An ammeter (from Am pere Meter ) is a measuring instrument used to measure current in a circuit. The electric current is measured in amperes (A), hence its name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters . Early ammeters are laboratory instruments that depend on the Earth's magnetic field for operation. At the end of the 19th century, improved instruments were designed that could be mounted in any position and allowed accurate measurements in electrical systems. Generally represented by the letter 'A' in the circle.
Video Ammeter
History
The relationship between electric current, magnetic field and physical strength was first noted by Hans Christian ÃÆ'ËÅ"rsted who, in 1820, observed the needle of the compass being deflected from the North when the current flows in adjacent wires. Galvanometer tangent is used to measure current using this effect, in which the restoring force returns the pointer to the zero position provided by the Earth's magnetic field. This makes this instrument usable only when aligned with the Earth's terrain. The sensitivity of the instrument is enhanced by the use of additional wire windings to double the effect - the instrument is called a "multiplier".
The word rheoscope as an electric current detector was invented by Sir Charles Wheatstone circa 1840 but is no longer used to describe electrical instruments. Word makeup is similar to rheostat (also created by Wheatstone) which is a device used to adjust the current in the circuit. Rheostat is a historical term for variable resistance, though unlike a rheoscope can still be found.
Maps Ammeter
Type
Move-coil
The D'Arsonval galvanometer is a moving coil ammunition. It uses magnetic deflection, where the current passing through the coil is placed in the magnetic field of the permanent magnet causing the coil to move. The modern form of the instrument was developed by Edward Weston, and used two spiral springs to provide the restoring power. The uniform air gap between the iron core and the permanent magnetic pole makes the deflection meter linearly proportional to the current. This meter has a linear scale. The base meter movement can have a full-scale deflection for currents of about 25 microamperes up to 10 milliamperes.
Because the magnetic field is polarized, the meter needle acts in opposite direction for each direction of the current. A DC ammeter is thus sensitive to which rotation direction is connected; most are marked with positive terminal, but some have zero-center mechanism and can display current in both directions. The moving coil meter shows the average (average) of the current varying through it, which is zero for AC. For this reason the moving-coil meter can only be used directly for DC instead of AC.
This type of motion meter is very common for both ammeters and other meters derived from them, such as voltmeters and ohmmeters.
Moving magnet
The moving magnetic ammeter operates in essentially the same principle as the moving coil, except that the coil is mounted in a meter box, and the permanent magnet moves the needle. The moving magnet Ammeters are capable of carrying a larger current than the moving coil instrument, often several tens of Amperes, since the coils can be made of thick wire and the current does not have to be carried by the hair springs. Indeed, some of these types of Ammeters do not have hairsprings at all, but instead use permanent permanent magnets to provide the restoring power.
Electrodynamics
Electrodynamic motion using electromagnet is not a permanent magnet of the d'Arsonval movement. This instrument can respond both alternating and direct current and also shows the correct RMS for AC. See Wattmeter for alternative use for this instrument.
Moving-iron
Moves the iron ampereter using a moving piece of iron when it is followed up by the electromagnetic force of the fixed wire coil. The iron-moving meter was invented by Austrian engineer Friedrich Drexler in 1884. This type of meter responds both to direct and alternating currents (as opposed to a moving-coil ammeter, acting on direct current only). The iron element consists of a propeller that moves attached to a pointer, and a fixed propeller, surrounded by a coil. As alternating or direct currents flow through the coils and inducing the magnetic fields in both the blades, the blades repel each other and the propellers move deflect against the restoring forces provided by the fine helical springs. The deflection of the iron meter moves in proportion to the square of the current. As a result, the meter usually has a non-linear scale, but the iron parts are usually modified in shape to make the scale quite linear in most of its range. Moving the iron instrument shows the RMS value of each applied AC wave. The moving iron ammber is generally used to measure the current in an industrial frequency AC circuit.
Heat wire
In the hot wire ammeter, the current passes through a wire that expands as it heats up. Although these instruments have slow response times and low accuracy, they are sometimes used in measuring current radio frequencies. It also measures the correct RMS for the applied air conditioner.
Digital
In much the same way as analog ammeter forms the basis for various kinds of derivative meters, including voltmeter, the basic mechanism for digital meters is a digital voltmeter mechanism, and other types of meters are built around this.
The digital ammeter design uses a shunt resistor to produce a calibrated voltage proportional to the current flowing. This voltage is then measured with a digital voltmeter, through the use of analog to digital converter (ADC); the digital display is calibrated to display the current through the shunt. The instrument is often calibrated to show the RMS value for the sine wave only, but many designs will show the correct RMS within the limitation of the wave peak factor.
Integrating
There are also various devices called as integrating ammeters. In this ammeters the current is summed over time, giving results of current and time; which is proportional to the electrical charge that is transferred with the current. This can be used to measure energy (the charge needs to be multiplied by the voltage to give energy) or to estimate the charge of a battery or capacitor.
Picoammeter
Picoammeter, or pico ammeter, measures very low electrical current, usually from the picoampere range at the lower end to the milliampere range at the top end. Picoammeters are used for sensitive measurements where measured currents are below the theoretical limits of the sensitivity of other devices, such as Multimeters.
Most picoammeters use the "virtual short" technique and have several different measurement ranges that must be diverted between to cover decades of measurement. Other modern picoammeters use log compression and "current sink" methods that negate the range of switching and related voltage spikes. Specific designs and considerations of use should be taken into account to reduce the leakage current that may be swamp measurements such as special insulators and shields driven. Triaxial cables are often used for probe connections.
Apps
Most of the ammeters are connected in series with the circuit carrying the current to be measured (for small shrapnel amps), or having a shunt resistor connected in series. In either case, the current passes through the meter or (mostly) through the door. Ammeters should not be connected directly at the voltage source because their internal resistance is very low and the overcurrent will flow. Ammeters are designed for low voltage drop in their terminals, less than a volt; the extra circuit losses generated by the ammeter are referred to as "load" on the measured circuit.
The usual Weston type meter movements can only measure milliamperes, because springs and coils can practically carry only limited current. To measure a larger current, a resistor called shunt is placed parallel to the meter. Shunts resistance is in integer to the fractional milliohm range. Almost all current flows through the shunt, and only a small portion flows through the meter. This allows the meter to measure large currents. Traditionally, the meter used with the shunt has a full-scale deflection (FSD) 50 mV , so the shunt is usually designed to produce a voltage drop of 50 mV when carrying their measurable current.
To create a multi-range ammeter, a selector switch can be used to connect one of a number of shunts along the length of the meter. This should be a make-before switch to avoid damage to current spikes through meter movement when switching ranges.
A better arrangement is the Ayrton shunt or universal shunt, created by William E. Ayrton, which does not require a make-before-break switch. It also avoids inaccuracy due to contact resistance. In the figure, assuming for example, a motion with a full-scale voltage of 50 mV and a desired current range of 10 mA, 100 mA, and 1 A, the resistance value would be: R1 = 4.5 ohm, R2 = 0.45 ohm, R3 = 0.05 ohm. And if the movement resistance is 1000 ohms, for example, R1 must be adjusted to 4.525 ohms.
Switched shunts are rarely used for currents above 10 amperes.
Zero-center ammeters are used for applications requiring current to be measured by polarity, common in scientific and industrial equipment. Zero center ammeters are also commonly placed in series with batteries. In this application, the battery charge switches the needle to one side of the scale (usually, the right side) and the battery usage switches the needle to the other side. A special type of zero-center ammeter for testing high currents in cars and trucks has a rotating bar magnet that moves the pointer, and a fixed rod magnet to keep pointers centered without a current. The magnetic field around the wire carrying the current to be measured deflects a moving magnet.
Because the ammeter shunt has very low resistance, the ammeter wiring fault in parallel with the voltage source will cause a short circuit, best blow the fuse, possibly damage the instrument and cable, and expose the observer to the injury.
In the AC circuit, the current transformer changes the magnetic field around the conductor into a small AC current, typically 1 A or 5 A in the measurable current, which can be read easily by the meter. In the same way, accurate non-contact AC/DC ammeters have been constructed using Hall effect magnetic field sensors. Portable handheld amemeter is a common tool for the maintenance of industrial and commercial electrical equipment, which is temporarily cut above the wire to measure the current. Some new types have a pair of parallel magnetic probes that are placed on both sides of the conductor.
See also
- Clamp meter
- An accuracy class in electrical measurement
- Electrical circuit
- Electrical measurements
- Electronics
- Electronic topic list
- Measurement categories
- Multimeter
- Ohmmeter
- Rheoscope
- Voltmeter
Note
References
External links
- Measurement Circuit DC chapter of Lesson In Electrical Vol 1 DC free ebook and Lessons in Circuit Electrical series.
Source of the article : Wikipedia
0 Comments