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Multimeter

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A digital multimeter

A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several measurement functions in one unit. A typical 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 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.

History

Avometer

Scientists originally used galvanometers to measure current. A galvanometer may be wired to measure resistance (given a known voltage source) or voltage (given a fixed resistance). While appropriate for primitive lab use, switching from one setup to another is inconvenient in the field.

Multimeters were invented in the early 1920s as radio receivers and other vacuum tube electronic devices became more common. The invention of the first multimeter is attributed to Post Office engineer Donald Macadie, who became dissatisfied with having to carry many separate instruments required for the maintenance of the telecommunication circuits.[1] Macadie invented an instrument which could measure amperes, volts and ohms, so the multifunctional meter was then named Avometer.[2] The meter comprised a galvanometer, voltage and resistance references, and a switch to select the appropriate circuit for the input under test.

Macadie took his idea to the Automatic Coil Winder and Electrical Equipment Company (ACWEEC, founded probably in 1923).[2] The first AVO was put on sale in 1923, and although it was initially a DC-only instrument many of its features remained almost unaltered right through to the last Model 8.

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.[3]

Quantities measured

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

Additionally, multimeters may also measure:

Digital multimeters may also include circuits for:

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

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.[4] 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 about three percent accuracy.[5] Standard portable digital multimeters claim to be capable of taking measurements with an accuracy of 0.5% on the DC voltage ranges. Mainstream bench-top multimeters make claims to have an accuracy of better than ±0.01%. Laboratory grade instruments can have accuracies in the parts per million figures.[6]

A multimeter's quoted accuracy is specified as being that of the lower (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.

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 smaller 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.[7]

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 milliamperes. 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 microampere or less...

Burden voltage

On both DC and AC current ranges a multimeter will cause voltage drop in the circuit under test. This is primarily due to the current shunt resistor used for measurement. This 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.[8]

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)

A bench-top multimeter from Hewlett-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 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.[9][10]
  • 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.[11]
  • Automotive circuit testers, including tests for automotive timing and dwell signals.[12]
  • Simple data acquisition features to record maximum and minimum readings over a given period, or to take a number of samples at fixed intervals.[13]
  • Integration with tweezers for surface-mount technology.[14]
  • A combined LCR meter for small-size SMD and through-hole components.[15]

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.[16]

The first digital multimeter was manufactured in 1955 by Non Linear Systems.[17][18]

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 the 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.[19]

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.[20] 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)[21] 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.

On meters that allow interfacing with computers, optical isolation protects the computer and operator from high voltage measurants.

Digital meters are category rated based on their intended application, as set forth by the CEN EN61010 standard.[22] 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.[23][24] Category-rated meters also feature protections from over-current faults.[25]

DMM Alternatives

A DMM is generally considered adequate for measurements at signal levels greater than one microvolt or one microampere, or below one gigaohm — levels far from the theoretical limits of sensitivity. Other instruments can be employed in applications in which a DMM is not considered the optimal choice. These include nanovoltmeters, electrometers and picoammeters. If the application demands greater voltage sensitivity and the source resistance is low, a nanovoltmeter is capable of measuring at levels much closer to the theoretical limits of measurement.

In measuring voltages with very high source resistance values (such as one tera-ohm), a DMM’s input resistance of ten megaohms to ten gigaohms is several orders of magnitude lower than the source resistance, which can lead to severe input loading errors. Additionally, DMM input currents are typically in the range of many picoamperes. That creates large voltage offsets. An electrometer, then, is generally the best option for measurement. Low-level current measurements pose a similar challenge for DMMs because they typically have a high input voltage drop (input burden), which affects low-level current measurements and makes DMM resolution generally no better than one nanoampere. Given their much lower input burden and better sensitivity, either an electrometer or a picoammeter is optimal.

See also

References

  1. ^ http://www.glias.org.uk/news/237news.html
  2. ^ a b www.gracesguide.co.uk / AVO
  3. ^ Fluke Manufacturing. "Fluke DTX CableAnaylzer Series". Retrieved 2007-01-28.
  4. ^ "Digital Multimeter Measurement Fundamentals". National Instruments. Retrieved 2008-01-26.
  5. ^ Milton Kaufman. Handbook of electronics calculations for engineers and technicians. McGraw-Hill.
  6. ^ Agilent Technologies. "Agilent 3458A Digital Multimeter Data Sheet" (PDF). Retrieved 2007-01-28.
  7. ^ Horn, Delton (1993). How to Test Almost Everything Electronic. McGraw-Hill/TAB Electronics. pp. 4–6. ISBN 0830641270.
  8. ^ "A Precision Current Adapter for Multimeters (Silicon Chip magazine April 2009)". alternatezone.com. Retrieved 2009-09-22.
  9. ^ Hewes, John. "Testing a diode with a multimeter". Retrieved 2007-01-28.
  10. ^ Goldwasser, Samuel. "Basic Testing of Semiconductor Devices". Retrieved 2007-01-28.
  11. ^ Extech Instruments. "Extech 5 MHz Dual Channel Multiscope". Retrieved 2007-01-28.
  12. ^ Snap-on Incorporated. "MT596AK Automotive Digital Multimeter". Retrieved 2007-01-28.
  13. ^ Extech Instruments. "Extech Dual Channel, Datalogging multimeter". Retrieved 2007-01-28.
  14. ^ Siborg Systems Inc. "Digital Multimeter Smart Tweezers from Siborg". Retrieved 2008-04-23.
  15. ^ Advance Devices Inc. "Smart Tweezers LCR Digital Multimeter" (PDF). Retrieved 2009-01-20.
  16. ^ Fluke Manufacturing. "Logging and analyzing events with FlukeView Forms Software" (PDF). Retrieved 2007-01-28.
  17. ^ "Gauging the impact of DVMs". EETimes.com. Retrieved 2008-01-26.
  18. ^ Dyer, Stephen (2001). Survey of Instrumentation and Measurement. p. 286. ISBN 047139484X.
  19. ^ Wilson, Mark (2008). The ARRL Handbook For Radio Communications. ISBN 0872591018.
  20. ^ Frank Spitzer and Barry Horwath Principles of Modern Instrumentation, Holt, Rinehart and Winston Inc., New York 1972, no ISBN, Library of Congress 72-77731, page 39
  21. ^ "The Incomplete Idiot's Guide to VTVMs". tone-lizard.com. Retrieved 2007-01-28.
  22. ^ Safety requirements for electrical equipment for measurement, control and laboratory use. General requirements. 1993. ISBN 0580224333.
  23. ^ Dyer, Stephen (2001). Survey of Instrumentation and Measurement. p. 285. ISBN 047139484X.
  24. ^ "Anatomy of a high-quality meter". Retrieved 2008-01-26.
  25. ^ Mullin, Ray (2005). Electrical Wiring: Residential. Thompson Delmar Learning. p. 6. ISBN 1401850200.
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