ammeters

Ammeter - a device for measuring the current, as the unit of current measurement is - amperes (A), the indications listed in amps. Ammeters scale calibrated in microamperes, milliamperes or amperes kA in accordance with the limits of the measurement device.

The ammeter electric circuit in series with the electric circuit portion, wherein the amperage is measured. Therefore, the lower the internal resistance ammeter (ideally - 0), the less will be the impact of the device on the object being studied, and the higher the measurement accuracy. To increase the detection limit ammeter is provided with a shunt (for AC and DC circuits), current transformer (AC chains only) or a magnetic amplifier (DC circuits).

General characteristics

On a design and method of use are divided ammeters: shield, portable, stationary and laboratory

According to the construction of the indicator are divided:
with an arrow of the measuring head without electronics;
a measuring head with an arrow using electronic circuits;
with digital display.

Appliances
arrow head The most common are ammeters, in which the moving part of the instrument with the arrow rotates the roll angle, which is proportional to the measured current.
Ammeters are magneto-electric, electromagnetic, electrodynamic, thermal, induction, detector, thermoelectric and photovoltaic.
Magnetoelectric ammeters measure the strength of the DC; and induction detector - power AC; ammeters other systems measure the strength of any current. The most accurate and sensitive are the magneto and electro-ammeters.
Devices with an arrow head may be provided with additional electronic circuitry to amplify the signal applied to the head (for measuring currents substantially lower than current full head deviation, which for most magnetoelectric device is 50 pA or more) protecting the head against overload and so on.

The devices with digital display
Recently, devices with arrow measuring head were replaced with digital display devices based on liquid crystal and LEDs.

The principle of operation of the measuring head arrow

The operating principle of the most common in the ammeter measuring systems:

The magnetoelectric device system arrows torque created by the interaction between the field permanent magnet and the current which passes through the winding frame (torque). With the frame is connected arrow, which moves along a scale. The angle of rotation of the arrow is directly proportional to the strength of the current, so the scale is linear coil instrument. arrow The direction of rotation depends on the direction of flow through the frame of the current, so magnetoelectric ammeters suitable for the direct measurement of AC current (arrow will tremble near zero), and require the correct polarity of the DC (otherwise the arrow will deviate to the left of zero).

The electromagnetic system device arrow torque created between the movable coil and ferromagnetic core, to which the meter needle is attached.

The electrodynamic system measuring head consists of a fixed and a movable coil connected in parallel or in series. The interaction between the currents that pass through the coils causes the movable deflection coil and connected with her arrows.
In all the above systems, the angle of rotation of the arrow is set at equal torque and torque resistance of the spring.

The inclusion of the ammeter to the electric circuit
In the circuit an ammeter is connected in series with the load, and at high currents - through the current transformer, a magnetic amplifier or a shunt. To measure the currents can also be applied and calibrated millivolt shunt (shunts primary currents may be selected from a standard range, standardized secondary voltage - often 75 mV). At high voltages (up to 1000V), - in AC circuits for galvanic isolation ammeters also used current transformers and DC circuits - magnetic amplifiers