An instrument used to measure direct (DC) or alternating (AC) electric current is called an ammeter. Current is the flow of electrons whose SI unit is ampere. Therefore, the instrument with which the flow of current in an ampere is measured is called the ampere meter or ammeter.
For an ideal ammeter, the internal resistance of the ammeter should be zero but in practice, the ammeter has little internal resistance which is considered to be negligible. The measurement value of the ammeter depends on the value of the resistance.
Symbol of Ammeter
In the circuit, the ammeter is denoted by the capital alphabet A.
Ammeter Connection in Circuit
In the circuit, the ammeter is connected in series so that the entire electron of measurement and current passes through the ammeter. The current measured in the ammeter and its internal resistance may result in loss of power. The ammeter circuit has low resistance due to which there is a small voltage drop in the circuit.
The resistance of the ammeter is kept low for two reasons.
- The entire measurement current passes through the ammeter.
- Less voltage drop occurs across the ammeter.
When a high-value current passes directly through the ammeter causing damage to its internal circuit. To overcome this problem, the shunt resistance is connected in parallel with the ammeter.
If a large measuring current passes through the circuit, then the bulk of the current passes through the shunt resistance. The shunt resistance does not affect the working of the ammeter, that is, the speed of the coil remains the same.
Effect of temperature in ammeter
Ammeter is a sensitive instrument which can be easily affected by temperature. Changes in temperature can cause reading errors and reduce resistance to swamp. The resistance having zero temperature coefficient is known as swamp resistance. It connects in series with the ammeter. The swamp resistance minimizes the effect of temperature on the meter.
An inbuilt fuse is present in the ammeter which protects the ammeter from heavy current. The fuse breaks when enough current flows through the ammeter. The ammeter is not able to measure the current until someone replaces the new fuse.
Types of Ammeter
The classification of an ammeter depends on its design and the type of current flowing through the ammeter.
Depending on Construction
Following are the types of an ammeter with respect to construction.
- Permanent moving coil ammeter.
- Moving iron ammeter.
- Electro-dynamometer ammeter.
- Rectifier type ammeter.
Permanent moving coil ammeter
This ammeter is used to measure Direct Current (DC), also known as the PMMC ammeter. The conductor is placed between the poles of a permanent magnet. When current flows through the coil, it starts deflecting and the deflection of the coil depends on the magnitude of the current flowing through it.
Moving iron ammeter
It is also called Mi Ammeter for short. It is used to measure both alternating current(AC) and direct current(DC). In this type of ammeter, the coil moves freely between the poles of a permanent magnet. When current passes through the coil, it starts deflecting at a certain angle. The deflection of the coil is proportional to the current passing through the coil.
This ammeter is also used to measure both AC and DC currents. The accuracy of this instrument is high as compared to PMMC and MI instruments. Calibration of the instrument is the same for both AC and DC, i.e. if DC calibrates the instrument, then it can be used for AC measurement without re-calibration.
Rectifier type ammeter
It is used to measure alternating current (AC). The equipment uses rectification equipment which changes the direction of the current and sends the converted current to the PMMC equipment. This type of instrument is used to measure the current in the communication circuit.
Depending on Current Ammeters
Following are the types of ammeters on the basis of current.
- DC Ammeter.
- AC Ammeter.
The rate of flow of charge is called electric current. If when this electric charge flows in only one direction, then the resultant current is called direct current (DC). The instrument used to measure direct current is called a DC ammeter.
If in a circuit, a resistor is connected in parallel with a Permanent Magnet Moving Coil (PMMC) galvanometer, then the entire combination will act as a DC ammeter. Parallel resistance, which is used in DC ammeters, is also called shunt resistance or simply, shunt. The value of this resistance is assumed to be negligible so that large value DC currents can be measured.
The circuit diagram of a DC ammeter is expressed as follows:
This DC ammeter is connected in series with an electrical circuit where DC current is to be measured. Elements connected in parallel have the same voltage. Then, the voltage across the shunt resistor, Rsh, and the voltage across the galvanometer resistance, Rm, remain the same, since those two elements are connected in parallel in the above circuit. It can be expressed something like this:
The KCL equation at node 1 is,
Substitute the value of Ish into Equation 1,
Im is the same in the denominator term, which is on the right-hand side of Equation 2.
- Rsh = the shunt resistance.
- Rm = the internal resistance of the galvanometer.
- I = total direct current to be measured.
- Im = the full scale deflection current.
The ratio of the total direct current to be measured, I, and the full-scale deflection current of the galvanometer, Im is treated as the multiplication factor, m. It can be represented as,
Multi Range DC Ammeter
A DC ammeter can be used to measure a special range of direct currents, called a multi-range DC ammeter.
In a DC ammeter, to measure multiple ranges of direct currents, multiple resistors will be used in parallel instead of a single resistor, and this entire combination of resistors will be connected in parallel with the PMMC galvanometer.
Multi-range DC ammeters are connected in series with an electric circuit where the required range of direct current is to be measured. The desired range of currents is selected by connecting the switch, s, to the corresponding shunt resistor.
Suppose, m1, m2, m3, and m4 are the multiplication factors of the DC ammeter, then the total direct currents are measured as I1, I2, I3, and I4 respectively. The following are the corresponding formulas for each of the multiplication factors.
In the above circuit, there are four shunt resistors Rsh1, Rsh2, Rsh3, and Rsh4. The following are the corresponding formulas for these four resistors.
The rate of flow of charge is the electric current. If the direction of the electric charge changes continuously, then the resultant current is called alternating current (AC).
The instrument used to measure the alternating current flowing in an electric circuit is called an AC ammeter.
Example: Thermocouple type AC ammeter.
Thermocouple Type AC Ammeter
When in an electric circuit, a thermocouple is connected in front of a PMMC galvanometer, the entire combination is called a thermocouple-type AC ammeter. The block diagram of thermocouple type AC ammeter is shown below:
In the block diagram there are mainly two blocks one is thermocouple and the other is PMMC galvanometer. Replacing each block with the appropriate corresponding component in the above block diagram gives the corresponding circuit diagram.
Whenever alternating current I, flows through the heater, the thermocouple produces an emf e. This emf E is directly proportional to the rms value of the current, which flows through the heater. Therefore, the scale of the PMMC instrument has to be calibrated to read the RMS value of the current.
Related Tutorial: Introduction to Measuring Instruments of Electronics