How Does a Digital Multimeter Work?

A multimeter measures voltage, resistance, current, capacitance, continuity, and temperature.

Unlike conventional analogue multimeters that display readings by means of a moving pointer, with the help of a micro ammeter, digital multimeters feature LCD or LED numeric display. Advanced models may also feature a graphical bar depicting the measured value.

Key Components of a Digital Multimeter

Digital multimeters consist of a combination of analogue and digital circuits. The core of the device is an analog-to-digital converter, which converts the underlying voltage levels to digital information.

The other important circuit elements in a digital multimeter include precise voltage reference generator, a voltage-to-current converter, a current-to-voltage converter, amplifiers that multiply the voltage or current, level shifters that converts a voltage level to another level, and a timer or counter that measures time, besides resistors, capacitors, diodes, and inductors. A typical digital multimeter also co-opts circuits for continuity tester, to emit a buzz when the circuit resistance is low to the desired point, and transistors to measure current gain and other parameters.

These components work to convert the measured signal to the voltage, enabling the analog-to-digital converter to derive a digital signal from the voltage. This digital signal is then displayed on an LCD/LED display.

A digital multimeter is a versatile device that may be used to take multiple measurements. Apart from the common voltage and other measurements, sensors may be attached to the device, to take various measurements, such as light levels, relative humidity, sound pressure level, pH levels, small current flow, and more. Fixing an adapter which uses inductance allows measuring large currents.

The Functioning

Digital multimeters work by the analogue to digital converter converting the voltage to a digitally displayed number. A scaling circuit accommodates different ranges. A small shunt resistor measures the current readings. The voltage across the shunt is proportional to the current.

Deploying an additional circuit allows the measurement of the average content of the waveform, for AC voltage and current.

How to Measure Voltage

A digital meter is in effect an analogue charging circuit, featuring a digital stopwatch. The basic circuit measures the time taken by a capacitor to charge up to the input voltage, with a reading of 1.500 volts denoting 1.500 milliseconds. Capacitors fed with constant current charge a measurable unit of volts for a determined time.

How to Measure Current

The basis to measure current with a digital multimeter is Ohm’s law. A resistor across the voltmeter terminals measures the current. Using a constant current source makes it possible to measure Ohm. A 1-ohm resistor will drop one volt, a 2-ohm resistor drops two volts, and so on.

How to Measure Resistance

To measure resistance, current is applied from a current source inside the device, usually a battery, to the resistor under test. The voltage drop that takes place is recorded. The developed voltage is proportional to the resistance.

How to Ensure Accuracy

The stability of the digital multimeter and the accuracy of the measurement depends on the quality of the switching elements. Stability depends on the age of the device and the state of the internal resistors. High-end digital multimeters have gold plated contacts. Cheaper models feature nickel plated contacts, and even cheaper models use printed circuit board solder traces as the switch contacts.

Another important consideration that determines the accuracy of digital multimeters is the frequency range. This depends on the circuit design and construction. The range is specified in the device, with high-end multimeters able to measure currents in milliamps and even micro amps.
Ever since Non-Linear Systems developed the first digital multimeter, in 1955, and the first handheld digital multimeter made its appearance in 1977, these devices have soared in popularity. Digital multimeters are now more common and popular than traditional analogue multimeters owing to its higher accuracy, with lesser cost. Digital multimeters are now the preferred tool for technicians across the world. Its compact design and durability make it versatile and handy, in almost all situations and locations.