Multimeter

A digital multimeter

Analog multimeter

A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several functions in one unit. A standard multimeter may include features such as the ability to measure voltage, current and resistance. There are two categories of multimeters, analog multimeters (or analogue multimeters in British English) and digital multimeters (often abbreviated DMM.)

A multimeter can be a hand-held device useful for basic fault finding and field service work or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such as batteries, motor controls, appliances, power supplies, and wiring systems.

Quantities measured

Contemporary multimeters can measure many quantities. The common ones are:

• Voltage in volts.
• Current in amperes.
• Resistance in ohms.

Additionally, multimeters may also measure:

• Capacitance in farads.
• Frequency in hertz
• Duty cycle as a percentage.
• Temperature in degrees Celsius or Fahrenheit.
• Conductance in siemens.
• Inductance in henrys
• Audio signal levels in decibels.

Digital multimeters may also include circuits for:

• Continuity that beeps when a circuit conducts.
• Diodes and Transistors

Various sensors can be attached to multimeters to take measurements such as:

• light level
• Acidity/Alkalinity(pH)
• Wind speed
• Relative humidity

Digital Multimeters (DMM)

A bench-top multimeter from Hewlett-Packard.

Modern multimeters are often digital due to their accuracy, durability and extra features.

In a DMM the signal under test is converted to a voltage and an amplifier with an electronically controlled gain preconditions the signal.

A DMM displays the quantity measured as a number, which prevents parallax errors.

The inclusion of solid state electronics, from a control circuit to small embedded computers, has provided a wealth of convenience features in modern digital meters. Commonly available measurement enhancements include:

• Auto-ranging, which selects the correct range for the quantity under test so that the most significant digits are shown. For example, a four-digit multimeter would automatically select an appropriate range to display 1.234 instead of 0.012, or overloading. Auto-ranging meters usually include a facility to 'freeze' the meter to a particular range, because a measurement that causes frequent range changes is distracting to the user.
• Auto-polarity for direct-current readings, shows if the applied voltage is positive (agrees with meter lead labels) or negative (opposite polarity to meter leads).
• Sample and hold, which will latch the most recent reading for examination after the instrument is removed from the circuit under test.
• Current-limited tests for voltage drop across semiconductor junctions. While not a replacement for a transistor tester, this facilitates testing diodes and a variety of transistor types.
• A graphic representation of the quantity under test, as a bar graph. This makes go/no-go testing easy, and also allows spotting of fast-moving trends.
• A low-bandwidth oscilloscope.
• Automotive circuit testers, including tests for automotive timing and dwell signals.
• Simple data acquisition features to record maximum and minimum readings over a given period, or to take a number of samples at fixed intervals.
• A miniature digital multimeter integrated with tweezers for Surface-mount technology.

Modern meters may be interfaced with a personal computer by IrDA links, RS-232 connections, USB, or an instrument bus such as IEEE-488. The interface allows the computer to record measurements as they are made. Some DMM's can store measurements and upload them to a computer.

The first digital multimeter was manufactured in 1955 by Non Linear Systems.

Analog Multimeters

A multimeter may be implemented with an analog meter deflected by an electromagnet, as a classic galvanometer, or with a digital display such as an LCD or vacuum fluorescent display.

Analog multimeters are not hard to find (though less common and often more expensive than low-end digital units), but are not considered as accurate as digital because of errors introduced in zeroing and reading the analog meter face.

Analog meters may be implemented with vacuum tubes to precondition and amplify the input signal. Such meters are known as vacuum tube volt meters (VTVM) or vacuum tube multimeters (VTMM).

Analog meters are sometimes considered better for detecting the rate of change of a reading; the ARRL handbook suggests that analog multimeters are often less susceptible to radio frequency interference.

The meter movement in a moving pointer analog multimeter is practically always a moving-coil galvanometer of the d'Arsonval type, using either jeweled pivots or taut bands to support the moving coil. In a basic analog multimeter the current to deflect the coil and pointer is drawn from the circuit being measured; it is usually an advantage to minimize the current drawn from the circuit. The sensitivity of an analog multimeter is given in units of ohms per volt. For example, an inexpensive multimeter would have a sensitivity of 1000 ohms per volt and would draw 1 milliampere from a circuit at the full scale measured voltage. More expensive, (and more delicate) multimeters would have sensitivities of 20,000 ohms per volt or higher, with a 50,000 ohms per volt meter (drawing 20 microamperes at full scale) being about the upper limit for a portable general purpose analog multimeter.

To avoid the loading of the measured circuit by the current drawn by the meter movement, later analog multimeters use an amplifier inserted between the measured circuit and the meter movement. While this increased the expense and complexity of the meter and required a power supply to operate the amplifier, by use of vacuum tubes or field effect transistors the input resistance can be made very high and independent of the current required to operate the meter movement coil. Such amplified multimeters are called VTVM (vacuum tube voltmeters) or TVM (transistor volt meter), and similar names.