Why Does a Voltmeter Reading on Open Circuit When Placed Across the Switch Twonterminals
Demonstration analog voltmeter
A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. Information technology usually has a high resistance then that information technology takes negligible current from the circuit.
Analog voltmeters move a pointer across a calibration in proportion to the voltage measured and can exist built from a galvanometer and series resistor. Meters using amplifiers can measure tiny voltages of microvolts or less. Digital voltmeters give a numerical display of voltage past utilize of an analog-to-digital converter.
Voltmeters are made in a broad range of styles, some separately powered (e.grand. past bombardment), and others powered past the measured voltage source itself. Instruments permanently mounted in a console are used to monitor generators or other fixed appliance. Portable instruments, usually equipped to too mensurate electric current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work. Whatever measurement that can exist converted to a voltage can exist displayed on a meter that is suitably calibrated; for instance, pressure, temperature, flow or level in a chemical procedure plant.
Full general-purpose analog voltmeters may have an accuracy of a few percentage of full scale and are used with voltages from a fraction of a volt to several thousand volts. Digital meters tin exist made with high accuracy, typically better than 1%. Particularly calibrated examination instruments have higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts per million. Part of the problem of making an accurate voltmeter is that of scale to check its accuracy. In laboratories, the Weston cell is used every bit a standard voltage for precision work. Precision voltage references are available based on electronic circuits.
Schematic symbol [edit]
In circuit diagrams, a voltmeter is represented by the alphabetic character V in a circle, with 2 emerging lines representing the two points of measurement.
Analog voltmeter [edit]
A moving coil galvanometer of the d'Arsonval type.
- The ruddy wire carries the electric current to be measured.
- The restoring jump is shown in dark-green.
- N and S are the n and south poles of the magnet.
A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with the instrument. The galvanometer has a gyre of fine wire suspended in a strong magnetic field. When an electric current is applied, the interaction of the magnetic field of the coil and of the stationary magnet creates a torque, tending to make the coil rotate. The torque is proportional to the current through the coil. The whorl rotates, compressing a spring that opposes the rotation. The deflection of the gyre is thus proportional to the current, which in turn is proportional to the applied voltage, which is indicated by a pointer on a scale.
One of the design objectives of the musical instrument is to disturb the excursion equally piffling equally possible and so the instrument should draw a minimum of current to operate. This is achieved by using a sensitive galvanometer in serial with a high resistance, and then the entire instrument is continued in parallel with the circuit examined.
The sensitivity of such a meter can be expressed equally "ohms per volt", the number of ohms resistance in the meter circuit divided by the full scale measured value. For example, a meter with a sensitivity of 1000 ohms per volt would draw 1 milliampere at total scale voltage; if the full scale was 200 volts, the resistance at the instrument's terminals would be 200000 ohms and at full scale, the meter would draw 1 milliampere from the circuit under test. For multi-range instruments, the input resistance varies as the instrument is switched to dissimilar ranges.
Moving-coil instruments with a permanent-magnet field respond just to direct current. Measurement of Air-conditioning voltage requires a rectifier in the circuit so that the scroll deflects in only 1 direction. Some moving-coil instruments are also fabricated with the zero position in the middle of the scale instead of at one end; these are useful if the voltage reverses its polarity.
Voltmeters operating on the electrostatic principle apply the mutual repulsion between two charged plates to deflect a pointer fastened to a jump. Meters of this type draw negligible current but are sensitive to voltages over about 100 volts and piece of work with either alternating or straight current.
Amplified voltmeter [edit]
The sensitivity and input resistance of a voltmeter can be increased if the current required to deflect the meter pointer is supplied by an amplifier and ability supply instead of by the excursion under test. The electronic amplifier between input and meter gives ii benefits; a rugged moving coil instrument can be used, since its sensitivity need non be high, and the input resistance tin can be made high, reducing the current drawn from the circuit under test. Amplified voltmeters frequently have an input resistance of ane, 10, or xx megohms which is contained of the range selected. A once-popular form of this instrument used a vacuum tube in the amplifier circuit and so was called the vacuum tube voltmeter (VTVM). These were almost e'er powered by the local Air conditioning line current and so were not particularly portable. Today these circuits use a solid-country amplifier using field-effect transistors, hence FET-VM, and appear in handheld digital multimeters equally well as in demote and laboratory instruments. These largely replaced non-amplified multimeters except in the least expensive price ranges.
Most VTVMs and FET-VMs handle DC voltage, AC voltage, and resistance measurements; modern FET-VMs add electric current measurements and frequently other functions also. A specialized form of the VTVM or FET-VM is the AC voltmeter. These instruments are optimized for measuring AC voltage. They have much wider bandwidth and meliorate sensitivity than a typical multifunction device.
Digital voltmeter [edit]
Two digital voltmeters. Note the 40 microvolt difference between the two measurements, an kickoff of 34 parts per million.
A digital voltmeter (DVM) measures an unknown input voltage past converting the voltage to a digital value and then displays the voltage in numeric form. DVMs are commonly designed around a special type of analog-to-digital converter called an integrating converter.
DVM measurement accuracy is affected by many factors, including temperature, input impedance, and DVM power supply voltage variations. Less expensive DVMs often have input resistance on the social club of 10 MΩ. Precision DVMs can have input resistances of ane GΩ or higher for the lower voltage ranges (due east.yard. less than xx V). To ensure that a DVM'south accuracy is within the manufacturer's specified tolerances, it must be periodically calibrated against a voltage standard such equally the Weston cell.
The first digital voltmeter was invented and produced by Andrew Kay of Non-Linear Systems (and later founder of Kaypro) in 1954.[1]
Simple AC voltmeters apply a rectifier connected to a DC measurement circuit, which responds to the average value of the waveform. The meter can be calibrated to display the root mean square value of the waveform, bold a stock-still relation betwixt the boilerplate value of the rectified waveform and the RMS value. If the waveform departs significantly from the sinewave causeless in the calibration, the meter will be inaccurate, though for simple wave shapes the reading can exist corrected by multiplying by a abiding cistron. Early on "true RMS" circuits used a thermal converter that responded but to the RMS value of the waveform. Modern instruments summate the RMS value past electronically calculating the foursquare of the input value, taking the boilerplate, and so computing the foursquare root of the value. This allows authentic RMS measurements for a diverseness of waveforms. .[2]
See also [edit]
- Ammeter
- Grade of accuracy in electrical measurements
- Electrical measurements
- Electrometer
- Electronic examination equipment
- Metrology
- Multimeter
- Ohmmeter
- Potentiometer (measuring musical instrument)
- Solenoid voltmeter
- Voltage divider
- Measurement category
References [edit]
- ^ Markoff, John (5 Sep 2014). "Andrew Kay, Pioneer in Calculating, Dies at 95". Obituary. New York Times. Retrieved seven September 2014.
- ^ "What is Truthful RMS Measurement? RMS Vs Truthful RMS". Electrical Volt. 2018-09-19. Retrieved 2021-10-14 .
External links [edit]
 | Wikimedia Commons has media related to Voltmeters. |
- DC Metering Circuits chapter from Lessons In Electric Circuits Vol 1 DC free ebook and Lessons In Electric Circuits series.
Source: https://en.wikipedia.org/wiki/Voltmeter
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