,Cut Out Fuse Carrier
Jilin Nengxing Electrical Equipment Co. Ltd. , https://www.nengxingelectric.com