We saw in our last tutorial that a permanent magnet moving-coil (PMMC) galvanometer is a type of instrument in which a current carrying coil is placed within a permanent magnetic field. When an electric current ( I ) passes through the coil, the electromagnetic field created around it reacts against the permanent magnetic field producing a deflecting torque that causes it to move. A pointer or needle attached to the coil indicates the amount of deflection, ( Φ ).
We also learnt that permanent-magnet moving-coil meters can be converted into an effective DC Voltmeter with the aid of series connected multiplier resistors. But we can also use PMMC meters to measure electrical current by connecting the resistors in parallel with the meter rather than in series and this forms the basis of Ammeters.
As its names implies, an Ammeter is an instrument used for measuring electrical current (I) and gets its name from the fact that the unit of measurement is “amps”, or more precise, Amperes. But in order to measure an electric current, an ammeter must be connected so that the total current of interest can pass through it. In other words, the ammeter should always be connected in series to the circuit or component being measured.
But here lies the problem. As we saw in the previous tutorial about voltmeters, the full-scale deflection (FSD) of a standard PMMC meter is very small so they can carry only small currents, 0 to IFSD, given in micro-amperes (uA), or milli-amperes (mA) due mainly to the small wire size used in the windings of the PMMC moving coil.
What if we wanted to measure a circuit current that is greater than this or up into the 10’s of amperes as a much higher current would force the meters pointer beyond its maximum FSD deflection which could potentially overheat or damage the coils windings, nevermind bend the pointer. So how can we use a standard PMMC meter to measure larger currents than those rated for FSD.
To measure a circuit current, the galvanometer has to be connected in series, and since it has a fairly large coil resistance, RG this will have an effect on the value of the current being measured. When using a PMMC meter as an ammeter, its range of measurement can be extended further with the help of parallel connected “Shunt Resistors”, thus allowing it to measure DC currents many times greater than its normal full-scale deflection current rating as only a fraction of the total current will pass through the meter.
Ammeter Shunt Resistors
An ammeter’s current sensitivity is determined by the amount of electric current required by the meters coil to produce the required FSD movement of the pointer. The amount by which the coil moves, called “deflection”, ( Φ ) is proportional to the strength of current flowing through the coil needed to produce the magnetic field required to deflect the needle by an amount, given in degrees (or radians) per ampere, o/A (or rad/A).
Therefore the smaller the amount of current required to produce the required deflection, the greater the sensitivity of the meter. Then the pointer of an ammeter moves in response to a current, so if the meter movement requires only 100uA for full scale deflection, it will have a greater sensitivity than a meter movement which requires 1mA for its FSD.
By connecting an external shunt resistors in parallel with the meter, rather than in series as was the case for the voltmeter, we can extend its usable range of the movement. This is because parallel connected resistances form current divider networks which, as their name suggests, divide the measured current by an amount determined by their resistive value as shown.
Here the low-resistance shunt is connected in parallel (shunted) with the PMMC meter terminals and is designed to carry the majority of the circuit current so that only a small portion of it flows through the meter’s wound-coil. Thus a shunt resistance increases the range of the ammeter with the meter’s current, IG being proportional to the total circuit current IT producing the required voltage drop across the meter for full scale deflection.
Let’s assume that we wish to use a 100uA, 200Ω galvanometer to measure a circuit current of up to 1.0 ampere. Clearly we cannot just connect the meter directly to measure one ampere but by using Ohm’s Law we can calculate the value of the shunt resistor, RS required which will produce a full-scale meter movement and corresponding IG x RG voltage drop across it when used to measure a circuit current of up to one ampere.
So if the current for which the galvanometer gives full scale deflection is given as 100uA, then the shunt resistance RS required is calculated as 0.02Ω. For a 20mV (V = I*R = 100µA x 200Ω) voltage drop, 100uA will flow through the PMMC meter and 999.9mA through the low-resistance shunt resistor. Therefore, nearly all of the circuit current (IT) passes through the shunt resistor with only a very small fraction of current required for FSD passing through the moving coil thereby converting the galvanometer into an ammeter by simply connecting a small enough resistance in parallel with it as shown.
Ammeter Shunt Resistance
Note that this shunt resistance, RS will always be lower than the coil’s internal resistance, RG to divert away the circuit current from the coil’s windings. Then the combination of the meter’s movement with this external shunt resistance forms the basis of a simple analogue ammeter no matter what the FSD is for a particular meter. For example, the same galvanometer can be used to measure currents of 0 to 1 amperes, 0 to 5 amperes, or 0 to 10 ampere, etc. just by using different values of shunt resistance with the same meter movement and modifying the meters scale accordingly.
Ammeter Example No1
A galvanometer has an internal moving-coil resistance of 100Ω and gives full-scale deflection for 3mA. Calculate the value of shunt resistance required to convert the PMMC meter into a DC ammeter with a range of 0 to 5 amperes.
Data given: RG = 100Ω, IG = 3mA and IT(max) = 5 Amperes
Thus a resistance of 0.06Ω, or 60 milli-ohms (60mΩ) is required to measure a maximum current strength of 5 amperes.
Ammeter Example No2
A PMMC meter has a coil resistance of 200Ω and a linear pointer scale marked with 25 divisions. If the meter has a sensitivity of 4mA per division, calculate the shunt resistance required to measure a maximum current of 20 amperes.
If 4mA = 1 Division, then 25 Divisions = 25*4mA = 100mA, or 0.1 ampere. Thus the PMMC meter has a FSD of 100mA.
Then hopefully we can see that the total resistance given by the ammeter is approximately equal to the value of the connected shunt resistance RS, and clearly becomes smaller as the circuit current being measured increases. Thus the loading effect of the ammeter when connected in series with the circuit component whose current is to be measured is greatly reduced. Ideally, the ammeter’s total resistance would be zero. As shunt resistors used for ammeters have resistive values which are very low, usually they have to be made from relatively large-diameter wire, or solid pieces of copper bar. High current shunts are commonly sold as calibrated copper bars to produce a particular voltage drop in mill-volts (mV).
Measurement of Current
As we saw previously in the tutorial about voltmeters, measuring instuments which use galvanometers can be converted into multi-range meters by the addition of a suitable range of resistors and a selector switch. Our simple DC ammeter can be further extended by having a number of shunt resistances, each one sized for a particular current range that can be selected one-by-one by a single multi-pole 4, or 5-position switch allowing our ammeter to measure a wider range of currents with a single movement. This type of ammeter configuration is called a multirange ammeter.
Direct Multi-range Ammeter Configuration
In this ammeter configuration each shunt resistor, RS of the multirange ammeter is connected in parallel (shunted) with the meter as before to give the desired ampere range. So if we assume our 100uA FSD meter from above is required to measure the following current ranges of 1mA, 10mA, 100mA, and 1A, then the required shunt resistors are calculated the same as before as:
Giving a direct multi-range ammeter circuit of:
While this direct voltmeter configuration would work, one of the major problems with its design is with the type of multi-position selector switch used. Most switches have a “break-before-make” (B-M) action, which means that as the switch is rotated from one position to another to read a different current, at one small instant in time the shunt resistor is actually disconnected from the meter so all the circuit current being measured is diverted through the moving-coil of the meter which may or may not damage it.
One way to overcome this issue is to either use a more expensive “make-before-break” (M-B) action switch, or configure the connection of the shunt resistors in such a way that when the selector switch is rotated they still remain connected in the circuit, thereby protecting the delicate meter movement. One way of achieving this is by using the indirect DC ammeter method.
Indirect Multi-range Ammeter Configuration
A more practical design is the indirect ammeter configuration in which one or more of the shunt resistances are connected together in series across the meter to give the desired current range. The advantage here is that as well as using standard preferred values for the shunt resistors, at any one time the delicate meter movement is shunted by a resistive value. So if we assume again our 50mV FSD meter and the current ranges of 1mA, 10mA, 100mA, and 1A as before, then the required resistor values are recalculated as:
Giving an indirect multi-range ammeter circuit of:
Then we have seen here in this indirect 5-position analogue ammeter configuration that the higher the current to be measured, the lower the value of the shunt resistance being selected by the switch. The total resistance connected in parallel with the PMMC meter will be the sum of the resistances, as RTOTAL = RS1 + RS2 + RS3 + RS4. Clearly then while the two circuits, direct and indirect ammeter configuration are both able to read the same current strengths, the indirect ammeter configuration is preferred as it protects the PMMC meter from an overcurrent condition when the selector switch is being rotated.
Analogue ammeters offer a quick and accurate reading of the amperes flowing around a circuit and the same galvanometer movement can be used to display a range of current strengths simply by changing the resistive value of the shunt. Zero-center ammeters are available and useful to show the direction of the flow of current, that is they can indicate either a “positive” current or a “negative” current flow.
The choice of shunt resistor values will ultimately depend on the FSD of the galvanometer being used as an ammeter as well as the current levels being measured whether the meter’s scale is calibrated in amperes, milliamperes, or microamperes. But what if we wanted to measure 10’s or even 100’s of amperes. Well the same principals apply except that the current shunt would need to be an extremely low-value resistor, usually in the milli-ohm (mΩ) or less value.
High current DC ammeters are available complete with calibrated shunts to provide the necessary voltage drop across the shunt to power the PMMC meter. Voltage drops as low as 10mV or 20mV are available to provide accurate conversion of the primary DC current for the meter to display with full-scale readings into the hundreds of amperes. Remember also that when sizing an ammeter shunt resistor to carry large amounts of current, its I2R power dissipation needs to be taken into account otherwise they may overheat and suffer damage.
The measurement of large AC currents requires the use of a current transformer. As we discussed in our tutorial about Current Transformers, a 5A full-scale ammeter can be used with the appropriate current transformer and are calibrated with the selected transformer.