PMMC Instrument: DC Voltmeter and Ammeter
Mohamad Hafizzudin Bin Hamdan
Faculty of Engineering Technology
Department of Electrical Engineering Technology
University Tun Hussein Onn Malaysia
Johor, Malaysia
Abstract—The permanent-magnet moving-coil (PMMC) instrument consist
basically of a light-weight coil of copper wire suspended in the field of a
permanent magnet. DC voltmeter and ammeter is a instrument that used to
measured the value of current and voltages in the electrical circuit. Ohm's
law and Kirchhoff's rules is fundamental for understanding of dc circuit.
This paper will describe about the DC voltmeter and ammeter in details.
Introduction
The permanent-magnet moving-coil (PMMC) instruments basically consist of a
light-weight coil of copper wire suspended in the field of a permanent
magnet. The current in the wire cause the magnetic field is produces that
interacts with the field from the permanent magnet. For the example , in DC
ammeter and voltmeter are used this system. DC Ammeter and Voltmeter is
used to measure the value of current in amperes in branch of an electric
circuit and voltage in the circuit. Both instrument are mainly used in
company, school, and etc. Before the instruments are used, the instrument
must calibrated to get the accurate measurement.
There are two types of ammeter which is analog ammeter and digital
ammeter. Analog voltmeters move a pointer across a scale in proportion to
the current of the circuit and the digital ammeter give a numerical display
of current by use of an analog to digital converter .This ammeter always
connected in series with a circuit in which current is to be measured. This
is because, the value of current flow is same at the circuit. The value of
currents changes when there is a junction in the circuits. Besides, to
avoid affecting the current level in the circuit, the ammeter must have a
resistance much lower than the circuit resistance.
A voltmeter is an instrument used for measuring electrical
potential difference between two points in an electric circuit. There are
two types of voltmeter which is analog and digital voltmeter. Analog
voltmeters move a pointer across a scale in proportion to the voltage of
the circuit and the digital voltmeters give a numerical display of voltage
by use of an analog to digital converter.
Construction And Basic Principle Operation
1 DC Voltmeter
The current is directly proportional to the voltage across the coil and
the scale of PMMC meter could be calibrates to indicate the value of
voltage. The coil resistance is normally very small and the coil voltage is
also usually very small value. Resistance must add to the PMMC instrument
to make the voltage measured value is larger. To increase range of the
voltmeter, connect the resistance in series with the PMMC instrument.
Figure 1 : construction of dc voltmeter
Figure 2 : Voltmeter circuit
Current in series;
Is = Im
From the Ohm's Law;
V = Im (Rs + Rm )
= ImRs + ImRm
Rs = V/Im - Rm
2 Multirange Voltmeter
In the multirange voltmeter, it consist of a deflecting instrument,
several multiplier resistors, and a rotary switch. The voltage can
measured from the smaller value until higher value by change the
multiplier resistor.
There are two types of Multirange voltmeter circuit;
multirange voltmeter using switched multiplier resistor
Formula for calculate the voltage (V1,V2,V3);
V1= I(Rm + R1)
V2 = I(Rm + R2)
V3 = I(Rm + R3
Figure 3: switched multiplier resistor
1. Multirange voltmeter using series-connected multiplier resistors.
Formula for calculate the voltages ( V1,V2,V3);
V1 = I(Rm +R1)
V2 = I(Rm + R1 + R2)
V3 = I(Rm + R1 + R2 + R3)
Figure 4: series-connected multiplier resistors
1 DC Ammeter
The DC ammeter is an instrument for measuring the electric current in
amperes in a branch of an electric circuit. It must be placed in series
with the measured branch, and must have very low resistance to avoid
significant alteration of the current that is needed to measure. The
pointer deflection is directly proportional to the current flowing in the
coil. The ammeter is also used the principle of d'Arsonval meter movement
with slight modification
Figure 5: construction of dc ammeter
Figure 6: dc ammeter circuit
For calculation;
Vm = ImRm
Vsh = IshRsh
ImRm = IshRsh
I = Ish + Im
Ish = I - Im
Rsh = ImRm/(I – Im)
Shunt Resistance
The smaller value of shunt resistance are used , the larger the shunt
current and the total meter current than the levels calculated because
current will flow at the less resistance.
Swamping Resistance
The swamping resistance is usually three times that of coil thereby
reducing a possible error. In the PMMC instrument, it used the small cooper
wire. When the temperature is changes, the resistance of the wire will
change too. The heating effect of the coil can cause the value of
resistance is change. Its mean have error happen. To minimize the error,
the swamping resistance is used. Swamping resistance made of manganin or
constantan is connected in series with the coil. The manganin and
constantan is used as swamping resistance because it have resistance
temperature coefficient very close to zero.
Figure 7: swamping resistance
2 Multirange DC Ammeter
In the multirange DC Ammeter, it have a rotary switched that can
select any one of several shunt having the different resistance values.
Make – before – break- switch must be used. The instrument is not left
without a shunt in parallel with it even for a brief instant. Besides,
the resistance of the instrument would affect the current flowing in the
circuit.
Figure 8:multirange ammeter using switched shunts
Based on figure 8 : formula for calculation of current, I;
When the rotary switch at terminal B
Is = Vs/Rs =Vm/Rs1
When the rotary switch at terminal D
Is = Vs/Rs =Vm/Rs1
Figure 9: make-before-break switch
Figure 10: multirange DC Ammeter using series connected resistor
Formula for calculation of current, I based on figure 10;
When rotary switch at terminal B
Vm = Im + Rm
Vm = Vs
IT = Im + Is
Is = Vs/Rs = Vm/(R1 + R2 + R3 )
When rotary switch at terminal C
Vm = Im + Rm
Vm = Vs
IT = Im + Is
Is = Vs/Rs = Vm/(R1 + R2 )
When rotary switch at terminal D
Vm = Im + Rm
Vm = Vs
IT = Im + Is
Is = Vs/Rs = Vm/(R1)
III. Advantage and limitation
1. Advantages:
a) The scale is uniformly divided (see at steady state , sθ=G/C*Is).
b) The power consumption can be made very low (25μW to 200μW).
c) The torque-weight ratio can be made high with a view to achieve high
accuracy.
d) A single instrument can be used for multi range ammeters and
voltmeters.
e) Error due to stray magnetic field is very small.
2. Limitations:
a) They are suitable for direct current only.
b) The instrument cost is high.
c) Variation of magnet strength with time.
d) The magnetic field will be decrease when the instrument fall.
e) The reading will not accurate when the instrument near with other
magnet.
IV. Measurement Errors of the Instrument
i) Frictional error,
the value measured change cause the instruments to have friction when
the component moving
ii) Magnetic decay,
- magnetic field will decrease when the instrument fall or in hot
environment.
iii) Thermo electric error,
iv) Temperature error. (systematic error)
- temperature is the main problem because the coil in the PMMC
instrument is very small. when the temperature change, the temperature
of the coil will change and cause the value of resistance change
V. CONCLUSION
PMMC system is used in the instrument that consist of coil. In
this case, DC ammeter and Voltmeter used the PMMC system to measured
the value of current and voltage in a circuit. PMMC also have
measurement error such as temperature error, magnetic decay, and etc.
The instrument had make it easier for the users to measured any
voltage or current. With this paper, basic operation and DC voltmeter
and ammeter had been explained with the aid of the diagram. The
advantages, limitations and its error of the instruments also had been
explained.
References
1] David A. Bell, "Electronic Instrumentation and Measurement," Prentice
Hall , Inc , second edition, pp. 39-51, 1994.
2] 21. Circuit and DC Instruments, :pp.21.3&21.4,. unpublished,
web:http://philschatz.com/physics-book/contents/m42720.html
3] , "textxbook,all about circuit," ammeter design, published under the
terms and condition of the Design Science License,.
web : http://www.allaboutcircuits.com/textbook/direct-current/chpt-
8/ammeter-design/
4] wikipedia, "arsene d'arsonval," unpublished.
web : https://fr.wikipedia.org/wiki/Ars%C3%A8ne_d%27Arsonval
5] , "Permanent Magnet Moving Coil Instrument or PMMC
Instrument,"unpublished.
web : http://www.electrical4u.com/permanent-magnet-moving-coil-
instrument/
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Im = full scale deflection current of the movement (Ifsd)
Rm = internal resistance of the movement
RS = multiplier resistance
V = full range voltage of the instrument
