An ampmeter is a device that measures the intensity of an electrical current in a circuit. The unit of measurement is the ampere, symbol : A.
There are several types :
- analog ampsmeters
- digital ampsmeters
- special ampsmeters
The most common analog ampmeter is magneto-electric, using a mobile frame galvanometer. It measures the average value of the current passing through it. For alternating current measurements, a diode straightener bridge is used to straighten the current, but this process can accurately measure only sinusoidal currents.
Analog amps are increasingly being replaced by digital amps. However, in practice, the observation of their needle can provide rapid visual information about the variations of the measured current that the digital display gives only difficultly.
The ferro-magnetic (or ferromagnetic) amperometer uses two soft iron pallets inside a coil. One of the pallets is fixed, the other is mounted on pivot. When the current passes through the coil, the two pallets attract and repel each other, regardless of the direction of the current.
This amp is therefore not polarized (it does not indicate negative values). Its accuracy and linearity are less good than those of the magneto-electric amp, but it can measure the effective value of alternating current of any form (but of low frequency < 1 kHz).
The thermal ampmeter is composed of a strong wire in which the current to be measured circulates. This thread heats up by Joule effect, its length varying according to its temperature, causes the rotation of the needle, to which it is attached.
The thermal amp is not polarized. It is not influenced by the surrounding magnetic fields, its indications are independent of the shape (alternative or continuous form of any form) and the frequency of the current. It can therefore be used to measure the effective value of alternating currents up to very high frequencies.
It very often incorporates a temperature offset designed to maintain its accuracy despite changes in ambient temperature.
measuring the voltage produced by the current to be measured in a resistance (called shunt). The value of the shunt depends on the caliber used.
Under Ohm's law, the measured U voltage is converted, based on the known resistance value R of the shunt, into an A value corresponding to the current.
The amperemetric clamp
It is a kind of electrical transformer whose primary is made up by the conductor whose current and secondary one wants to know by a coiled coil on a magnetic circuit formed by the two jaws of the clamp.
It is used to measure high alternating currents without inserting anything into the circuit. It cannot measure continuous currents.
The Hall-effect current sensor amperus clamp
It allows to measure any currents (alternative or continuous) and high intensity without inserting into the circuit or interrupting it. The clamp is composed of a magnetic circuit (an intensity transformer) that closes on a semiconductor pellet. This pellet will be subjected to the induction generated by the wire (current to be measured).
Induction is measured because it has the advantage of existing regardless of the type of current. The semiconductor pellet is subjected to a current perpendicular to the induction that passes through it.
All this to cause thanks to The force of Lorentz a load shift in the pellet that will result in a difference in potential that is proportional to the field and therefore aware, a counter-reaction system requires the transformer to operate at zero flow and it is the flow cancellation current that, converted into voltage using an operational amplifier converter , gives its output a tension image of the measured current.
Fiber optic ampmeter
They are used in the field of THT (very high voltage), large currents and when the bandwidth of Hall-effect sensors is insufficient (study of violent transient regimes, those for which the di/dt is greater than 108 A/s).
This measurement technique uses the Faraday effect : the plane of polarization of light in glass rotates under the effect of an axial magnetic field.
This effect does not depend on the direction of light propagation but depends on the direction of intensity.
Effect amperemeters Néel
They are able to measure continuous and alternating currents, with great precision whether for weak or strong currents. These sensors consist of several coils and nuclei made of nanostructured composite material with superparmagnetic properties, hence the absence of magnetic remanence over a wide range of temperature.
An arousal coil detects the presence of current through the modulation by néel effect. A counter-reaction coil delivers the measurement current, directly proportional to the primary current and the ratio of the number of primary/secondary spires.
The Néel current sensor therefore behaves like a simple, linear and precise current transformer.
An ampmeter plugs into the circuit in series. This means opening the circuit where you want to measure the intensity and placing the ampmeter between the two terminals created by this circuit opening.
Sense of connection and polarity
An amp meter measures the flowing intensity of Terminal A (or terminal) to the COM (or terminal) terminal, taking into account its sign. In general, the needle of analog amperometers can only deviate in one direction.
This requires thinking about the direction of the current and requires the amperemeter to be wired in order to measure a positive intensity : we then check that the terminal - of the amperemeter is connected (possibly by crossing one or more dipoles) to the pole - of the generator and that the terminal - of the amperemeter is connected (possibly by crossing one or more dipoles) to the pole of the generator.
The highest intensity caliber can be measured by the amperemeter.
All modern devices are multi-calibre : you change caliber either by turning a switch or by moving a plug. The newest devices are self-calibrated and require no manipulation.
When using an analog amp, avoid using a caliber smaller than the current intensity. This requires calculating an order of magnitude of this intensity and choosing the caliber accordingly. If you have no idea of the order of magnitude of the intensity that you will measure, it is desirable to start from the highest caliber, usually sufficient. This gives you an idea of the current flowing through the circuit.
Then the caliber is reduced to the smallest possible caliber, while keeping a value higher than the measured current. However, it is necessary to change the caliber carefully, for example by cutting off the current or shunting the ampmeter during the change of caliber of the device, especially if the circuit is inductive.
The reading of a digital device is direct and depends on the caliber selected.
For the analog amperemeter, the needle moves on a common graduation of several calibers. The indication read represents only a number of divisions. It is therefore necessary to deduce the intensity from this number taking into account the value of the caliber by making a calculation, knowing that the maximum graduation corresponds to the caliber.