Multimeter
A multimeter or a multitester is an electronic measuring instrument that combines several functions in one unit. The most basic instruments include an ammeter, voltmeter, and ohmmeter.
Analog multimeters are sometimes referred to as "volt-ohm-meters", abbreviated VOM. Digital multimeters are usually referred to as "digital-multi-meters", abbreviated DMM.
A multimeter can be a handheld device useful for basic fault finding and field service work or a bench instrument which can measure to a very high degree of accuracy. Such an instrument will commonly be found in a calibration lab and can be used to characterise resistance and voltage standards or adjust and verify the performance of multi-function calibrators.
Current, voltage, and resistance measurements are considered standard features for multimeters. AVO multimeters, a manufacturer of early multimeters, derived their name from amperes, volts, and ohms, the units used for the measurement of current, voltage, and resistance.
Multimeters are available in a wide ranges of features and prices (in the United States, inexpensive multimeters can be purchased for less than US$20). They are extremely handy devices to have around the house as they can be used to troubleshoot electrical problems in a wide array of household devices such as possibly dead batteries, washing and drying machines, kitchen appliances, and even automobile electronics and electrical systems.
Measured quantities
Contemporary equipment can measure many quantities besides voltage and resistance. Some common additional measured quantities and the units in which they are measured:
- Capacitance in farads.
- Frequency in hertz.
- Duty cycle as a percentage.
- Temperature in degrees Celsius or Fahrenheit.
- Conductance in siemens.
- Inductance in henrys.
A multimeter may be implemented with an analog meter deflected by an electromagnet, as a classic galvanometer, or with a digital display such as an LCD or vacuum fluorescent display.
Analog multimeters are not hard to find (though less common and often more expensive than low-end digital units), but are not considered as accurate as digital because of errors introduced in zeroing and reading the analog meter face.
Analog meters may be implemented with vacuum tubes to precondition and amplify the input signal. Such meters are known as vacuum tube volt meters (VTVM) or vacuum tube multimeters (VTMM).[1]
Resolution
The resolution of a multimeter is often specified in "digits" of resolution. The use of a digit count dates back to the 1970's when multimeter vendors were very proud of how many digits their products could display (this was important, because readout displays were costly). The vendors started to specify the maximum resolution of the multimeter based on the digital display. 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.
Accuracy
Similarly, better circuitry and electronics have improved meter accuracy. Older analog meters might have basic accuracies of five to ten percent. Modern portable DMMs may have accuracies as good as ±0.025%, and high-end bench-top instruments can have accuracies in the hundredths of parts per million figures.[2] At the other end of the spectrum, meters with ±1% basic accuracy are available for less than US$20.
Manufacturers may provide calibration services so that a new meter 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.
Common features
Modern multimeters are most commonly digital, and identified by the term DMM or digital multimeter. In such an instrument, the signal under test is converted to a voltage and an amplifier with an electronically controlled gain preconditions the signal. Since the digital display directly indicates a quantity as a number, there is no risk of parallax causing an error when viewing a reading. A few analog units are available, however, and are sometimes considered better for detecting the rate of change of a reading; electronics expert Forrest Mims suggests keeping both analog and digital meters in one's toolbox, and the 2003 ARRL handbook suggests that analog multimeters are often less susceptible to radio frequency interference.
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:
- Autoranging, 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. Autoranging 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.
- 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.[3][4]
- 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 continuity tester that beeps when a circuit conducts.
- A low-bandwidth oscilloscope.[5]
- Automotive circuit testers, including tests for automotive timing and dwell signals.[6]
- Simple data acquisition features to record maximum and minimum readings over a given period, or to take a number of samples at fixed intervals.[7]
Digital meters often feature circuitry or software to accurately measure the AC voltage at any frequency within a specified range. These meters integrate the input signal using the root mean square method, and will correctly read the true voltage of an input signal even if it is not a perfect sine wave.
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 or for the instrument to upload a series of results to the computer.[8]
As modern appliances and systems become more complicated, the multimeter is becoming less common in the technician's toolkit. More complicated and specialized equipment replaces it. Where a service man might have used an ohmmeter to measure resistance while testing an antenna, a modern technician may use a hand-held analyzer to test several parameters in order to determine the integrity of a network cable.[9]
Probes
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.
See also
References
- ^ "The Incomplete Idiot's Guide to VTVMs". tone-lizard.com. Retrieved 2007-01-28.
- ^ Agilent Technologies. "Agilent 3458A Digital Multimeter Data Sheet" (PDF). Retrieved 2007-01-28.
- ^ Hewes, John. "Testing a diode with a multimeter". Retrieved 2007-01-28.
- ^ Goldwasser, Samuel. "Basic Testing of Semiconductor Devices". Retrieved 2007-01-28.
- ^ Extech Instruments. "Extech 5MHz Dual Channel Multiscope". Retrieved 2007-01-28.
- ^ Snap-on Incorporated. "MT596AK Automotive Digital Multimeter". Retrieved 2007-01-28.
- ^ Extech Instruments. "Extech Dual Channel, Datalogging multimeter". Retrieved 2007-01-28.
- ^ Fluke Manufacturing. "Logging and analyzing events with FlukeView Forms Software" (PDF). Retrieved 2007-01-28.
- ^ Fluke Manufacturing. "Fluke DTX CableAnaylzer Series". Retrieved 2007-01-28.
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