What Is a Digital Multimeter?
A digital multimeter is generally used to measure basic electrical characteristics such as DC voltage, AC voltage, DC current, and resistance. Higher-end models can measure more exotic power profiles, including those that capture complex AC-based data. For example, AC (Alternating Current) power is frequency-based. High-end digital multimeters are capable of capturing peak currents and average currents, which are stored at different positions on an AC waveform. This kind of data can be used to troubleshoot impedance issues, power factor correction problems, and much more.
On the device, while conventional voltmeters, ammeters, and resistance meters have analog displays in which the meter pointer indicates the measured value, a digital multimeter is called a digital multimeter because it has multiple measurement functions and three- to eight-digit numerical displays. Models with extended measurement functions such as capacitance, AC frequency, and temperature are also available.
Compact and lightweight models suitable for use at construction sites are also called digital testers. The number of digits displayed is about 4 digits, and the measurement accuracy is generally 0.05 to 0.1% for DC voltage and 0.5 to 1% for AC voltage. Although the accuracy is insufficient for precise measurements in the laboratory, they are easy to use for outdoor applications. In anticipation of such use, models with a sturdy construction to withstand drops are also available.
Uses of Digital Multimeters
Digital multimeters are used in a variety of situations, including measurements in laboratories, electrical adjustment of products on factory production lines, and construction and maintenance inspections of electrical facilities.
They are often incorporated into power-receiving equipment and power control panels. In such cases, in addition to basic parameters such as current, voltage, and resistance, some have built-in functions to measure capacitance, frequency, and temperature.
In addition to the specialized applications described above, inexpensive models are also available for use in general household electronic construction.
Principle of Digital Multimeters
The core of a digital multimeter (DMM) consists of a high-precision, high-resolution analog-to-digital converter (A/D converter) and a processor that calculates measurement values based on digital output. The A/D converter converts the analog signal acquired by the test procedure into a digitally recognizable measurement that’s then processed by a microchip. The means of collecting the analog measurement data are described as follows.
1. DC Voltage Measurement
The voltage between two probes is converted to a voltage within the dynamic range through an amplifier or attenuator that affects the voltage by either amplifying it (for low voltage) or attenuating it (for high voltage) to become the input voltage that’s transferred to the A/D converter. The processor calculates the voltage between the probes based on the digital value, amplifier gain, and attenuation factor of the attenuator, and displays the DC voltage value on the display unit.
2. AC Voltage Measurement
The AC voltage is converted to DC voltage through a rectifier circuit, then input to the A/D converter, and the AC voltage value is displayed on the display unit through the same process as DC voltage.
3. Resistance Measurement
A constant current is applied to the resistance to be measured through two probes from the constant current power supply built into the digital multimeters. The DC voltage appearing at both ends of the probes is input to the A/D converter to measure the voltage at both ends of the resistor to be measured. From this voltage value and the current value of the constant-current power supply, the processor calculates the resistance value to be measured.
4. Current Measurement
To measure DC current, the voltage at both ends of the micro resistor generated by the measured current flowing through in the digital multimeters is input to an A/D converter. The processor calculates the current value from the output value of the A/D converter and displays the current value on the display unit. For AC current, the AC voltage at both ends of the micro resistor is converted to DC voltage by a rectifier circuit and input to the A/D converter.
5. A/D Converter
The A/D converter of digital multimeters requires very high precision (high resolution), e.g., 24 bits or more for a 7-digit display, so a double-integral type is generally used. Therefore, the time required for conversion is relatively long, and several measurements per second is the most that can be done. However, by reducing the number of displayed digits and shortening the conversion time of the A/D converter, it is possible to shorten the measurement time.
How to Use the Digital Multimeters
A description of how to use digital multimeters follows
1. Voltage and Current Measurements
In digital multimeters, the system to be measured is connected between the two input terminals, the Hi and Lo terminals. When measuring DC voltage, connect the Hi terminal to the high voltage side and the Lo terminal to the constant voltage side, and the voltage on the Hi terminal side will be displayed based on the potential on the Lo terminal side. When measuring DC current, if the current to be measured flows in from the Hi terminal and out from the Lo terminal, the current value is displayed as positive, and in the opposite direction, it is displayed as negative. In AC voltage, current, and resistance measurements, polarity need not be taken into consideration.
2. Measurement Range Setting
For general use, the AutoRange function automatically switches to the optimum range for the voltage and current within the maximum input rating, so there is no need to search for the optimum range. However, if you need to reduce measurement time, such as when adjusting a production line, you will need to manually set the range based on the expected measurement value.
3. Effect on the Circuit to be Measured
Connecting digital multimeters may affect the system under measurement and cause fluctuations in measured values. For example, if digital multimeters are connected to a circuit with very high impedance, such as when measuring the output voltage of an optical sensor in a dark environment, its internal impedance may load the measurement system, resulting in a lower value than the original output voltage.
Similarly, when measuring the current of a circuit with a small impedance, the minute resistance for voltage detection present in the digital multimeters may cause a non-negligible error in the circuit under measurement. Therefore, the influence of the digital multimeters on the circuit under measurement should be considered before deciding whether or not to use digital multimeters.
4. Low Resistance Measurement
There are digital multimeters that can perform 4-terminal measurements for resistance measurement. As the term “4-terminal” implies, it consists of a pair of constant-current power supplies and a pair of voltmeters. A constant-current power supply is connected to both ends of the resistor to be measured. The voltmeter is connected to the constant-current terminals of the resistor to be measured.
The voltmeter measures the voltage at both ends of the resistor by placing a probe inside the constant-current terminals, at a point on the resistor side. The resistance is calculated from this measured voltage and the constant current value. Since the contact resistance of the constant-current terminal does not affect the measured voltage and the contact resistance of the probe of the voltmeter is negligible compared to the internal resistance of the voltmeter, which is typically as high as 10-MΩ, low resistance can be accurately measured.