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 seven or eight and a half digits 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 multimeter. 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.
Newer equipment can measure many other quantities. 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 in the used market, but are not very accurate 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).
The resolution of a multimeter is often specified in "digits" of resolution. The term "digits" 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. 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. This digital multimeter could show positive or negative values from 0 to 199,999. For a modern DMM, such as a PC-based multimeter, the term "digits" actually maps to the noise performance of the device.
Modern multimeters are exclusively digital, and identified by the term DMM or digital multimeter. In such an instrument, the signal under test is converted to a digital 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.
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 bench-top instruments have accuracies in the single-digit parts per million figures.
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 display 1.234 instead of 0.012.
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 continuity tester that beeps when a circuit conducts.
A low-bandwidth oscilloscope.
A telephone test set.
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.
Digital meters often feature circuitry or software to accurately measure the AC voltage at any frequency. 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 isn't 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.
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.
Monday, January 8, 2007
Ammeter
An ammeter is a measuring instrument used to measure the flow of electric current in a circuit. Electric currents are measured in amperes, hence the name. The word "ammeter" is commonly misspelled or mispronounced as "ampmeter" by some.
The earliest design is the D'Arsonval galvanometer or moving coil ammeter. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The voltage drop across the coil is kept to a minimum to minimize resistance in any circuit into which the meter is inserted.
Moving iron ammeters use a piece or pieces of iron which move when acted upon by the electromagnetic force of a fixed coil of (usually heavy gauge) wire. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only).
To measure larger currents, a resistor called a shunt is placed in parallel with the meter. Most of the current flows through the shunt, and only a small fraction flows through the meter. With this solution, arbitrarily large currents can be measured with a single meter. Traditionally, the meter used with a shunt reaches full-scale deflection when a voltage of 50mV is placed across its coil, so shunts are typically designed to produce a voltage drop of 50mV when carrying their full rated current.
More modern ammeter designs are non-mechanical, or digital, and use an analog to digital converter to measure the voltage across the shunt resistor. The ADC is read by a microcomputer that performs the calculations to display the current through the resistor.
One problem with the use of an ammeter is the need for the meter to be inserted into the circuit and become part of it. Mistakenly placing the ammeter in parallel with a circuit will blow the fuse, possibly damaging the meter and causing injury. In AC circuits, an inductive coupling adapter converts the magnetic field around a conductor into a small AC current that can be easily read by a meter. See clamp meter. In a similar way, accurate DC non-contact ammeters have been constructed using Hall effect magnetic field sensors.
The earliest design is the D'Arsonval galvanometer or moving coil ammeter. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The voltage drop across the coil is kept to a minimum to minimize resistance in any circuit into which the meter is inserted.
Moving iron ammeters use a piece or pieces of iron which move when acted upon by the electromagnetic force of a fixed coil of (usually heavy gauge) wire. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only).
To measure larger currents, a resistor called a shunt is placed in parallel with the meter. Most of the current flows through the shunt, and only a small fraction flows through the meter. With this solution, arbitrarily large currents can be measured with a single meter. Traditionally, the meter used with a shunt reaches full-scale deflection when a voltage of 50mV is placed across its coil, so shunts are typically designed to produce a voltage drop of 50mV when carrying their full rated current.
More modern ammeter designs are non-mechanical, or digital, and use an analog to digital converter to measure the voltage across the shunt resistor. The ADC is read by a microcomputer that performs the calculations to display the current through the resistor.
One problem with the use of an ammeter is the need for the meter to be inserted into the circuit and become part of it. Mistakenly placing the ammeter in parallel with a circuit will blow the fuse, possibly damaging the meter and causing injury. In AC circuits, an inductive coupling adapter converts the magnetic field around a conductor into a small AC current that can be easily read by a meter. See clamp meter. In a similar way, accurate DC non-contact ammeters have been constructed using Hall effect magnetic field sensors.
Potentiometer
A voltmeter may also be realized using a potentiometer, which is a length of uniform resistance material (wire or carbon film, for instance) and a "wiper" that can short-circuit any portion of the material, thereby changing effective resistance between the wiper and an end terminal of the potentiometer. The unknown voltage source may be connected to a current detector, which is in turn connected to the potentiometer's wiper, while the known voltage source is connected to an end terminal of the potentiometer. Then the wiper position is adjusted to change the potentiometer's effective resistance until a balance is obtained and no current is detected. At this time, record the potentiometer's wiper position. For example, if our potentiometer were a length of very long wire and our wiper were some sort of metal wand in contact with that wire, record the length of wire between the wiper and the end of the wiper that is in our circuit. Now replace the unknown voltage supply with the known voltage supply and repeat the procedure. The unknown voltage is then given by the product of the known voltage and the recorded used length of wire corresponding to the unknown voltage, divided by the recorded length of wire corresponding to the reference voltage.
Voltmeter
A voltmeter is an instrument used for measuring the potential difference between two points in an electric circuit.
The voltage can be measured by allowing it to pass a current through a resistance; therefore, a voltmeter can be seen as a very high resistance ammeter. One of the design objectives of the instrument is to disturb the circuit as little as possible and hence the instrument should draw a minimum of electric current to operate. This is achieved by using a sensitive ammeter or microammeter in series with a high resistance.
The moving coil galvanometer is one example of this type of voltmeter. It employs a small coil of fine wire suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current that is flowing through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.
The voltage can be measured by allowing it to pass a current through a resistance; therefore, a voltmeter can be seen as a very high resistance ammeter. One of the design objectives of the instrument is to disturb the circuit as little as possible and hence the instrument should draw a minimum of electric current to operate. This is achieved by using a sensitive ammeter or microammeter in series with a high resistance.
The moving coil galvanometer is one example of this type of voltmeter. It employs a small coil of fine wire suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current that is flowing through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.
Voltage
Voltage is the difference of electrical potential between two points of an electronic circuit, expressed in volts. It is a measure of the capacity (not the technical meaning) of an electric field to cause an electric current in an electrical conductor. Depending on the difference of electrical potential it is called extra low voltage, low voltage, high voltage or extra high voltage.
Between two points in an electric field, such as exists in an electrical circuit, the difference in their electrical potentials is known as the electrical potential difference. This difference is proportional to the electrostatic force that tends to push electrons or other charge-carriers from one point to the other. Potential difference, electrical potential, and electromotive force are measured in volts, leading to the commonly used term voltage. Voltage is usually represented in equations by the symbols V, U, or E. (E is often preferred in academic writing, because it avoids the confusion between V and the SI symbol for the volt, which is also V.)
Electrical potential difference can be thought of as the ability to move electrical charge through a resistance. At a time in physics when the word force was used loosely, the potential difference was named the electromotive force or EMF—a term which is still used in certain contexts.
Voltage is a property of an electric field, not individual electrons. An electron moving across a voltage difference experiences a net change in energy, often measured in electron-volts. This effect is analogous to a mass falling through a given height difference in a gravitational field.
When using the term 'potential difference' or voltage, one must be clear about the two points between which the voltage is specified or measured. There are two ways in which the term is used. This can lead to some confusion.
Between two points in an electric field, such as exists in an electrical circuit, the difference in their electrical potentials is known as the electrical potential difference. This difference is proportional to the electrostatic force that tends to push electrons or other charge-carriers from one point to the other. Potential difference, electrical potential, and electromotive force are measured in volts, leading to the commonly used term voltage. Voltage is usually represented in equations by the symbols V, U, or E. (E is often preferred in academic writing, because it avoids the confusion between V and the SI symbol for the volt, which is also V.)
Electrical potential difference can be thought of as the ability to move electrical charge through a resistance. At a time in physics when the word force was used loosely, the potential difference was named the electromotive force or EMF—a term which is still used in certain contexts.
Voltage is a property of an electric field, not individual electrons. An electron moving across a voltage difference experiences a net change in energy, often measured in electron-volts. This effect is analogous to a mass falling through a given height difference in a gravitational field.
When using the term 'potential difference' or voltage, one must be clear about the two points between which the voltage is specified or measured. There are two ways in which the term is used. This can lead to some confusion.
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