digital multimeter

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Multimeter

[muhl-tim-i-ter]

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 or DVOM.)

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.

Multimeters are available in a wide ranges of features and prices. Cheap multimeters can cost less than US$10, while the top of the line multimeters can cost more than US$5000.

Quantities measured

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

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

Resolution

Digital

The resolution of a multimeter is often specified in "digits" of resolution. For example, the term 5½ digits refers to the number of digits displayed on the readout of a multimeter.

By convention, a half digit can display either a zero or a one, while a three-quarters digit can display a numeral higher than a one but not nine. Commonly, a three-quarters digit refers to a maximum value of 3 or 5. The fractional digit is always the most significant digit in the displayed value. A 5½ digit multimeter would have five full digits that display values from 0 to 9 and one half digit that could only display 0 or 1. Such a meter could show positive or negative values from 0 to 199,999. A 3¾ 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 in precision, 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.

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' display can show, ignoring a decimal separator. For example, a 5½ 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. In some designs the underlying analog-to-digital converter mechanism may have more or less digits of precision than displayed.

Analog

Resolution of analog multimeters is limited by the width of the scale pointer, vibration of the pointer, the accuracy of printing of scales, zero calibration, number of ranges, and errors due to non-horizontal use of the mechanical display. Accuracy of readings obtained is also often compromised by miscounting division markings, errors in mental arithmetic, parallax observation errors, and less than perfect eyesight. Mirrored scales and larger meter movements are used to improve resolution; two and a half to three digits equivalent resolution is usual (and may be adequate for the limited precision actually necessary for most measurements).

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.

Accuracy

Digital multimeters generally take measurements with accuracy superior to their analog counterparts. Analog multimeters typically measure with three to five percent accuracy. Standard portable digital multimeters claim to be capable of taking measurements with an accuracy of 0.5% on DC voltage and current scales. Mainstream bench-top multimeters make claims to have as great accuracy as ±0.01%. Laboratory grade instruments can have accuracies in the parts per million figures.

Manufacturers can provide calibration services so that new meters may be purchased with a certificate of calibration indicating the meter has been adjusted to standards traceable to the National Institute of Standards and Technology. Such manufacturers usually provide calibration services after sales, as well, so that older equipment may be recertified. Multimeters used for critical measurements may be part of a metrology program to assure calibration.

Sensitivity and input impedance

The current load, or how much current is drawn from the circuit being tested may affect a multimeter's accuracy. A small current draw usually will result in more precise measurements. With improper usage or too much current load, a multimeter may be damaged therefore rendering its measurements unreliable and substandard.

Meters with electronic amplifiers in them, such as all digital multimeters and analog meters using a transistor for amplification, have an input impedance that is usually considered high enough not to disturb the circuit tested. This is often one million ohms, or ten million ohms. The standard input impedance allows use of external probes to extend the direct-current measuring range up to tens of thousands of volts.

Most analog multimeters of the moving pointer type are unbuffered, and draw current from the circuit under test to deflect the meter pointer. The impedance of 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 20,000 ohms/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). Lower sensitivity meters are useful for general purpose testing especially in power circuits, where source impedances are low compared to the meter impedance. Some measurements in signal circuits require higher sensitivity so as not to load down the circuit under test with the meter impedance.

Sometime sensitivity is confused with resolution of a meter, which is defined as measure of the lowest voltage, current or resistance that can change measurement reading. For general-purpose digital multimeters, a full-scale range of several hundred millivolts AC or DC is common, but the minimum full-scale current range may be several hundred milliamps. Since general-purpose multimeters have only two-wire resistance measurements, which do not compensate for the effect of the lead wire resistance, measurements below a few tens of ohms will be of low accuracy. The upper end of multimeter measurement ranges varies considerably by manufacturer; generally measurements over 1000 volts, over 10 amperes, or over 100 megohms would require a specialized test instrument, as would accurate measurement of currents on the order of 1 microamp or less.

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 multimeters may utilize a rectifier circuit, calibrated to evaluate the average value of a rectified sine wave. User guides for such meters will give correction factors for some simple waveforms, to allow the correct root mean square (RMS) equivalent value to be calculated for the average-responding meter. More expensive multimeters will include an AC to DC converter that responds to the RMS value of the waveform for a wide range of possible waveforms; the user manual for the meter will indicate the limits of the crest factor and frequency for which the meter calibration is valid. RMS sensing is necessary for measurement s of non-sinusoidal quantities, such as found in audio signals, or in variable-frequency drives.

Digital Multimeters (DMM or DVOM)

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 an electronically controlled gain preconditions the signal.

A Digital Multimeter 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 a galvanometer meter movement, or with a bar-graph or simulated pointer such as an LCD or vacuum fluorescent display. Analog multimeters are common, although a quality analog instrument will be about the same cost as a digital multimeter. Analog multimeters have the precision and reading accuracy limitations described above, and so are not built to provide teh same accuracy as digital instruments.

Analog meters are sometimes considered better for detecting the rate of change of a reading; some digital multimeters include a fast-responding bar-graph display for this purpose. 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, non-amplified 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) TVM (transistor volt meter), FET-VOM, and similar names.

Probes

A multimeter can utilise 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. The connectors are attached to flexible, thickly-insulated leads that are terminated with connectors appropriate for the meter. Handheld meters typically use shrouded or recessed banana jacks, while benchtop meters may use banana jacks or BNC connectors. 2mm plugs and binding posts have also been used at times, but are not so common today.

Meters which measure high voltages or current may use non-contact attachment mechanism to trade accuracy for safety. Clamp meters provide a coil that clamps around a conductor in order to measure the current flowing through it.

Safety

Some multimeters include a fuse, which will sometimes prevent damage to the multimeter if it is overloaded. However the fuse often only protects the highest current range on the multimeter. 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; meters without protection are quickly damaged by such errors, and can sometimes explode causing injury to the operator.

Digital meters are category rated based on their intended application, as set forth by the CEN EN61010 standard. There are four categories:

Category I: used where current levels are low.
Category II: used on residential branch circuits.
Category III: used on permanently installed loads such as distribution panels, motors, and appliance outlets.
Category IV: used on locations where current levels are high, such as service entrances, main panels, and house meters.

Each category also specifies maximum transient voltages for selected measuring ranges in the meter. Category-rated meters also feature protections from over-current faults.

History

Multimeters were invented in the early 1920s as radio receivers and other vacuum tube electronic devices became more common. As modern systems become more complicated, the multimeter is becoming more complex or may be supplemented by more specialized equipment in a technician's toolkit. For example, where a general-purpose multimeter might only test for short-circuits, conductor resistance and some coarse measure of insulation quality, a modern technician may use a hand-held analyzer to test several parameters in order to validate the performance of a network cable.