Download a) Objective

a) Objective :To construct a multi-range voltmeter and ammeter using the PMMC .
b)Theory :In the PMMC, a coil is suspended so that it can rotate freely in a magnetic field.
The EM torque causes the coil to rotate. This EM torque is counter balanced by
the mechanical torque of control springs attached to the movable coil. The
balance of torque and therefore the angular position of the movable coil is
indicated by a pointer against a fixed reference called (scale). The equation for the
developed torque is given by
T = BAIN
Where. T is the torque in N-m
B is the flux density in the air gap in Tesla
I is the coil current in Amps.
N is the number of coil turns.
A is the effective area enclosed by the coil in m
The PMMC, therefore, is a linear-reading DC (polarized) device. Pointer
deflection is directly proportional to the coil current when other parameters are
constant. It is unsuitable for AC measurements unless current is rectified. The
PMMC requires low current for full-scale pointer deflection. When large current
is to be measured it is necessary to bypass the major part of the current through a
resistor called (shunt). The shunt resistance should have a low-temperature
coefficient. Current range of a DC ammeter may be extended further by a number
of shunts selected by a (range) switch. Such an ammeter is called (multi-range)
ammeter.
The addition of a resistor in series with the PMMC movement converts it into a
DC voltmeter. This resistor limits the coil current so that the full-scale deflection
(FSD) current is not exceeded. A multi-range voltmeter can be constructed using
a number of resistors connected in series string and a range selector, which
switches the appropriate amount of resistance in series with the movement.
The quotient of total circuit resistance and the range voltage is called the
sensitivity (S) of the voltmeter. It is the reciprocal of the FSD current Ifsd of the
movement
S = 1/ Ifsd (Ω/V)
For AC measurements the PMMC is used in combination with some rectifier
arrangement. Silicon diodes are usually used. The rectifier produces a pulsating
unidirectional current through the meter movement over a cycle. The meter will
indicate a steady deflection proportional to the average value. The meter scale is
calibrated in terms of the RMS value of a sinusoidal waveform. The form factor
is used for calibration for full wave rectification.
c) Procedure & Results:Full scale = 5000 Div.
DC ammeter
1) measure Rm using the ohmmeter. Read Im
Rm = 0.31 kΩ
Im = 1 mA
2) connect the following circuit. Vary the DC supply for full-scale deflection for
the PMMC. Read the PMMC and the ammeter readings.
PMMC = 5000 Div.
VDC = 2.87 V
Im = 1 mA
3) calculate Rsh to convert the PMMC to 10mA full scale, and then connect the
following circuit.
Rsh = ImRm / (I-Im) = (.001*310) / (0.010-0.001) = 34.4 Ω
4) Adjust the supply voltage to 5V , and then record the meters readings .
I = 1.9 mA
PMMC = 850 Div.
Deflection = 850 / 5000 = 0.17
5) disconnect the circuit.
DC voltmeter
6) Calculate Rs to convert the PMMC to 10V DC full scale. Connect the
following circuit, record the meter reading. Remove the designed voltmeter ,
and then check the above measurement using a voltmeter on 10V DC full
scale .
1mA ( Rs + 310 ) = 10
 Rs = 9.69 kΩ
V = 4.9 V
Deflection = 2370 / 5000
7) Disconnect the circuit .
Loading effects
8) connect the following circuit.
9) Using the above designed 10V DC full scale measure Vo.
Vo = 1.292 V
Deflection = 600 / 5000
10) design 100V DC full scale. Repeat step 10 measurement
Vo = 2.78 V
Deflection = 100 / 5000
series type ohmmeter
11) Build up the following circuit
12) Short circuit the terminals a-b. adjust R2 for full scale deflection on the
movement. Remove the short circuit .
R2 = 620 Ω
13) Calculate R1 for half scale deflection on the movement for 2000Ω reading on
the ohmmeter adjust R1 to the value calculated.
R1 = Rh – ( Rm || R2 ) = 1793.33 Ω
14) Put he load resistance on 2000Ώ. Connect it to the a-b terminals. Check for
half scale deflection.
15) Reduce the supply voltage to 2.8V. zero the ohmmeter. Note the value of R2
R2 = 200 kΩ
d) Questions :1) Calculate the sensitivity of the used PMMC.
S = 5000 div / 1mA = 5000000 div/A
2) Calculate the percentage error for all measurements.
3) Design a 10Vrms voltmeter using half wave rectifier circuit.
4) Design a 10Vrms voltmeter using full wave rectifier circuit.
e) Discussions :As we have seen from the results, the PMMC may be used to design a voltmeter,
ammeter, or ohmmeter. To design an ammeter, we connect a small shunt
resistance with the PMMC to maximize the measured current, because the full
scale current of the PMMC is constant & usually has a small value.
To use it in a voltmeter, we connect series resistances, to divide voltage. The
series resistance usually has a great value. We noticed the loading effect which
makes an error in reading when measuring the voltage across a resistance having
a very large resistance compared with the resistance connected to the PMMC.
When using the PMMC as an ohmmeter we needed an external voltage source to
be connected to the circuit. This is done to generate current in the
PMMC to measure the desired resistance.
In all of the above parts we saw that the reading is more accurate when the
measured value is closer to the value that gives full scale.
f) Conclusions :During this experiment, we constructed a multi-range voltmeter, ohmmeter and
ammeter using the PMMC .
We saw that :1) The PMMC is a linear-reading DC device in which the pointer deflection is
directly proportional to the coil current.
2) The PMMC requires low current for full scale pointer deflection.
3) The PMMC is not suitable for AC readings.
4) When the PMMC is used as an ammeter that measures high currents, a shunt
resistance is connected to bypass the major current.
5) The addition of a resistor in series with the PMMC converts it into a DC
voltmeter.
6) The quotient of total circuit resistance and the range voltage is called the
sensitivity (S) of the voltmeter.
7) For AC measurements the PMMC is used in combination with some rectifier
arrangement.