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 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.
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.
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 count 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.
Resolution of analog multimeters is limited by the width of the scale pointer, vibration of the pointer, parallax observation errors, and the accuracy of printing of scales. Resistance measurements, in particular, are of low precision due to the typical resistance measurement circuit which compresses the scale at the higher resistance values. 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).
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.
Digital multimeters generally take measurements with superior accuracy to their analog counterparts. Analog multimeters typically measure with three to five percent accuracy.[citation needed] 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 claim es 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.
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 transistorized analog meters, have a standardized input impedance 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.
Analog multimeters of the moving pointer type 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 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). Low-sensitivity meters are useful for general purpose testing especially in power circuits, where source impedances are low compared to the meter impedance. Measurements in signal circuits generally require higher sensitivity so as not to load down the circuit under test with the meter impedance.
The sensitivity of a meter is also a measure of the lowest voltage, current or resistance that can be measured with it. 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 mulitmeters 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 microamperes or less.
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.
Modern multimeters are often digital due 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.
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.
Modern multimeters are often digital due 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.
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.
A 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. 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.
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.
Almost every multimeter includes a fuse, which will generally prevent damage to the multimeter if it is overloaded. 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 may cause 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.
Multimeters were invented in the early 1920's 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.