Digital Voltmeters

Digital voltmeters are instruments that measure voltage or voltage drop in a circuit. They use solid-state components and display values digitally. Typically, digital voltmeters (digital volt meters) are used to locate excessive resistance that may indicate an open circuit or ground. They are also used to identify low voltage or voltage drops that may indicate a poor connection. Digital voltmeters are connected in parallel (and never in series) with the circuit being tested so that the meter can tap a small amount of current. The positive lead is connected to the circuits positive side and the negative lead is connected to the circuits ground. The digital voltmeters internal resistance is the impedance, which is usually expressed in ohms per volt. This amount is relatively high in order to prevent the device from drawing significant current and disturbing the operation of the circuit being tested. The sensitivity of the current meter and the value of the series resistance determine the range of voltages that digital voltmeters can measure.

Digital voltmeters can measure a range of alternating current (AC) voltages, direct current (DC) voltages, or both AC and DC voltages. Devices typically display between three and seven digits. Some digital voltmeters can capture minimum and maximum voltages called spike readings. Others measure the root mean square (RMS), a range of frequencies, or the signal power in decibels. Digital voltmeters are also used to monitor resistance temperature detectors (RTDs), thermocouples, transistors, and diodes. Benchtop, rack mounted, and handheld devices are commonly available. Battery powered units do not require plug-in power. Digital voltmeters with audibility continuity beep when the probes touch. Devices with analog bar graph capabilities display status readings such as battery power, signal level, and continuity.

Digital voltmeters interface to computers and include integral monitoring software for applications such as data acquisition. Programmable devices allow users to set values that trigger monitoring routines. Data storage, logging, and removable data storage devices are often available. Some digital voltmeters allow users to adjust the sampling rate or provide internal memory. Others include an auto-ranging feature that automatically adjusts the measurement range. Output options include general-purpose interface bus (GPIB), binary coded decimal (BCD), and digital-to-analog (DA). RS232 is a standard communication protocol for serial ports. IEEE 488 is a standard communication protocol for parallel ports.

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