This article difference between analog and digital multimeter pdf about Voltmeter. Voltmeters are made in a wide range of styles.
Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. General purpose analog voltmeters may have an accuracy of a few percent of full scale, and are used with voltages from a fraction of a volt to several thousand volts. Specially calibrated test 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 calibration to check its accuracy. Precision voltage references are available based on electronic circuits. N and S are the north and south poles of the magnet. The galvanometer has a coil 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 coil rotates, compressing a spring that opposes the rotation. The deflection of the coil 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 instrument is to disturb the circuit as little as possible and so the instrument should draw a minimum of current to operate. This is achieved by using a sensitive galvanometer in series with a high resistance, and then the entire instrument is connected in parallel with the circuit examined. The sensitivity of such a meter can be expressed as “ohms per volt”, the number of ohms resistance in the meter circuit divided by the full scale measured value. 1 milliampere from the circuit under test.
For multi-range instruments, the input resistance varies as the instrument is switched to different ranges. Moving-coil instruments with a permanent-magnet field respond only to direct current. Meters of this type draw negligible current but are sensitive to voltages over about 100 volts and work with either alternating or direct current. 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 power supply instead of by the circuit under test. Amplified voltmeters often have an input resistance of 1, 10, or 20 megohms which is independent 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, or VTVM. These were almost always powered by the local AC line current and so were not particularly portable. These are now so ubiquitous that they have largely replaced non-amplified multimeters except in the least expensive price ranges. FET-VMs add current measurements and often other functions as well. 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 better sensitivity than a typical multifunction device. 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 order of 10 MΩ. This page was last edited on 14 January 2018, at 14:19. Please forward this error screen to 69. Big List of 8051 Micro Controller based Projects, Circuits and Project Kits.
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