Amultimeteror amultitester, also known as aVOM(Volt-Ohm meter), is anelectronicmeasuring instrumentthat combines several measurement functions in one unit. A typical multimeter would include basic features such as the ability to measurevoltage,current, andresistance.Analog multimetersuse amicroammeterwhose pointer moves over a scale calibrated for all the different measurements that can be made.Digital multimeters(DMM, DVOM) display the measured value in numerals, and may also display a bar of a length proportional to the quantity being measured. Digital multimeters are now far more common but analog multimeters are still preferable in some cases, for example when monitoring a rapidly varying value.

A multimeter can be a hand-held device useful for basicfaultfinding and field service work, or abench instrumentwhich 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 aselectronic equipment, motor controls,domestic appliances,power supplies, and wiring systems.

Multimeters are available in a wide range of features and prices. Cheap multimeters can cost less than US$10, while laboratory-grade models with certifiedcalibrationcan cost more than US$5,000.

History

1920s Pocket Multimeter

AvometerModel 8

The first moving-pointer current-detecting device was thegalvanometerin 1820. These were used to measure resistance and voltage by using aWheatstone bridge, and comparing the unknown quantity to a reference voltage or resistance. While useful in the lab, the devices were very slow and impractical in the field. These galvanometers were bulky and delicate.

TheD'Arsonval/Westonmeter movement used a fine metal spring to give proportional measurement rather than just detection, and built-in permanent field magnets made deflection independent of the orientation of the meter. Instead of balancing a bridge, values could be directly read off the instruments's scale, which made measurement quick and easy. By adding a series orshunt resistor, more than one range of voltage or current could be measured with one movement.

Multimeters were invented in the early 1920s asradio receiversand othervacuum tubeelectronic devicesbecame more common. Theinventionof the first multimeter is attributed to British Post Office engineer,Donald Macadie, who became dissatisfied with having to carry many separate instruments required for the maintenance of thetelecommunicationscircuits.Macadie invented an instrument which could measureamperes( amps), volts andohms, so the multifunctional meter was then namedAvometer. The meter comprised a moving coil meter, voltage and precision resistors, and switches and sockets to select the range.

Macadie took his idea to theAutomatic Coil Winder and Electrical Equipment Company(ACWEEC, founded in ~1923). The first AVO was put on sale in 1923, and many of its features remained almost unaltered through to the last Model 8.

Pocket watch style meters were in widespread use in the 1920s, at much lower cost thanAvometers. The metal case was normally connected to the negative connection, an arrangement that caused numerous electric shocks. The technical specifications of these devices were often crude, for example the one illustrated has aresistanceof just 33 ohms per volt, anon-linearscale and no zero adjustment.

Any meter will load the circuit under test to some extent. For example,a multimeter using a movement with full-scale current of 50microamps, the highest sensitivity commonly available, must draw at least 50 microamps from the circuit under test to deflect fully. This may load a high-impedance circuit so much as to affect the circuit, and to give a low reading.

Vacuum Tube Voltmetersor valvevoltmeters(VTVM, VVM) were used for voltage measurements in electronic circuits where highimpedancewas necessary. The VTVM had a fixed input impedance of typically 1megohm or more, usually through use of acathode followerinput circuit, and thus did not significantly load the circuit being tested. VTVMs were used before the introduction of digital electronic high-impedanceanalogtransistorandfield effect transistor(FET) voltmeters. Modern digital meters and some modern analog meters use electronic input circuitry to achieve high-input impedancetheir voltage ranges are functionally equivalent to VTVMs.

Additional scales such asdecibels, and measurement functions such ascapacitance,transistor gain,frequency,duty cycle, display hold, andbuzzerswhich sound when the measured resistance is small have been included on many multimeters. While multimeters may be supplemented by more specialized equipment in a technician's toolkit, some multimeters include additional functions for specialized applications (temperature with athermocoupleprobe,inductance, connectivity to acomputer, speaking measured value, etc.).

Operation[A multimeter is a combination of a multirange DC voltmeter, multirange AC voltmeter, multirangeammeter, and multirangeohmmeter. An un-amplified analog multimeter combines a meter movement, range resistors and switches.

For an analog meter movement, DC voltage is measured with a series resistor connected between the meter movement and the circuit under test. A set of switches allows greater resistance to be inserted for higher voltage ranges. The product of the basic full-scale deflection current of the movement, and the sum of the series resistance and the movement's own resistance, gives the full-scale voltage of the range. As an example, a meter movement that required 1milliampere for full scale deflection, with an internal resistance of 500ohms, would, on a 10-volt range of the multimeter, have 9,500ohms of series resistance. For analog current ranges, low-resistanceshuntsare connected in parallel with the meter movement to divert most of the current around the coil. Again for the case of a hypothetical 1-mA, 500-ohm movement on a 1-Ampere range, the shunt resistance would be just over 0.5ohms.

Moving coil instruments respond only to the average value of the current through them. To measure alternating current, arectifierdiode is inserted in the circuit so that the average value of current is non-zero. Since the rectified average value and the root-mean-square value of a waveform need not be the same, simple rectifier-type circuits may only be calibrated for sinusoidal waveforms. Other wave shapes require a different calibration factor to relate RMS and average value. Since practical rectifiers have non-zero voltage drop, accuracy and sensitivity is poor at low values.

To measure resistance, a small battery within the instrument passes a current through the device under test and the meter coil. Since the current available depends on the state of charge of the battery, a multimeter usually has an adjustment for the ohms scale to zero it. In the usual circuit found in analog multimeters, the meter deflection is inversely proportional to the resistance; so full-scale is 0ohms, and high resistance corresponds to smaller deflections. The ohms scale is compressed, so resolution is better at lower resistance values.

Amplified instruments simplify the design of the series and shunt resistor networks. The internal resistance of the coil is decoupled from the selection of the series and shunt range resistors; the series network becomes avoltage divider. Where AC measurements are required, the rectifier can be placed after the amplifier stage, improving precision at low range.

Digital instruments, which necessarily incorporate amplifiers, use the same principles as analog instruments for range resistors. For resistance measurements, usually a small constant current is passed through the device under test and the digital multimeter reads the resultant voltage drop; this eliminates the scale compression found in analog meters, but requires a source of significant current. An autoranging digital multimeter can automatically adjust the scaling network so that the measurement uses the full precision of the A/D converter.

In all types of multimeters, the quality of the switching elements is critical to stable and accurate measurements. Stability of the resistors is a limiting factor in the long-term accuracy and precision of the instrument.

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

Voltage,alternatinganddirect, involts.

Current, alternating and direct, inamperes.Thefrequencyrange for which AC measurements are accurate must be specified.

Resistanceinohms.

Additionally, some multimeters measure:

Capacitanceinfarads.

Conductanceinsiemens.

Decibels.

Duty cycleas apercentage.

Frequencyinhertz.

Inductanceinhenries.

Temperaturein degreesCelsiusorFahrenheit, with an appropriate temperaturetest probe, often athermocouple.

Digital multimeters may also include circuits for:

Continuity tester; sounds when a circuit conducts

Diodes(measuring forward drop of diode junctions), andtransistors(measuringcurrent gainand otherparameters)

Battery checking for simple 1.5-volt and 9-volt batteries. This is a current loaded voltage scale which simulates in-use voltage measurement.

Varioussensorscan be attached to multimeters to take measurements such as:

Light level Acidity/Alkalinity(pH) Relative humidityResolution[Resolution and accuracyThe resolution of a multimeter is the smallest part of the scale which can be shown. The resolution is scale dependent. On some digital multimeters it can be configured, with higher resolution measurements taking longer to complete. For example, a multimeter that has a 1mV resolution on a 10V scale can show changes in measurements in 1mV increments.

Absolute accuracy is the error of the measurement compared to a perfect measurement. Relative accuracy is the error of the measurement compared to the device used to calibrate the multimeter. Most multimeter datasheets provide relative accuracy. To compute the absolute accuracy from the relative accuracy of a multimeter add the absolute accuracy of the device used to calibrate the multimeter to the relative accuracy of the multimeter

DigitalThe resolution of a multimeter is often specified in the number of decimaldigitsresolvedand displayed. If the most significant digit cannot take all values from 0 to 9 is often termed a fractional digit. For example, a multimeter which can read up to 19999 (plus an embedded decimal point) is said to read 4 digits.

By convention, if the most significant digit can be either 0 or 1, it is termed a half-digit; if it can take higher values without reaching 9 (often 3 or 5), it may be called three-quarters of a digit. A 5 digit multimeter would display one "half digit" that could only display 0 or 1, followed by five digits taking all values from 0 to 9.Such a meter could show positive or negative values from 0 to 199,999. A 3 digit meter can display a quantity from 0 to 3,999 or 5,999, depending on the manufacturer.

While a digital display can easily be extended inprecision, the extra digits are of no value if not accompanied by care in the design and calibration of the analog portions of the multimeter. Meaningful high-resolution measurements require a good understanding of the instrument specifications, good control of the measurement conditions, and traceability of the calibration of the instrument. However, even if its resolution exceeds theaccuracy, a meter can be useful for comparing measurements. For example, a meter reading 5 stable digits may indicate that one nominally 100,000 ohm resistor is about 7 ohms greater than another, although the error of each measurement is 0.2% of reading plus 0.05% of full-scale value.

Specifying "display counts" is another way to specify the resolution. Display counts give the largest number, or the largest number plus one (so the count number looks nicer) the multimeter's display can show, ignoring adecimal separator. For example, a 5 digit multimeter can also be specified as a 199999 display count or 200000 display count multimeter. Often the display count is just called the count in multimeter specifications.

The accuracy of a digital multimeter may be stated in a two-term form, such as "1% of reading +2 counts", reflecting the different sources of error in the instrument. Analog]

Display face of an analog multimeter

Analog meters are older and still preferred by many engineers. One reason for this is that analog meters are more sensitive to what is happening in the circuit that is being measured. A digital multimeter samples the quantity being measured at a particular time and displays it. Analog multimeters sample a quantity as it is happening. If there are slight changes in DC voltage, the needle of an analog multimeter will track them -- the needle moves -- while digital multimeters often miss them. This continuous tracking feature becomes important when testing capacitors or coils. A properly functioning capacitor should allow current to flow when voltage is applied, and the current slowly decreases to zero -- this "signature" is easy to see on an analog multimeter but not on a digital multimeter. It is similar when testing a coil, except the current starts small and increases.

Resistance measurements, in particular, are of low precision due to the typical resistance measurement circuit which compresses the scale heavily at the higher resistance values. Inexpensive analog meters may have only a single resistance scale, seriously restricting the range of precise measurements. Typically an analog meter will have a panel adjustment to set the zero-ohms calibration of the meter, to compensate for the varying voltage of the meter battery.

Accuracy[Digital multimeters generally take measurements withaccuracysuperior to their analog counterparts. Standard analog multimeters measure with typically 3% accuracy,[7]though instruments of higher accuracy are made. Standard portable digital multimeters are specified to have an accuracy of typically 0.5% on the DC voltage ranges. Mainstream bench-top multimeters are available with specified accuracy of better than 0.01%.Laboratory gradeinstruments can have accuracies of a fewparts per million. Accuracy figures need to be interpreted with care. The accuracy of an analog instrument usually refers to full-scale deflection; a measurement of 30V on the 100V scale of a 3% meter is subject to an error of 3V, 10% of the reading. Digital meters usually specify accuracy as a percentage of reading plus a percentage of full-scale value, sometimes expressed in counts rather than percentage terms.

Quoted accuracy is specified as being that of the lower millivolt (mV) DC range, and is known as the "basic DC volts accuracy" figure. Higher DC voltage ranges, current, resistance, AC and other ranges will usually have a lower accuracy than the basic DC volts figure. AC measurements only meet specified accuracy within a specified range offrequencies.

Manufacturers can providecalibrationservices so that new meters may be purchased with a certificate of calibration indicating the meter has been adjusted to standards traceable to, for example, the USNational Institute of Standards and Technology(NIST), or other nationalstandards organization.

Test equipment tends todriftout of calibration over time, and the specified accuracy cannot be relied upon indefinitely. For more expensive equipment, manufacturers and third parties provide calibration services so that older equipment may be recalibrated and recertified. The cost of such services is disproportionate for inexpensive equipment; however extreme accuracy is not required for most routine testing. Multimeters used for critical measurements may be part of ametrologyprogram to assure calibration.

A multimeter can be assumed to be "average responding" to AC waveforms unless stated as being a "True RMS" type. An average responding mulimeter will only meet its specified accuracy on AC volts and amps for purely sinusoidal waveforms. A True RMS responding multimeter on the other hand will meet its specified accuracy on AC volts and current with any waveform type up to a specifiedcrest factor.

A meter's AC voltage and current accuracy may have different specifications for different ranges of frequency.

Sensitivity and input impedance[When used for measuring voltage, the input impedance of the multimeter must be very high compared to the impedance of the circuit being measured; otherwise circuit operation may be changed, and the reading will also be inaccurate.

Meters with electronic amplifiers (all digital multimeters and some analog meters) have a fixed input impedance that is high enough not to disturb most circuits. This is often either one or tenmegohms; thestandardizationof the input resistance allows the use of external high-resistanceprobeswhich form avoltage dividerwith the input resistance to extend voltage range up to tens of thousands of volts. High-end multimeters generally provide an input impedance >10Gigaohms for ranges less than or equal to 10V. Some high-end multimeters provide >10Gigaohms of impedance to ranges greater than 10V. Most analog multimeters of the moving-pointer type areunbuffered, and draw current from the circuit under test to deflect the meter pointer. Theimpedanceof the meter varies depending on the basic sensitivity of the meter movement and the range which is selected. For example, a meter with a typical 20,000ohms/volt sensitivity will have an input resistance of two million ohms on the 100-volt range (100 V * 20,000 ohms/volt = 2,000,000 ohms). On every range, at full scale voltage of the range, the full current required to deflect the meter movement is taken from the circuit under test. Lower sensitivity meter movements are acceptable for testing in circuits where source impedances are low compared to the meter impedance, for example,power circuits; these meters are more rugged mechanically. Some measurements in signal circuits require higher sensitivity movements so as not to load the circuit under test with the meter impedance. Sensitivity should not be confused withresolutionof a meter, which is defined as the lowest signal change (voltage, current, resistance...) that can change the observed reading.

For general-purpose digital multimeters, the lowest voltage range is typically several hundred millivolts AC or DC, but the lowest current range may be several hundred microamperes, although instruments with greater current sensitivity are available. Multimeters designed for (mains) "electrical" use instead of generalelectronics engineeringuse will typically forego the microamps current ranges.

Measurement of low resistance requires lead resistance (measured by touching the test probes together) to be subtracted for best accuracy. This can be done with the "delta", "Zero", or "null" feature of many digital multimeters.

The upper end of multimeter measurement ranges varies considerably; measurements over perhaps 600volts, 10amperes, or 100megohmsmay require a specialized test instrument.

Burden voltage[Any ammeter, including a multimeter in a current range, has a certain resistance. Most multimeters inherently measure voltage, and pass a current to be measured through ashunt resistance, measuring the voltage developed across it. The voltage drop is known as the burden voltage, specified in volts per ampere. The value can change depending on the range the meter selects, since different ranges usually use different shunt resistors. The burden voltage can be significant in very low-voltage circuit areas. To check for its effect on accuracy and on external circuit operation the meter can be switched to different ranges; the current reading should be the same and circuit operation should not be affected if burden voltage is not a problem. If this voltage is significant it can be reduced (also reducing the inherent accuracy and precision of the measurement) by using a higher current range.

Alternating current sensing[Since the basic indicator system in either an analog or digital meter responds to DC only, a multimeter includes an AC to DC conversion circuit for making alternating current measurements. Basic meters utilize arectifier circuitto measure the average or peak absolute value of the voltage, but are calibrated to show the calculatedroot mean square(RMS) value for asinusoidalwaveform; this will give correct readings for alternating current as used in power distribution. User guides for some such meters givecorrection factorsfor some simple non-sinusoidalwaveforms, to allow the correctroot mean square(RMS) equivalent value to be calculated. More expensive multimeters include an AC to DC converter that measures the true RMS value of the waveform within certain limits; the user manual for the meter may indicate the limits of thecrest factorand frequency for which the meter calibration is valid. RMS sensing is necessary for measurements on non-sinusoidalperiodicwaveforms, such as found in audio signals andvariable-frequency drives.

Digital multimeters (DMM or DVOM)[

A bench-top multimeter fromHewlett-Packard.

Modern multimeters are often digital due to their accuracy, durability and extra features. In a digital multimeter the signal under test is converted to a voltage and an amplifier with electronically controlled gain preconditions the signal. A digital multimeter displays the quantity measured as a number, which eliminatesparallaxerrors.

Modern digital multimeters may have anembedded computer, which provides a wealth of convenience features. Measurement enhancements available include:

Auto-ranging, which selects the correct range for the quantity under test so that the mostsignificant digitsare 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 hold the meter to a particular range, because a measurement that causes frequent range changes can be distracting to the user.

Auto-polarityfor 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 forvoltage dropacrosssemiconductor junctions. While not a replacement for atransistor tester, this facilitates testing diodes and a variety of transistor types. Agraphic representationof the quantity under test, as abar graph. This makes go/no-go testing easy, and also allows spotting of fast-moving trends.

A low-bandwidthoscilloscope. Automotive circuit testers, including tests for automotive timing and dwell signals. Simpledata acquisitionfeatures to record maximum and minimum readings over a given period, or to take a number ofsamplesat fixed intervals. Integration with tweezers forsurface-mount technology. A combinedLCR meterfor small-size SMD and through-hole components.[18]Modern meters may be interfaced with apersonal computerbyIrDAlinks,RS-232connections,USB, or an instrument bus such asIEEE-488. The interface allows the computer to record measurements as they are made. Some DMMs can store measurements and upload them to a computer. The first digital multimeter was manufactured in 1955 by Non Linear Systems. Analog multimeters[

Inexpensive analog multimeter with a galvanometer needle display

A multimeter may be implemented with agalvanometermeter movement, or less often with abargraphor simulated pointer such as anLCDorvacuum fluorescent display. Analog multimeters are common; a quality analog instrument will cost about the same as a DMM. Analog multimeters have the precision and reading accuracy limitations described above, and so are not built to provide the same accuracy as digital instruments.

Analog meters are also useful in situations where it is necessary to pay attention to something other than the meter, and the swing of the pointer can be noticed without looking directly at it. This can happen when accessing awkward locations, or when working on cramped live circuitry.

Analog meter movements are inherently more fragile physically and electrically than digital meters. Many analog meters have been instantly broken by connecting to the wrong point in a circuit, or while on the wrong range, or by dropping onto the floor. Many analog multimeters feature a switch position marked "transit" to protect the meter movement during transportation. This feature works by placing a low resistance across the movement winding, resulting indynamic braking. Sensitive meter movements may be protected in the same manner by connecting a shorting or jumper wire between the terminals when not in use. Meters which feature a shunt across the winding such as an ammeter may not require further resistance to arrest uncontrolled movements of the meter needle because of the low resistance of the shunt.

TheARRLhandbook also says that analog multimeters, with no electronic circuitry, are less susceptible toradio frequency interference.[22]The meter movement in a moving pointer analog multimeter is practically always a moving-coilgalvanometerof thed'Arsonvaltype, 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, a very low cost multimeter with a sensitivity of 1000 ohms per volt would draw 1 milliampere from a circuit at full scale deflectionMore expensive, (and mechanically more delicate) multimeters typically have sensitivities of 20,000 ohms per volt and sometimes higher, with a 50,000 ohms per volt meter (drawing 20microamperes at full scale) being about the upper limit for a portable, general purpose, non-amplified analog multimeter.

To avoid the loading of the measured circuit by the current drawn by the meter movement, some analog multimeters use an amplifier inserted between the measured circuit and the meter movement. While this increased the expense and complexity of the meter, by use ofvacuum tubesorfield effect transistorsthe input resistance can be made very high and independent of the current required to operate the meter movement coil. Such amplified multimeters are called VTVMs (vacuum tube voltmeters),[24]TVMs (transistor volt meters), FET-VOMs, and similar names.

Probes[Main article:Test probeA multimeter can utilize a variety of test probes to connect to the circuit or device under test.Crocodile clips, retractable hook clips, and pointed probes are the three most common attachments.Tweezer probesare used for closely spaced test points, as insurface-mount devices. The connectors are attached to flexible, thickly insulated leads that are terminated with connectors appropriate for the meter. Probes are connected to portable meters typically by shrouded or recessedbanana jacks, while benchtop meters may usebanana jacksorBNC connectors. 2mm plugs andbinding postshave also been used at times, but are less common today.

The banana jacks are typically placed with a standardized center-to-center distance of 0.75" (19.05mm), to allow standard adapters or devices such as voltage multiplier or thermocouple probes to be plugged in.

Clamp metersclamp around aconductorcarrying a current to measure without the need to connect the meter in series with the circuit, or make metallic contact at all. Types to measure AC current use the transformer principle; clamp-on meters to measure small current or direct current require more complicated sensors.

Safety[

An example of input protection on the CAT-IV rated Fluke 28 Series II Multimeter

Most multimeters include afuse, or two fuses, which will sometimes prevent damage to the multimeter from a current overload on the highest current range. A common error when operating a multimeter is to set the meter to measure resistance or current, and then connect it directly to a low-impedance voltage source. Unfused meters are often quickly destroyed by such errors; fused meters often survive. Fuses used in meters must carry the maximum measuring current of the instrument, but are intended to disconnect if operator error exposes the meter to a low-impedance fault. Meters with inadequate or unsafe fusing were not uncommon; this situation has led to the creation of theIEC61010 categoriesto rate the safety and robustness of meters.

Digital meters are rated into four categories based on their intended application, as set forth by IEC 61010-1and echoed by country and regional standards groups such as theCENEN61010 standard. Category I: used where equipment is not directly connected to the mains

Category II: used on single phase mains final sub-circuits

Category III: used on permanently installed loads such as distribution panels, motors, and 3-phase appliance outlets

Category IV: used on locations where fault current levels can be very high, such as supply service entrances, main panels, supply meters, and primary over-voltage protection equipment

Each category also specifies maximum transient voltages for selected measuring ranges in the meter.Category-rated meters also feature protections from over-current faults. n meters that allow interfacing with computers,optical isolationmay be used to protect attached equipment against high voltage in the measured circuit.

Good quality multimeters designed to meet CAT II and above ratings will include High Rupture Capacity ceramic fuses typically rated at more than 20kA breaking capacity.[30]They will also include high energy overvoltage MOV (Metal OxideVaristor) protection, and circuit over-current protection in the form of aPolyswitch. DMM alternatives

A general-purpose electronics DMM is generally considered adequate for measurements at signal levels greater than one millivolt or one microampere, or below about 100 megohmslevels far from the theoretical limits of sensitivity. Other instrumentsessentially similar, but with higher sensitivityare used for accurate measurements of very small or very large quantities. These include nanovoltmeters,electrometers(for very low currents, and voltages with very high source resistance, such as one teraohm) andpicoammeters. These measurements are limited by available technology, and ultimately by inherentthermal noise.

Power supply]Analog meters can measure voltage and current using power from the test circuit, but require internal power from the meter for resistance testing; electronic meters always require an internal power supply. Hand-held meters use batteries, while bench meters usually use mains power; either arrangement allows the meter to test devices not connected to an active circuit. Testing often requires that the component under test be isolated from the circuit, as otherwise stray or leakage current paths may distort measurements.

Meters intended for testing inhazardous locationsor for use onblasting circuitsmay require use of a manufacturer-specified battery to maintain their safety rating.