Updated October 20th 2011

How does an ammeter work?

First some words about electricity. If electricity is compared to water, then the current can be compared to the flow whereas voltage compares to the pressure. If the current increases, usually the voltage decreases and vice versa.
An ammeter could be called a current flow meter and the voltmeter could be an electricity pressure meter.

AC vs DC ammeter

AC ammeter full wave diode bridge rectifier circuit

Any ammeter is in itself a DC ammeter because it can only show DC current. So, as in the shown circuit, an AC ammeter has a rectifier, usually a full wave diode bridge rectifier, otherwise the meter would go up and down at low frequencies or not move at all at higher frequencies.

Just as with any analog meter, in a passive ammeter the current passes through a set of coils, which causes rotation of a central armature. Attached on this armature, an indicator needle shows the amount of deflection against a graduated scale. Ammeters migh be designed to have a full-scale deflection as small as a few microamperes but as for automotive use, up to several amperes. Thus the used coil wire is heavier.

To extend the range of an ammeter, allowing it to register very large currents, a resistor of precisely determined value is placed in parallell with the coils of the meter. This resistor diverts much, or most, of the current so that the meter actually reads only a fraction of the current.

Ammeter shunt

An ammeter shunt is a low ohm resistor or component (which acts as a resistor) in series where a current flows. It allows the measurement of large currents. A components or resistor of accurately known resistance, is placed in series with the load so that all of the current to be measured will flow through it. The voltage drop across the shunt is proportional to the current flowing through it and since its resistance is known, a millivoltmeter connected across the shunt can be scaled to directly display the current value.

Used as a voltmeter

Ammeters might be used as voltmeters by placing a resistor in series with the meter coils. Then, even a very high voltage will cause a small deflection of the needle. Thre greatest accuracy is obtained when a sensitive ammeter is used with a large-valued resistor. Then, the current drawn from the circuit is minimized. Ammeters should never be connected across a source of a voltage without a series resistor because the meter mechanism might be damaged as a result.

Ammeter types

Apart from the old analog ammeter, there are also digital ammeters which shows the current on a display, and then clamp on ammeters, which are also digital but work in another way.

Digital ammeter

A digital ammeter doesn't necessarily let the current go through a coil (which becomes resistive along its length), it can pass the current through a shunt, a very low ohm component (doesn't have to be a typical resistor) where there will be a voltage drop. This voltage is then amplified and measured and then put to an ad converter. With other electronics the value is shown on a display. A digital AC ammeter also has a rectifier.

Clamp on ammeter

This instrument can be clamped around a conductor via its two jaws, which then shows a reading of amperage. There are two types of clamp on meters. The common ones measure AC currents on the principle of electromagnetic induction caused by the AC that flows in the conductor which reverses direction causing a dynamically changing magnetic field. However an inductive sensor cannot sense a static magnetic field as in DC conductors, where the current always flows in a fixed polarity. Consequently, the magnetic field around the conductor is fixed and does not change. Hence, a conventional clamp meter will register no reading for DC.

However there are DC ammeters using another technique.

DC clamp on ammeter

DC clamp ammeters use Hall effect sensors known as Hall elements. A Hall effect sensor is basically just a kind of magnetometer, it senses the strength of an applied static (not changing) magnetic flux, a magnetic field caused by the flow of current. This causes a small voltage across the Hall element. This voltage is amplified and measured.
A DC clamp ammeter basically consists of a iron core which is toroidal, which clamps together with a Hall effect chip in the gap between the two halves, so that the induced magnetic flux from the current-carrying wire is channelled through it.
A DC clamp on meter can also work for AC because it can work for alternating magnetic fields too.

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