Editing Multimeter (section)

== Designs ==

=== Analog ===
[[File:Multimeter-4254e.jpg|thumb|right|Inexpensive analog multimeter with a galvanometer needle display]]

An un-amplified analog multimeter combines a meter movement, range resistors and switches;  VTVMs are amplified analog meters and contain active circuitry. For an analog meter movement, DC voltage is measured with a series resistor connected between the meter movement and the circuit under test. A switch (usually rotary) allows greater resistance to be inserted in series with the meter movement to read higher voltages. 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 1&nbsp;mA for full-scale deflection, with an internal resistance of 500&nbsp;Ω, would, on a 10&nbsp;V range of the multimeter, have 9,500&nbsp;Ω of series resistance.<ref>Frank Spitzer, Barry Howarth ''Principles of modern instrumentation'', Holt, Rinehart and Winston, 1972 {{ISBN|0-03-080208-3}} pp. 32–40</ref> For analog current ranges, matched low-resistance [[Shunt (electrical)|shunts]] are connected in parallel with the meter movement to divert most of the current around the coil. Again for the case of a hypothetical 1&nbsp;mA, 500&nbsp;Ω movement on a 1&nbsp;A range, the shunt resistance would be just over 0.5&nbsp;Ω.

Moving coil instruments can respond only to the average value of the current through them.  To measure alternating current, which changes up and down repeatedly, a [[rectifier]] is inserted in the circuit so that each negative half cycle is inverted;  the result is a varying and nonzero DC voltage whose maximum value will be half the AC peak to peak voltage, assuming a symmetrical waveform.  Since the rectified average value and the [[root mean square]] (RMS) value of a waveform are only the same for a square wave, simple rectifier-type circuits can only be calibrated for sinusoidal waveforms.  Other wave shapes require a different calibration factor to relate RMS and average value. This type of circuit usually has fairly limited frequency range. Since practical rectifiers have non-zero voltage drop, accuracy and sensitivity is poor at low AC voltage values.<ref>Stephen A. Dyer,  ''Wiley Survey of Instrumentation and Measurement'', John Wiley & Sons, 2004 {{ISBN|0471221651}},  pp. 277–281</ref>

To measure resistance, switches arrange for a small battery within the instrument to pass a current through the device under test and the meter coil. Since the current available depends on the state of charge of the battery which changes over time, a multimeter usually has an adjustment for the ohm scale to zero it. In the usual circuits found in analog multimeters, the meter deflection is inversely proportional to the resistance, so full-scale will be 0&nbsp;Ω, and higher resistance will correspond 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 thus becomes a [[voltage divider]]. Where AC measurements are required, the rectifier can be placed after the amplifier stage, improving precision at low range.

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, which implies delicate mechanisms. 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 1,000&nbsp;Ω/V would draw 1&nbsp;mA from a circuit at full-scale deflection.<ref>Frank Spitzer and Barry Horwath ''Principles of Modern Instrumentation'', Holt, Rinehart and Winston Inc., New York 1972, no ISBN, Library of Congress 72-77731, p. 39</ref> More expensive, (and mechanically more delicate) multimeters typically have sensitivities of 20,000 ohms per volt and sometimes higher, with 50,000 ohms per volt (drawing 20&nbsp;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, some analog multimeters use an amplifier inserted between the measured circuit and the meter movement. While this increases the expense and complexity of the meter, by use of [[vacuum tube]]s or [[field effect transistor]]s 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 VTVMs (vacuum tube voltmeters),<ref>{{cite web | work = tone-lizard.com | url = http://www.tone-lizard.com/VTVM.htm | archive-url = https://web.archive.org/web/20031006061034/http://www.tone-lizard.com/VTVM.htm | url-status = dead | archive-date = 2003-10-06 | access-date = 2007-01-28 | title = The Incomplete Idiot's Guide to VTVMs }}</ref> TVMs (transistor volt meters), FET-VOMs, and similar names.

Analog meters are intuitive where the trend of a measurement was more important than an exact value obtained at a particular moment. A change in angle or in a proportion is easier to interpret than a change in the value of a digital readout. For this reason, some digital multimeters additionally have a bar graph as a second display, typically with a more rapid sampling rate than used for the primary readout. These fast sampling rate bar graphs have a superior response than the physical pointer of analog meters, obsoleting the older technology. With rapidly fluctuating DC, AC or a combination of both, advanced digital meters are able to track and display fluctuations better than analog meters whilst also having the ability to separate and simultaneously display DC and AC components.<ref name="Joe_Smith">{{Cite web|url=https://www.youtube.com/watch?v=K-4L2JarVxA| archive-url=https://ghostarchive.org/varchive/youtube/20211117/K-4L2JarVxA| archive-date=2021-11-17 | url-status=live|title="Brymen BM869s vs Fluke"|last=Smith|first=Joe|date=August 24, 2014|website=YouTube|access-date=March 17, 2020}}{{cbignore}}</ref>

Because of the absence of amplification, ordinary analog multimeter are typically less susceptible to [[radio frequency interference]], and so continue to have a prominent place in some fields even in a world of more accurate and flexible electronic multimeters.<ref>{{cite book|title=The ARRL Handbook for Radio Communications|year=2008|isbn=978-0-87259-101-1|last=Wilson|first=Mark|publisher=American Radio Relay League }}</ref>

Analog meter movements are inherently more fragile physically and electrically than digital meters. Many analog multimeters feature a range switch position marked "off" to protect the meter movement during transportation which places a low resistance across the meter movement, resulting in [[dynamic braking]]. Meter movements as separate components 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.

High-quality analog multimeters continue to be made by several manufacturers, including Chauvin Arnoux (France), Gossen Metrawatt (Germany), and Simpson and Triplett (USA).{{citation needed|date=March 2023}}

=== Digital ===
[[File:Benchtop multimeter.jpg|thumb|A bench-top multimeter]]
[[File:PowerbankEnergizerQE10007PQ20210904191417pixelated20210905.jpg|thumb|USB-powered multimeter for examining [[battery charging]] of [[portable electronics]]]]
[[File:Fluke87-V Multimeter.jpg|thumb|upright=.6|A {{frac|4|1|2}}-digit digital multimeter]]

Digital instruments, which necessarily incorporate amplifiers, use the same principles as analog instruments for resistance readings. 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 precise current. An autoranging digital multimeter can automatically adjust the scaling network so the measurement circuits use the full precision of the A/D converter.

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 eliminates [[parallax]] errors.

Modern digital multimeters may have an [[embedded 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 most [[significant digits]] are shown. For example, a four-digit multimeter would automatically select an appropriate range to display 12.34 mV instead of 0.012 V, 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-polarity for direct-current readings, shows if the [[electric polarity]] of 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 [[P-n junction|semi conductor junctions]]. While not a replacement for a proper [[transistor tester]], and most certainly not for a swept [[curve tracer]] type, this facilitates testing diodes and a variety of transistor types.<ref>{{cite web|last=Goldwasser|first=Samuel| url=http://www.repairfaq.org/sam/semitest.htm#stttm|title=Basic Testing of Semiconductor Devices|access-date=2007-01-28}}</ref>
* 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]].<ref>{{cite web|last=Extech Instruments|url=http://www.extech.com/instrument/products/310_399/381295_381296.html | access-date = 2007-01-28|title=Extech 5&nbsp;MHz Dual Channel Multiscope}}</ref>
* Automotive circuit testers, including tests for automotive timing and dwell signals (dwell and engine rpm testing is usually available as an option and is not included in the basic automotive DMMs).
* Simple [[data acquisition]] features to record maximum and minimum readings over a given period, or to take a number of [[Sample (statistics)|samples]] at fixed intervals.<ref>{{cite web |series=Extech Instruments | url = http://www.extech.com/instrument/products/310_399/380900.html | access-date = 2007-01-28 |title=Extech Dual Channel, Datalogging multimeter |archive-url=https://web.archive.org/web/20070403235544/http://www.extech.com/instrument/products/310_399/380900.html |archive-date=2007-04-03}}</ref>
* Integration with tweezers for [[surface-mount technology]].<ref>{{cite web|last=Siborg Systems Inc|url=http://www.siborg.com/smarttweezers| access-date = 2008-04-23|title=Digital Multimeter Smart Tweezers from Siborg}}</ref>{{Better source needed|reason=Citation given is a primarily commercial website. Surely, a better source must be available.|date=January 2015}}
* A combined [[LCR meter]] for small-size SMD and through-hole components.<ref>{{ cite web | last = Advance Devices Inc | url = http://www.advancedevices.com/docs/SmartTweezersDs.pdf | archive-url = https://web.archive.org/web/20070109171501/http://www.advancedevices.com/docs/SmartTweezersDs.pdf | url-status = dead | archive-date = 2007-01-09 | access-date = 2009-01-20 | title = Smart Tweezers Digital Multimeter/LCR Meter }}</ref>

Modern meters may be interfaced with a [[personal computer]] by [[IrDA]] links, [[RS-232]] connections, [[USB]], [[Bluetooth]], or an instrument bus such as [[IEEE-488]]. The interface allows the computer to record measurements as they are made. Some DMMs can store measurements and upload them to a computer.<ref>{{cite web|last=Fluke Manufacturing | url = http://support.fluke.com/find-sales/download/asset/2386842_a_w.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://support.fluke.com/find-sales/download/asset/2386842_a_w.pdf |archive-date=2022-10-09 |url-status= live | title = Logging and analyzing events with FlukeView Forms Software|access-date=2007-01-28}}</ref>