Ammeter

An ammeter measures electric current in a circuit- direct current (DC) or alternating current (AC). Since electric currents are measured in amperes (A), ammeters are sometimes referred to as amp meters too. An ammeter can measure different current values because at high values only a small part of the current is directed through the meter mechanism and the major portion of current is carried by a shunt in parallel with the meter.

 

Ammeter Uses

An ammeter has various uses in a number of different fields. It is used for measuring the flow of current through the cable wiring systems of new buildings for ensuring that the wiring is according to local electrical safety codes. Electrical equipment manufacturer companies use ammeters for testing their electrical products before they are sent to the market for sale. Electricians use ammeter to identify problems with wiring systems in older buildings.

 

Ammeters are also used to measure current with both polarities that are common in scientific equipment as well as industrial equipment. In such cases, a zero-center ammeter is used which is also placed in series with a battery. An ammeter can be utilized in almost any situation where it is possible to tap into a circuit and execute a measurement.

 

Ammeter Working

An ammeter works by measuring the voltage dropped across the shunt resistor. For measuring the current flowing in a particular closed circuit, the circuit is first broken and the ammeter is connected in series. When the circuit is reactivated, current flows through the ammeter's shunt resistor which causes a voltage drop across the resistor. This voltage drop is then displayed on a voltmeter that is calibrated in amps. If the amount of current is unknown, a fuse is used in series with the shunt resistor. This protects the shunt resistor from getting damaged if the current range is exceeded. An ammeter set to its highest current range that progressively moves down the scale until the proper range is found is safe to work with.

 

Experienced and careful ammeter manufacturers make ideal ammeters having zero input impedance. Ammeters are always placed in series with the circuit to be measured. Therefore an ammeter has to have a low input impedance so that there is minimal effect on the circuit under examination.

 

Ammeter Types

There are different types of ammeter for measuring wide range of current values. They differ in their operating principles and accuracies. Smaller values of current are measured with the help of a milliameter or a microammeter. For measuring larger currents, a resistor known as shunt is placed in parallel with the meter. Most of the current flows through the ammeter shunt and only a small portion flows through the meter. This helps ammeter in measuring large currents.

 

Galvanometer
A galvanometer can be termed as oldest type of ammeter. It is a DC ammeter which reads direct current flow. There is an indicator needle in galvanometer. When current flows through coil and acts on spring, a magnetic field is created which in turn moves the indicator needle.
D'Arsonval Ammeter

 

A moving coil ammeter is known as D'Arsonval galvanometer. 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 across the ammeter in any circuit into which it is inserted. The popular form of D'Arsonval ammeter which is used in recent times was developed by Edward Weston. This model uses two spiral springs to provide the restoring force. The D�Arsonval-movement ammeter measures direct current with accuracies ranging from 0.1 to 2.0 percent by maintaining a uniform air gap between the iron core of the instrument and the poles of its permanent magnet.

 

Moving Iron Ammeter

A moving iron ammeter has a piece of iron (in place of spring and pointer) which moves when the electromagnetic force of a fixed coil of wire acts upon it. This type of ammeter measures both direct and alternating currents as opposed to the moving coil ammeter which is a DC ammeter and works on direct current only. The iron element in a moving iron ammeter consists of a moving vane attached to a pointer, and a fixed vane, surrounded by a coil. When current (AC or DC) flows through the coil, it induces a magnetic field in both vanes. The vanes repel each other and the moving vane deflects against the restoring force provided by fine helical springs. However, this type of ammeter is not liked by many people due to the non-linear scale of these meters.

 

Electrodynamic Ammeter
In such types of ammeters, there is electrodynamic movement which uses an electromagnet instead of the permanent magnet that is used in the D'Arsonval movement. An electrodynamic ammeter works on both- alternating and direct current.

 

Hot Wire Ammeter
In a hot-wire ammeter, the electrical current passes through a wire which expands upon heating. Due to their slow response time and low accuracy, hot wire ammeters are not very popular. This type of ammeter had been generally used in measuring radio-frequency current.

 

Digital Ammeter
Digital ammeters do not have any moving part. They use a circuit like dual slope integrator to convert a measured analogue (continuous) current to its digital equivalent which is referred to as Analog to Digital Converter (ADC). The ADC in a digital ammeter measures voltage across the shunt resistor and the digital display is calibrated to read the current through the shunt.

 

Zero-Center Ammeter

A zero-center ammeter is used in systems where the voltage has to be monitored in two directions. This type of ammeter is typically used along with a battery.
Clamp-on Ammeter

 

"Clamp on Ammeter" is a portable hand-held ammeter generally used for maintenance of industrial and commercial electrical equipment. Clamp-on ammeter is temporarily clipped over a wire to measure current. It is equipped with two jaws that can be fixed around the conductor. The magnetic field of the conductor moves an iron cylinder inside the ammeter to create a reading.
Integrating Ammeters

 

Integrating ammeters are a whole range of devices in which the amount of current is summed over time. As a result, an integrating ammeter gives the product of current and time, which is proportional to the energy transferred with that current. Integrating ammeters can be used for energy meters (watt-hour meters) or for estimating the charge of battery or capacitor.