Ammeter

Amperimeter.

An ammeter in general terms, is a simple galvanometer (instrument to detect small amounts of current), with a resistance in parallel, called a "shunt resistance". The ammeter is used to measure the intensity of electrical currents. Having a range of shunt resistors, you can have an ammeter with various ranges or measurement intervals. Ammeters have a very small internal resistance, below 1 ohm, so that their presence does not decrease the current to be measured when connected to an electrical circuit.

The device described corresponds to the original design, since currently ammeters use an analog/digital converter to measure the voltage drop in a resistor through which the current to be measured flows. The converter reading is read by a microprocessor that performs the calculations to present the value of the circulating electric current on a numeric display.

Classes of ammeters

The most important measurement systems are the following: magnetoelectric, electromagnetic, electrodynamic and digital, each with its respective type of ammeter.

Magnetoelectric Ammeters

To measure the current flowing through a circuit, the ammeter must be connected in series with the power source and with the current receiver. Thus, all the current that circulates between these two points will pass through the ammeter first. These devices have a mobile coil that is made of a very fine wire (approximately 0.05 mm in diameter) and whose turns, through which the current to be measured will pass, are very small. For all this, it can be said that the current intensity, which will be able to measure an ammeter whose measurement system is magnetoelectric, will be limited by the physical characteristics of the elements that make up said device. The limit value of what can be measured without fear of introducing errors is going to be around 100 milliamps, so the measurement scale to be used cannot be amps but must be milliamps. To increase the scale of values that can be measured, shunt resistors can be placed, being able to measure amps (approximately up to 300 amps). Shunt resistors can be connected directly inside the apparatus or can be connected externally.

Electromagnetic ammeters

The most widely used ammeters are based either on the interactions between magnetic fields and currents, or on their thermal effects. Among the first, those with a mobile frame, electrodynamics and those with moving iron stand out. Moving frame ammeters consist of a fixed magnet between whose poles is a moving coil. When the current that is measured passes through the coil, the magnetic field of the fixed magnet determines the appearance of a couple of forces that move the coil in direct proportion to the intensity of the current. Electrodynamic ammeters are similar to the previous ones, except that in this case the magnetic field is created by a fixed coil connected in series to the mobile one. The deviation is then proportional to the square of the intensity. In moving iron ammeters, the field created by a fixed coil attached to the circuit deflects a piece of soft iron fitted with a recoil spring. Thermal ammeters take advantage of temperature changes associated with the release of heat due to the Joule effect in some element of the circuit. The most modern ones are provided with a thermoelectric couple connected to a millivoltmeter.

They are made up of a coil that has few turns but a large section. The power required by these devices to produce a maximum deflection is about 2 watts. In order for this power to be absorbed, there must be a sufficient voltage drop at the ends of the coil, the value of which will depend on the range of the ammeter. The range of values covered by this type of ammeter goes from 0.5 A to 300 A. Shunt resistors cannot be used here as they would produce heating that would lead to measurement errors. Both direct and alternating current can be measured with them. Alternating current measurements are only valid for frequencies below 2620 Hz. Ammeters of other efficient measurements can also be added.

Electrodynamic Ammeters

Ammeters with "electrodynamic#34; They are made up of two coils, one fixed and one mobile.

Digital ammeters

These ammeters use a shunt resistor and an analog-to-digital converter (ADC)

Mechanism of a rectifying amperimeter

Rectifying ammeters

They are used for the measurement of alternating current that is connected to the secondary current of a current transformer, the secondary current is much less than the primary and is connected with a bridge rectifier to a moving coil ammeter.

Usage

To carry out the measurement it is necessary that the intensity of the current circulates through the ammeter, for which reason it must be placed in series, so that it is traversed by said current. The ammeter must have an internal resistance that is as small as possible in order to avoid an appreciable voltage drop (because it is very small, it will allow a greater passage of electrons for its correct measurement). For this, in the case of instruments based on the electromagnetic effects of electric current, they are equipped with coils of thick wire and with few turns.

In some cases, to allow the measurement of currents higher than those that the delicate windings and mechanical parts of the device could withstand without being damaged, they are provided with a very small value resistor placed in parallel with the winding, so that only a fraction of the main current passes through it. This additional resistor is called a shunt. Although most of the current passes through the shunt resistor, the small amount that flows through the meter is still proportional to the total current so the galvanometer can be used to measure currents of several hundred amps.

The current clamp is a special type of ammeter that avoids the inconvenience of having to open the circuit in which you want to measure the intensity of the current.

Figure 1.- Connection of an amperimeter in a circuit

Figure 1 shows the connection of an ammeter (A) in a circuit, through which a current of intensity (I) circulates, as well as the connection of the shunt resistor (R_S).

The value of R_S is calculated based on the multiplier power (n) that you want to obtain and the internal resistance of the ammeter (R_A) according to the following formula:

RS=RAn− − 1{displaystyle R_{S}={frac {R_{A}}{n-1}}}

So, suppose you have an ammeter with 5 Ω internal resistance that can measure a maximum of 1 A (full scale reading). If it is desired that it can measure up to 10 A, which implies a multiplier power of 10. The resistance R_S of the shunt must be:

RS=59=0.555Ω Ω {displaystyle R_{S}={frac {5}{9}}}=0.555Omega }