multimeter or multitester , also known as VOM (volt-ohm-milliammeter), is an electronic measuring device that combines multiple measurement functions in a single unit. Common multimeters can measure voltage, current, and resistance. Analog multimeters use microammeter with moving pointer to display readings. Digital multimeters (DMM, DVOM) have numeric views, and may also display a graphical bar that represents measurable values. Digital multimeters are now much more common because of their cost and precision, but analog multimeters are still preferred in some cases, for example when monitoring rapidly changing values.
Multimeters can be handheld devices useful for finding basic errors and field service work, or bench instruments that can measure to a very high degree of accuracy. They can be used to solve electrical problems in a variety of industrial and household appliances such as electronic equipment, motor controls, household appliances, power supplies, and cabling systems.
Multimeters are available in various features and prices. Inexpensive multimeters can cost less than US $ 10, while certified classroom lab models can cost more than US $ 5,000.
Video Multimeter
History
The first moving-pointer pointing device was a galvanometer in 1820. It was used to measure resistance and stress by using the Wheatstone bridge, and comparing unknown quantities with voltage or reference resistance. Although useful in the lab, the device is very slow and not practical in the field. This galmometer is large and smooth.
The D'Arsonval/Weston meter movement uses a moving coil that carries a pointer and spins on a fast pivot or ligament band. The coil spins in a permanent magnetic field and is retained by a fine spiral spring that also serves to bring the current to the moving coil. This provides a proportional measurement rather than just detection, and the deflection does not depend on the orientation of the meter. Instead of balancing the bridge, the value can be read directly from the scale of the instrument, which makes measurements quick and easy.
The basic moving coil meter is only suitable for direct current measurements, typically in the range of 10 microamperes to 100 mA. It's easily adapted for reading heavier currents by using shunts (resistance in parallel with basic motion) or reading voltages using a series of resistances known as multipliers. To read the current or alternating voltage, a rectifier is required. One of the most suitable rectifiers is a copper oxide rectifier developed and produced by Union Switch & amp; Signal Company, Swissvale, Pennsylvania, the next part of Westinghouse Brake and Signal Company, from 1927.
Multimeters were invented in the early 1920s as radio receivers and electronic tube vacuum devices became more common. The invention of the first multimeter was associated with British Post Office engineer Donald Macadie, who became dissatisfied with the need to bring as many separate instruments as necessary for the maintenance of telecommunication circuits. Macadie invented instruments that could measure amperes (amps), volts and ohms, so that the multifunctional meter was then named Avometer. The meter consists of a driving coil, a voltage and precision resistor, and a switch and a socket to select a range.
Automatic Coil Winder and Electrical Equipment Company (ACWEECO), established in 1923, was established to manufacture Avometer and coil rolling machines which are also designed and patented by MacAdie. Despite the shareholders of ACWEECO, Mr. MacAdie continued to work for the Post Office until his retirement in 1933. His son, Hugh S. MacAdie, joined ACWEECO in 1927 and became Technical Director. AVO was first sold in 1923, and many of its features remain virtually unchanged until the last 8 models.
Maps Multimeter
General properties of multimeters
Each meter will load the circuit being tested to some extent. For example, a multimeter using a moving coil motion with a current full-scale deflection of 50 microampok, the highest sensitivity normally available, should draw at least 50 microampels from the circuit tested for meters to reach the upper end of the scale. It may contain a high impedance circuit that affects the circuit, thus providing a low reading. A full-scale deflection can also be expressed in terms of "ohms per volt". The ohm number per volt is often called the "sensitivity" of the instrument. So a meter with a 50 microampere movement will have a "sensitivity" of 20,000 ohms per volt. "Per volt" refers to the fact that the impedance that a given meter for the circuit under test will be 20,000 ohm multiplied by the full-scale voltage set by the meter. For example, if the meter is set to a full scale 300 volt range, the impedance meter will be 6 megabms. 20,000 ohms per volt is the best (highest) sensitivity available for common analog multimeters that do not have internal amplifiers. For meters that have internal amplifiers (VTVMs, FETVMs, etc.), the input impedance is set by the amplifier circuit.
The first avometer has a sensitivity of 60 ohms per volt, three direct current range (12 mA, 1.2 A, and 12 A), three direct voltage ranges (12, 120, and 600 V or optionally 1200 V), and 10,000 ohm resistance Range. The improved 1927 version enhances this to 13-range and 166.6 ohms per volt (6 mA) movement. The "Universal" version has additional alternating current and alternating voltage ranges offered from 1933 and in 1936 the dual sensitivity Avometer Model 7 offers 500/100 ohms per volt. Between the mid-1930s and the 1950s, 1,000 ohms per volt became the de facto standard of sensitivity for radio work and this figure is often quoted on the service sheet. However, some manufacturers such as Simpson, Triplett and Weston, all in the US, produced 20,000 ohm per volt VOM before the Second World War and some were exported. After 1945/6, 20,000 ohms per volt became the expected standard for electronics but some makers offer more sensitive instruments. For industry and other "heavy current" uses, low-sensitivity multimeters continue to be produced and this is considered to be stronger than the more sensitive types.
High-quality analog (analog) multimeters continue to be made by several manufacturers, including Chauvin Arnaux (France), Gossen Metrawatt (Germany), and Simpson and Triplett (USA).
Pocket watch gauges were widely used in the 1920s. Metal casing is usually connected to a negative connection, a setting that causes many electric shocks. The technical specifications of these devices are often crude, for example those described as having a resistance of only 33 ohms per volt, non-linear scales and zero adjustment.
Vacuum Tube Voltmeters or valve voltmeters (VTVM, VVM) are used for voltage measurement in electronic circuits where high input impedance is required. VTVM has a fixed input impedance usually 1 megohm or more, usually through the use of cathode follower input circuits, and thus not significantly loading the tested circuit. VTVM is used before the introduction of high-impedance analog transistors and voltage field effect transistors (FETVOMs). Modern digital meters (DVMs) and some modern analog meters also use electronic input circuits to achieve high input impedance - the voltage range is functionally equivalent to VTVM. The input impedance of some poorly designed DVMs (especially some initial designs) will vary during the internal measurement cycle of the sample and hold, causing interference on some sensitive circuits being tested.
Additional scales such as decibels, and measurement functions such as capacitance, transistor gain, frequency, duty cycle, display retention, and continuity are buzzer when a small measured resistance has been inserted on many multimeters. While multimeters can be equipped with more specialized equipment in the technician's toolkit, some multimeters include additional functions for special applications (temperature with thermocouple probe, inductance, connectivity to computer, speech measurable value, etc.).
Operation
Multimeters are a combination of DC multirange voltmeter, multirange AC voltmeter, multirange ammeter, and multirange ohmmeter. Un-reinforced analog amplifiers incorporate motion meters, resistors and range switches; VTVMs are amplified analog meters and contain active circuits.
For analog meter movement, the DC voltage is measured with series resistors connected between the meter movement and the circuit under test. Switches (usually rotating) allow for greater resistance to be inserted in series with the motion meter to read higher voltages. The product of the full-scale deflection of the base of the movement, and the sum of the series resistance and the movement resistance itself, provides a full-scale voltage of the range. For example, the motion meter required 1 milliampere for full-scale deflection, with a 500 ohm internal resistance, would, in the 10-volt range of the multimeter, have 9,500 ohm of series resistance.
For analog current ranges, the corresponding low-resistance shunts are connected in parallel with the motion of the meter to switch most of the current around the coil. Again for a hypothetical 1 mA case, 500 ohm movement at 1 ampere range, shunt resistance will be more than 0.5 ohms.
The moving instrument can only respond to the average value of the current through it. To measure the alternating current, which changes up and down repeatedly, the rectifier is inserted into the circuit so that every half of the negative cycle is reversed; The result is a variable and non-zero DC voltage whose maximum value will be half of the peak AC to peak voltage, assuming a symmetrical waveform. Because the improved mean values ​​and the square-square values ​​of the waveforms are just the same for square wave, simple rectifier type circuits can only be calibrated for sinusoidal waveforms. Other waveforms require different calibration factors to connect RMS and average values. This type of circuit usually has a fairly limited frequency range. Because the practical rectifier has a non-zero voltage drop, accuracy and poor sensitivity at low AC voltage values.
To measure resistance, the switch regulates the small battery inside the instrument to pass through the current through the device being tested and the meter coil. Since it is currently available depending on the state of battery charge that changes over time, the multimeter usually has an adjustment for the ohm scale to zero it. In ordinary circuits found in analog multimeters, the deflection meter is inversely proportional to resistance, so the full scale will be 0 ohms, and the higher resistance will correspond to the smaller deflection. The ohms scale is compressed, so the resolution is better at lower resistance values.
The amplified instrument simplifies the series design and the shunt resistor network. The internal resistance of the coils is separated from series selection 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, increasing the precision in the low range.
Digital instruments, which need to include amplifiers, use the same principles as analog instruments for resistance readings. For measurement of resistance, usually a small constant current is passed through the device under test and the digital multimeter reads the resulting voltage drop; this eliminates the compression scales found in analog meters, but it requires the right current source. Autoranging digital multimeters can automatically adjust the network scale so that the measurement circuit uses full precision of the A/D converter.
In all types of multimeters, the quality of switching elements is essential for stable and accurate measurements. The best DMMs use gold-plated contacts on their switches; meter cheaper using nickel plating or not at all, relying on the printed solder printed circuit board for contacts. Accuracy and stability (eg, temperature variation, or aging, or history of voltage/current) of the internal meter resistor (and other components) is a limiting factor in the accuracy and long-term accuracy of the instrument.
Measurable amount
Contemporary multimeters can measure a lot of quantity. The most common are:
- Voltage, back and forth, in volts.
- Current, alternately and instantly, in amperes.
Accurate frequency range of AC measurements is important, depending on circuit design and construction, and should be determined, so that users can evaluate the readings they take. Some meters measure current as low as milliamps or even microamps. All meters have a load voltage (caused by a combination of the shunt used and the meter circuit design), and some (even expensive) meters have a high enough load voltage so that low current readings are severely disrupted. The meter specification should include the meter load voltage. - Resistance in ohms.
In addition, some multimeters also measure:
- Capacitance in farads, but usually the range range is between a few hundred or thousand micro farads and some pico farads. Very few common multimeters can measure other important aspects of capacitor status such as ESR, dissipation factor, or leakage.
- Conductance in siemens, which is the opposite of the measured resistance.
- Decibel in the circuit, rarely in sound.
- Duty cycle as a percentage.
- Frequency in hertz.
- Inductance in henry. As with capacitance measurement, this is usually better handled by purpose of designed inductance/capacitance meter.
- Temperatures in degrees Celsius or Fahrenheit, with the appropriate temperature test probe, are often thermocouples.
Digital multimeters can also include circuits for:
- Continuity tester; buzzer sound when the circuit resistance is low enough (how far enough to vary from meter to meter), so testing should be treated as improper.
- Diodes (measure down the diode connection forward).
- Transistor (measure current gain and other parameters in some transistor types)
- Checking the battery for a simple 1.5-volt and 9-volt battery. This is the current loaded measurement, which simulates the battery load used; normal voltage range draws a very small current from the battery.
Various sensors can be attached to (or included in) a multimeter to perform measurements such as:
- light level
- sound pressure level
- acidity/alkalinity (pH)
- relative humidity
- very small current (to nanoamps with multiple adapters)
- very small resistance (up to micro ohms for some adapters)
- large currents - available adapters that use inductance (AC currents only) or Hall effect sensors (AC and DC current), usually through isolated threaded clips to avoid direct contact with dangerous high-capacity circuits, for meter and to operator
- Very high voltage - available adapters that form voltage dividers with internal meter resistance, allowing measurements into thousands of volts. However, very high voltages often have surprising behavior, apart from effects on the operator (possibly fatal); a high voltage that actually reaches the internal meter circuit can damage the internal parts, may damage the meter or damage its performance permanently.
Resolution
Resolution and accuracy
The multimeter resolution is the smallest part of the scale that can be shown, which depends on the scale. In some digital multimeters can be configured, with higher resolution measurements take longer to complete. For example, a multimeter having a resolution of 1 mV on a scale of 10Ã, V can show a change in measurement in an increase of 1 mV.
Absolute accuracy is a measurement error compared to a perfect measurement. The relative accuracy is a measurement error compared to the device used to calibrate the multimeter. Most multimeter data sheets provide relative accuracy. To calculate the absolute accuracy of 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.
Digital
Multimeter resolution is often specified in the number of decimal digits completed and displayed. If the most significant digits can not retrieve all values ​​from 0 to 9, generally, and are confusing, they are called fractional digits. For example, a multimeter that can read until 19999 (plus embedded decimal point) is said to read 4 ½ digits.
Under the convention, if the most significant digit can be 0 or 1, it is called a half digit; if it can take a higher value without reaching 9 (often 3 or 5), it can be called three quarters of the digits. A 5.5 million multimeter will display a "half digit" which can only display 0 or 1, followed by five digits taking all values ​​from 0 to 9. Such a meter can show a positive or negative value from 0 to 199.999. The 3¾¾ digit gauge can display quantities from 0 to 3,999 or 5,999, depending on the manufacturer.
While digital displays can be easily extended in resolution, extra digits are not valuable if not accompanied by care in the design and calibration of analog parts of the multimeter. Significant measurements (ie, high accuracy) require a good understanding of instrument specifications, good control of the measurement conditions, and traceability of instrument calibration. However, even if the resolution exceeds accuracy, meters can be useful for comparing measurements. For example, a stable 5 ½-digit meter reading can show that a nominal 100,000 ohm resistor is about 7 ohms larger than the other, although the error of each measurement is 0.2% of the readings plus 0.05% of the full-scale value.
Specifying "display count" is another way of determining resolution. The number of screens gives the largest number, or the largest number plus one (to include a view all zeros) multimeter view can show, ignore the decimal separator. For example, a 5 ½ digit multimeter can also be specified as a screen count of 199999 or a multimeter count of 200000 displays. Often the number of views is simply called a 'count' in the multimeter spec.
The accuracy of digital multimeters can be expressed in two-term terms, such as "Â ± 1% reading 2 counts", which reflects the various sources of error in the instrument.
Analog
Analog meters are older designs, but are still favored by many engineers. One reason is that analog meters are more sensitive to changes in the circuit being measured. Digital multimeter samples of quantity are measured and then display them. The analog multimeter keeps reading the test values. If there is a slight change in the reading, the analog multimeter needle will track it while the digital multimeter may be misses or hard to read. This continuous tracking feature becomes important when testing capacitors or coils. A well-functioning capacitor should allow the current to flow when the voltage is applied, then the current slowly decreases to zero and the "signature" is easily seen in the analog multimeter but not the digital multimeter. This is similar when testing the coil, except when it starts low and rises.
Resistance measurements in analog meters, in particular, have low accuracy due to typical resistance measurement circuits that condense the weight scale at higher resistance values. Cheap analog meters may only have a single resistance scale, severely limiting the exact range of measurements. Usually the analog meter will have a panel adjustment to adjust the zero-ohm meter calibration, to compensate for the various voltage meter batteries.
Accuracy
Digital multimeters generally take measurements with an accuracy that is superior to their analog. Standard analog multimeters measure with an accuracy usually  ± 3%, although higher accuracy instruments are made. A standard portable digital multimeter is specified to have an accuracy of typically  ± 0.5% in the DC voltage range. Mainstream bench-top multimeters are available with a predetermined accuracy better than Ã,  ± 0.01%. Laboratory class instruments can have an accuracy of several parts per million.
Accuracy figures need to be interpreted with caution. The accuracy of analog instruments usually refers to a full-scale deflection; measurements of 30Ã, V on a scale of 100Ã,V of 3% meters are subject to errors 3Ã, V, 10% of readings. The digital meter usually determines the accuracy as a percentage of readings plus a percentage of full-scale value, sometimes expressed in quantities rather than percentage terms.
Known accuracy is determined as that of the lower millivolt (mV) DC range, and is known as the "baseline DC voltage accuracy" number. Higher DC voltage ranges, currents, resistance, AC and other ranges will usually have lower accuracy than the basic DC voltage figure. AC measurements only meet certain accuracy within a certain frequency range.
Manufacturers can provide calibration services so that new meters can be purchased with calibration certificates indicating the meter has been adjusted to standards that can be traced to, for example, the US National Institute of Standards and Technology (NIST), or other national standards organizations.
Test equipment tends to drift from calibration over time, and the specified accuracy is unreliable without limit. For more expensive equipment, manufacturers and third parties provide calibration services so that older equipment can be recalibrated and re-certified. The cost of such services is not proportional to cheap equipment; However extreme accuracy is not required for most routine testing. The multimeters used for critical measurement can be part of the metrology program to ensure calibration.
A multimeter can be assumed to be an "average response" to an AC waveform unless otherwise specified as the "True RMS" type. The average response multimeter will only meet the accuracy specified on the volt and the AC ampere for a pure sinusoidal waveform. True RMSs that respond to multimeters on the other hand will meet the accuracy specified on AC volt and current with any wave type up to a certain peak factor; The RMS performance is sometimes claimed for meters that report accurate RMS readings only on certain frequencies (usually low) and with certain waveforms (basically always sine waves).
AC voltage and meter current accuracy may have different specifications at different frequencies.
Sensitivity and input impedance
When used to measure voltage, the input impedance of the multimeter must be very high compared to the measured circuit impedance; otherwise, the circuit operation can be changed, and the readings are also inaccurate.
Meters with electronic amplifiers (all digital multimeters and several analog meters) have a fixed input impedance high enough not to disturb most circuits. This is often one or ten megohms; standardization of input resistance allows the use of external high resistance probes that form a voltage divider with input resistance to extend the voltage range up to tens of thousands of volts. High-end multimeters generally provide input impedance & gt; 10 Gigaohms for ranges less than or equal to 10Ã, V. Some high-end multimeters provide & gt; 10 Gigaohms impedance to a range greater than 10Ã, V.
Most analog multimeters of the move pointer type are not supported, and draw current from the circuit being tested to deflect the meter pointer. The meter impedance varies depending on the base motion sensitivity of the meter and the selected range. For example, a meter with a typical sensitivity of 20,000 ohm/volt will have an input resistance of two million ohms in the 100-volt range (100 V * 20,000 ohm/volt = 2,000,000 ohm). At each range, at the full-scale voltage of the range, the full current required to deflect the meter movement is taken from the circuit under test. The lower sensitivity meter movement is acceptable for testing in circuits where the source impedance is low compared to the impedance meter, for example, the power circuit; This meter is more mechanically coarse. Some measurements in signal circuits require a higher sensitivity movement in order not to load the circuit tested by impedance meter.
The sensitivity should not be confused with the resolution of the meter, which is defined as the lowest signal change (voltage, current, resistance...) that can alter the observed readings.
For multipurpose digital multimeters, the lowest voltage range is usually 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 electrical use (major) rather than the use of common electronic techniques will usually sacrifice the current range of microamps.
Low resistance measurements require lead resistance (measured by touching the test probe together) to be reduced for best accuracy. This can be done with the "delta", "Zero", or "zero" features of many digital multimeters.
The upper end of the multimeter measurement range varies greatly; measurements over maybe 600 volts, 10 amperes, or 100 megohms may require special test instruments.
Voltage load
Each inline series connected ammeter, including multimeters in the current range, has a certain resistance. Most multimeters inherently measure the voltage, and pass the current to be measured through the shunt resistance, measuring the voltage developed through it. The voltage drop is known as the load voltage, which is determined in volts per ampere. Values ​​may change depending on the selected range of meters, since different ranges usually use different shunt resistors.
The load voltage can be significant in the area of ​​low voltage circuits. To check the effect on the accuracy and operation of the external circuit, meters can be diverted to different ranges; the current reading should be the same and the circuit operation should not be affected if the load voltage is not a problem. If this voltage can be significantly reduced (also reduces the inherent accuracy and measurement accuracy) by using a higher current range.
Sensing alternating current
Since the basic indicator system in either analog or digital meter responds only to DC, the multimeter includes the AC to DC conversion circuit to make alternating current measurements. The base meter uses a rectifier circuit to measure the average absolute value or peak voltage, but calibrated to show the mean square root value (RMS) calculated for the sinusoidal waveform; this will give the correct readings for alternating current as used in the power distribution. The user guide for several meters provides a correction factor for some simple non-sinusoidal waveforms, to allow the exact equivalent value of the root mean square (RMS) to be calculated. The more expensive multimeters include AC to DC converters that measure the true RMS values ​​of waveforms within certain limits; the user manual for the meter can show the limits of the peak factor and the frequency at which the meter calibration is valid. RMS sensing is required for measurements on non-sinusoidal periodic waveforms, such as those found in audio signals and variable-frequency drives.
Digital Multimeter (DMM or DVOM)
Modern multimeters are often digital because of their accuracy, durability and additional features. In a digital multimeter, the tested signal is converted to a voltage and an amplifier with electronically controlled gain initiates the signal. The digital multimeter displays quantities that are measured as numbers, which eliminates parallax errors.
Modern digital multimeters may have an embedded computer, which provides many convenience features. Increases in available measurements include:
- Auto start , which selects the exact range for the quantity being tested so that the most significant digits are displayed. For example, a four-digit multimeter will automatically select the appropriate range to display 1,234 instead of 0.012, or overloaded. The auto-moving meter usually includes a facility to hold the meter to a certain range, since the measurements that cause the range changes can often interfere with the user.
- Automatic polarity for current direct reading, indicating whether the applied voltage is positive (according to meter introduction label) or negative (opposite polarity with meter direction).
- Sample and hold , which will install the latest reading for review after the instrument is removed from the circuit under test.
- The test current is limited to voltage drop across the semiconductor junction. Although not a substitute for a proper transistor tester, and certainly not for the sweep type tracer curve, it facilitates testing of diodes and different types of transistors.
- The graph representation of the number being tested, as a bar graph. This makes testing unusable, and also allows finding fast-moving trends.
- A oscilloscope is low voltage
- Automotive circuit testers, including test for automotive occupant time and signals (dwell and engine rpm testing are usually available as an option and are not included in the basic automotive DMM).
- Simple data acquisition feature to record maximum and minimum readings over a given period, or to take samples at fixed intervals.
- Integration with tweezers for surface-mount technology.
- A combined LCR gauge for SMD components and through small holes.
The modern meter can be connected to a personal computer with an IrDA link, RS-232 connection, USB or instrument bus such as IEEE-488. The interface allows the computer to record measurements when created. Some DMMs can store measurements and upload them to a computer.
The first digital multimeter was produced in 1955 by Non Linear Systems. It is claimed that the first digital multimeter handheld was developed by Frank Bishop of Intron Electronics in 1977, which at the time presented a major breakthrough for service and error discovery in the field.
Analog multimeter
Multimeters can be implemented with galvanometer meter movements, or more rarely with bargraph or simulation pointers such as LCD or vacuum fluorescent screens. Analog multimeters are common; quality analog instruments cost the same as DMM. The analogue multimeter has the precision and read accuracy described above, and is therefore not built to provide the same accuracy as a digital instrument.
Analogue gauges are also useful where measurement trends are more important than fixed values ​​obtained at a given moment. Changes in angles or in proportions are easier to interpret than changes in digital readings. For this reason, digital multimeters often approach this with bargraph (usually with a faster reading response than primary reading); the most effective is arranged in the arc, to simulate the pointer of the analogue meter.
The movement of an analog meter is inherently more physically and electrically fragile than a digital meter. Many analog multimeters display a switch position of the range marked "off" to protect the movement of meters during transport where placing low resistance along the movement of the meter, resulting in dynamic braking. The movement of meters as separate components can be protected in the same way by connecting shorting or jumper cables between terminals when not in use. Meters that have a shunt across windings such as an ammeter may not require further resistance to capture uncontrolled needle meter movement due to the low resistance of the shunt.
Motion meter movement in practically analog multimeters is always a moving-coil galvanometer of d'Arsonval type, either using a gem pivot or a fast band to support the moving coil. In a basic analogue multimeter, the current to bend the coil and the pointer is taken from the measured circuit; it is usually an advantage to minimize the current drawn from the circuit, which implies a complicated mechanism. The sensitivity of the analog multimeter is given in ohms per volt. For example, a very low cost multimeter with a 1000 ohm per volt sensitivity would pull 1 milliampere from the circuit on a full-scale deflection. The more expensive, (and mechanically more sensitive) multimeters typically have a sensitivity of 20,000 ohms per volt and sometimes higher, with 50,000 ohms per volt (drawing 20 microamperes at full scale) to about the upper limit for portable, general purpose, amplified analog multimeter.
To avoid loading the circuit measured by the current drawn by the motion meter, some analog multimeters use an amplifier that is inserted between the measured circuit and the meter movement. While this increases the cost and complexity of the meter, using a vacuum tube or field effect transistor, the input resistance can be made very high and independent of the current required to operate the meter motion coil. The reinforced multimeters are called VTVM (vacuum tube voltmeter), TVM (transistor volt meter), FET-VOM, and similar names.
Due to the absence of amplification, ordinary analogue multimeters are usually less susceptible to radio frequency interference, thus continuing to have a prominent place in some areas even in a more accurate and flexible world of electronic multimeters.
Probe
Multimeters can use many different test probes to connect to the circuit or device under test. Crocodile clips, retractable hook clips, and pointed probes are the three most common types. Clamp probes are used for melee test points, such as surface mount devices. The connectors are mounted on a flexible, well-insulated end that ends with a corresponding connector for the meter. The probe is connected to a portable meter usually with a hidden or hidden banana jack, while a benchtop meter can use a banana jack or BNC connector. 2 mm plugs and post bindings have also been used at times, but are much less used today. Indeed, the security rating now requires a veiled banana jack.
The banana jack is usually placed with a standard center-to-center range of 0.75 in (19 mm), to allow a standard adapter or device such as a voltage multiplier or thermocouple probe to be installed.
Clamp meters clamp around the conductor carrying current to measure without the need to connect the meter in series with the circuit, or make metal contacts altogether. They are for AC measurement using transformer principle; clamp-on meters for measuring small currents or direct currents require more exotic sensors such as hall-based systems that measure unchanged magnetic fields to determine current.
Security
Most multimeters include a fuse, or two fuses, which sometimes prevent multimeter damage from excess currents at the current highest range. (For additional security, available test cables with available fuses.) A common mistake 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. Unused meters are often quickly destroyed by such mistakes; fused meters often survive. The fuse used in the meter must carry the maximum measurement current from the instrument, but is intended to disconnect if the operator error exposes the meter to a low impedance error. Meters with inadequate or unsafe fuses are not uncommon; this situation has led to the creation of the IEC61010 category to assess the security and robustness of meters.
The digital meter is ranked into four categories based on the intended application, as defined by IEC 61010-1 and echoed by state and regional standard groups such as the CEN EN61010 standard.
- Category I : used if the equipment is not directly connected to a power source
- Category II : used on a single phase parent end sub-circuit
- Category III : used on permanent installed loads such as distribution panels, motors, and 3-phase tool outlets
- Category IV : used at locations where the current error rate can be very high, such as supply service entrance, main panel, supply meter and main high voltage protection equipment
Each Rank Category also sets the maximum safe transient voltage for the selected measurement range in meters. The category-ranking meter also shows protection from excessive errors. In meters that allow interacting with computers, optical isolation can be used to protect attached equipment against high voltages in measured circuits.
Good quality multimeters designed to meet CAT II standards and above including High Pressure ceramic capacities typically rated over 20 kA capacity; this is less likely to fail explosively than the more common glass fuse. They will also include high-voltage MOV (Metal Oxide Varistor) protection, and overcurrent protection circuits in the form of Polyswitch.
DMM alternatives
The general quality of DMM electronics is generally considered adequate for measurements at signal levels greater than one millivolt or one microampere, or below about 100 megabms; these values ​​are far from the theoretical limits of sensitivity, and are very interesting in some circuit design situations. Other instruments - essentially similar, but with higher sensitivity - are used for accurate measurements in 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) and picoammeters. Accessories for the more common multimeters allow some of these measurements as well. The measurement is limited by available technology, and ultimately by inherent thermal noise.
Power supply
The analog meter can measure voltage and current using the power of the test circuit, but requires an additional internal voltage source for endurance testing, while the electronic meter always needs an internal power supply to run its internal circuitry. The handheld meter uses the battery, while the bench meter usually uses electric power; fine setting allows the meter to test the device. Testing often requires that the component being tested is isolated from the circuit in which they are installed, since the irregular path or leakage current may distort the measurement. In some cases, the voltage of the multimeter may activate the active device, distort the measurement, or in extreme cases even damage the elements in the circuit under investigation.
Security
This is most secure (for multimeters, tested circuits, and operators) to disconnect components from its circuit, and almost always, to remove power from the device under investigation. Removing all electrical connections from electrical equipment before testing (and ensuring that all large capacitance devices are disposed of safely) is the safest option. Leaving equipment attached to the power supply while making measurements should only be an alternative choice that is very carefully considered. Among other issues, there is an interaction between the basic settings for wall-tested test devices, and tested devices, which are unsafe, and may damage test equipment and devices under test. This is especially true when there are errors, suspected or not, on one connected device. Battery test devices can be the safest option in such situations.
Meters intended for testing in hazardous locations or for use on blasting circuits may require the use of factory-specified batteries to maintain their safety ratings.
See also
- Electronic test equipment
References
External links
- How to use multimeter, sparkfun
- High definition video tutorial video
- How to select Multimeter - Discusses the key considerations for choosing the right multimeter.
- Multimeter Tutorial - Lots of useful information about using digital multimeters.
- Beginner's Guide to 830B Multimeters - Discusses the abundance of cheap digital multimeters at home.
- How to Determine Digital Multimeter Accuracy
- ABC Multimeter Security - (PDF)
- Digital Multimeter - From theory to use
- Multimeters and How to Use It
- Measurement Circuit DC chapter of Lessons In Electrical Circuit Vol 1 DC Free eBook and Lessons In Circuit Electrical series.
Source of the article : Wikipedia
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