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CHAPTER 2 : DC METERS
2.1
BASIC PRINCIPLE OF ANALOG METER
This permanent magnet moving coil meter movement is the basic movement in most
analog (meter with a pointer indicator hand) measuring instruments. It is commonly called
d'Arsonval movement because it was first employed by the Frenchman d'Arsonval in making
electrical measurements.
This type of meter movement is a current measuring device which is used in the
ammeter, voltmeter, and ohmmeter. Basically, both the ammeter and the voltmeter are current
measuring instruments, the principal difference being the method in which they are connected
in a circuit. While an ohmmeter is also basically a current measuring instrument, it differs from
the ammeter and voltmeter in that it provides its own source of power and contains other
auxiliary circuits.
2.1.1
Basic Principle Operation Of Permanent-Magnetic Moving-Coil Movement
a) Basic Construction
b) The Permanent-Magnetic Moving-Coil
Movement Used In A Meter.
Figure 2.1 : Permanent-Magnetic Moving-Coil
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The compass and conducting wire meter can be considered a fixed-conductor moving-magnet
device since the compass is, in reality, a magnet that is allowed to move. The basic principle of
this device is the interaction of magnetic fields: the field of the compass (a permanent magnet)
and the field around the conductor (a simple electromagnet).
A permanent-magnet moving-coil movement is based upon a fixed permanent magnet
and a coil of wire which is able to move, as in figure 2.2. When the switch is closed, causing
current through the coil, the coil will have a magnetic field which will react to the magnetic field
of the permanent magnet. The bottom portion of the coil will be the north pole of this
electromagnet. Since opposite poles attract, the coil will move to the position shown in figure
2.3.
Figure 2.2 : A movable coil in a magnetic
field (with current).
Figure 2.3. : A movable coil in a magnetic
field (no current).
The coil of wire is wound on an aluminum frame, or bobbin, and the bobbin is
supported by jeweled bearings which allow it to move freely. This is shown in figure 2.4. To use
this permanent-magnet moving-coil device as a meter, two problems must be solved. First, a
way must be found to return the coil to its original position when there is no current through
the coil. Second, a method is needed to indicate the amount of coil movement. The first problem
is solved by the use of hairsprings attached to each end of the coil as shown in figure 2.5. These
hairsprings can also be used to make the electrical connections to the coil.
Figure 2.4. : A basic coil arrangement.
Figure 2.5. : Coil and hairsprings.
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With the use of hairsprings, the coil will return to its initial position when there is no
current. The springs will also tend to resist the movement of the coil when there is current
through the coil. When the attraction between the magnetic fields (from the permanent magnet
and the coil) is exactly equal to the force of the hairsprings, the coil will stop moving toward the
magnet.
As the current through the coil increases, the magnetic field generated around the coil
increases. The stronger the magnetic field around the coil, the farther the coil will move. This is
a good basis for a meter. But, how will you know how far the coil moves? If a pointer is
attached to the coil and extended out to a scale, the pointer will move as the coil moves, and the
scale can be marked to indicate the amount of current through the coil. This is shown in figure
2.6.
Figure 2.6. - A complete coil.
Figure 2.7 : Complete Construction of
Permanent Magnet Moving Coil (PMMC)
Two other features are used to increase the accuracy and efficiency of this meter
movement. First, an iron core is placed inside the coil to concentrate the magnetic fields.
Second, curved pole pieces are attached to the magnet to ensure the turning force on a coil
increases steadily as the current increases. These same curved pole pieces are found in a motor.
2.1.3. Deflection Torque
It has been mentioned that interaction between the induced field and the field produced by the
permanent magnet causes a deflecting torque, which results in rotation of the coil. Deflection
torque is controlling torque controls the deflection and tries to stop the pointer at its final
position. But due to inertia, the pointer oscillates around its final position before coming to rest.
Hence damping torque is provided to avoid this oscillation and bring the pointer quickly to its
final position.
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Thus the damping torque is never greater than the controlling torque. In fact it is the condition
of critical damping which is sufficient to enable the pointer rising quickly to its deflected
position without overshooting.
The deflecting torque produced is described below in mathematical form:
Deflecting Torque, Td = BINA
(Equation 2.1)
Where
B = flux density in Wb/m2 (Tesla)
I = current (A).
N = number of turns of the coils.
A = area ( length X wide), (m2).
Example 1:
Given frame of permanent moving coil is 6m2. The number of winding around coil is 50 and
flux 0.12 wb/m2. If 1mA current through the coil, calculate the deflection torque.
Solution
Td
= BINA
= (0.12 wb/m2)( 1mA)(50)(6m2)
= 36mNm
2.1.5 Damping
A problem that is created by the use of a rectifier and d’Arsonval meter movement is that the
pointer will vibrate (oscillate) around the average value indication. In physics, damping is any
effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the
harmonic oscillator.
This oscillation will make the meter difficult to read. The process of "smoothing out" the
oscillation of the pointer is known as DAMPING. There are two basic techniques used to damp
the pointer of a d’Arsonval meter movement.
i.
The first method of damping comes from the d’Arsonval meter movement itself. In the
d’Arsonval meter movement, current through the coil causes the coil to move in the
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magnetic field of the permanent magnet. This movement of the coil (conductor) through
a magnetic field causes a current to be induced in the coil opposite to the current that
caused the movement of the coil. This induced current will act to damp oscillations. In
addition to this method of damping, which comes from the movement itself, most
meters use a second method of damping.
ii.
The second method of damping used in most meter movements is an airtight chamber
containing a vane (like a windmill vane) attached to the coil.
As the coil moves, the vane moves within the airtight chamber. The action of the vane against
the air in the chamber opposes the coil movement and damps the oscillations.
There are two general classes of damped motion, as follows:
1.
2.
Periodic, in which the pointer oscillates about the final position before coming to rest.
Aperiodic, in which the pointer comes to rest without overshooting the rest position.
The point of change between periodic and aperiodic damping is called "critical
damping." An instrument is considered to be critically damped when overshoot is
present but does not exceed an amount equal to one half the rated accuracy of the
instrument.
A problem that is created by the use of a rectifier and d’Arsonval meter movement is that the
pointer will vibrate (oscillate) around the average value indication. This oscillation will make
the meter difficult to read. The value of the damping ratio ζ determines the behavior of the
system. A damped harmonic oscillator can be:
i.
Critical damping (ζ = 1)
When ζ = 1, there is a double root γ (defined above), which is real. The system is said to
be critically damped. A critically damped system converges to zero faster than any
other, and without oscillating. An example of critical damping is the door closer seen on
many hinged doors in public buildings. The recoil mechanisms in most guns are also
critically damped so that they return to their original position, after the recoil due to
firing, in the least possible time.
ii.
Over-damping (ζ > 1)
When ζ > 1, the system is over-damped and there are two different real roots. An overdamped door-closer will take longer to close than a critically damped door would
iii.
Under-damping (0 ≤ ζ < 1)
Finally, when 0 ≤ ζ < 1, γ is complex, and the system is under-damped. In this situation,
the system will oscillate at the natural damped frequency ωd, which is a function of the
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natural frequency and the damping ratio. To continue the analogy, an underdamped
door closer would close quickly, but would hit the door frame with significant velocity,
or would oscillate in the case of a swinging door.
Figure 2.8 : Damping Curve
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2.1.6
DC Meter / 7
Common Damping System In Indicating Instrument
a.
Air friction damping
Figure 2.9 : Air Friction Damping .
b.
Liquid damping
Similar principle as air damping only the vane moves in a liquid chamber with a
proper concentration.
c.
Eddy current damping
Eddy currents are currents induced in conductors to oppose the change in flux
that generated them. It is caused when a conductor is exposed to a changing
magnetic field due to relative motion of the field source and conductor; or due to
variations of the field with time.
This can cause a circulating flow of electrons, or a current, within the body of the
conductor. These circulating eddies of current create induced magnetic fields
that oppose the change of the original magnetic field due to Lenz's law, causing
repulsive or drag forces between the conductor and the magnet.
The stronger the applied magnetic field, or the greater the electrical conductivity
of the conductor, or the faster the field that the conductor is exposed to changes,
then the greater the currents that are developed and the greater the opposing
field.
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Figure 2.12 : Eddy Current Damping
2.2
DC VOLTMETER
A basic d’Arsonval movement can be converted into dc voltmeter by adding in series resistor
multiplier as shown in figure 2.9.
Figure 2.9 : DC Voltmeter circuit.
IM
RM
RS
V
= full scale deflection current of the movement (Ifsd)
= internal resistance of the movement
= multiplier resistance
= full range voltage of the instrument
Current in series;
IS = IM
(Equation 2.2)
From Ohm Law;
V = IM (RS + RM)
=
IM RS + IMRM
(Equation 2.3)
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(Equation 2.4)
Exercise 2
A basic D’ Arsonval movement with a full-scale deflection of 50 µA and internal resistance of
500Ω is used as a DC voltmeter. Determine the value of the multiplier resistance needed to
measure a voltage range of 0-10V.
Solution:
= 500Ω
= 50 uA
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2.2.3 Multi-Range Voltmeter
A DC voltmeter can be converted into a multirange voltmeter by connecting a number of
resistors (multipliers) in series with the meter movement. A practical multi-range DC voltmeter
is shown in Figure 2.6.
R1
R2
Rm
V1
V2
+
Im
_
Figure 2.10 : DC Multi-range Voltmeter circuit.
(Equation 2.5)
(Equation 2.6)
(Equation 2.7)
Exercise 3
Convert a basic D’ Arsonval movement with an internal resistance of 100Ω and a full scale
deflection current of 1mA into a multirange dc voltmeter with voltage ranges of 0-15V and 050V.
Solution
i.
Range 0 – 15V
= 15KΩ
Ω
EE 101 MEASUREMENT
ii.
Range 0 – 50V
2.2.7
DC Meter / 11
"#
Ω
!
$ %Ω
Loading Effects in DC Voltmeter
When a voltmeter is used to measure the voltage across a circuit component, the voltmeter
circuit itself is in parallel with the circuit component. Total resistance will decrease, so the
voltage across component will also decrease. This is called voltmeter loading. The resulting
error is called a loading error. The voltmeter loading can be reduced by using a high sensitivity
voltmeter.
The exercise below can show the loading effect when using voltmeter with two value
sensitivity.
Exercise 4
R1 = 200KΩ
200
R2 = 50KΩ
v
Find the voltage across the resistor 50KΩ as shown in figure above if using
a. Voltmeter with sensitivity 1000Ω/V
b. Voltmeter with sensitivity 20000Ω/V
And voltmeter range for both measurements is 0 – 50V.
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Solution
a. At range 50V and the sensitivity voltmeter is 1000Ω/V
Actual voltage value across R2,
R2
&'
&( )&'
V2 = -------- X V
*
R1 + R2
+,Ω
+,Ω)++,Ω
*-
= 40 V
Analysis voltmeter with sensitivity 1000Ω/V across R2.
Impendent voltmeter, Rin = V (range) x sensitivity
= 50V x 1000 Ω/V
= 50KΩ
When voltmeter connected, the resistance in circuit as s how in figure
R voltmeter
R1 = 200KΩ
200V
V Rin = 50KΩ
R2 = 50KΩ
Req = -----------------R2 selari dengan Rin
The circuit can be simplify as show in figure below
R1 = 200KΩ
200V
./ 012
3 12
0
./ Req = 25KΩ
3 ./ -Ω
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So, the reading at voltmeter is
Voltage across Req, &45
&45 )&(
*
,
,)++, *-
$$ $$V
6789:9;9:< =9:>;?@:!7:7A
*C
=9:>?B:;?@:!7:7A
----
*C
"
C
DAA?A C 6789:9;9:<
C C
"""C
"""C
b. At range 50V and sensitivity voltmeter is 20000Ω/V
The calculation actual voltage value across R2 is same with a.
Analysis voltmeter with sensitivity 20000Ω/V across R2.
Impendent voltmeter, Rin = V (range) x sensitivity
= 50V x 20000 Ω/V
= 1MΩ
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When voltmeter connected, the resistance in circuit as show in figure
R voltmeter
R1 = 200KΩ
200V
V Rin = 1MΩ
R2 = 50KΩ
R2 selari dengan Rin
The circuit can be simplify as show in figure below
R1 = 200KΩ
./ 200V
012
E 12
Req = 47.62KΩ
./ 0
E ./ "FG-Ω
So, the reading at voltmeter is
Voltage across Req, &45
&45 )&(
*
"FG-
*-
"FG- E -
%H IV
6789:9;9:< =9:>;?@:!7:7A
*C
=9:>?B:;?@:!7:7A
JK"G
*C
"
I C
DAA?A C 6789:9;9:<
DAA?A C #GC
= JKC
EE 101 MEASUREMENT
2.3
DC Meter / 15
DC AMMETER
The PMMC galvanometer constitutes the basic movement of a dc ammeter. The coil winding of
a basic movement is small and light, so it can carry only very small currents. The PMMC can
use to build an ammeter with connected the shunt resistor and meter in parallel. A low value
resistor (shunt resistor) is used in DC ammeter to measure large current. Basic DC ammeter:
IM
I
ISH
RM
RSH
Figure 2.11 : DC Ammeter circuit.
RM
RSH
ISH
IM
I
= internal resistance of the movement
= shunt resistance
=shunt current
= full scale deflection current of the movement
= full scale current of the ammeter + shunt (i.e. total current)
* RSH is smaller than RM
&
L &
(Equation 2.8)
&
L L (Equation 2.9)
L &
MN
(Equation 2.10)
From Ohm’s Laws
E L
L Therefore
L &
O
(Equation 2.11)
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DC Meter / 16
Exercise 5
A 1mA meter movement with an internal resistance of 100Ω is to be converted into a 0-100 mA.
Calculate the value of shunt resistance required.
Solution
L L !Ω
! !
= 1.01Ω
2.3.3 Multirange Ammeter – Individual Shunt
The range of the dc ammeter is extended X by a number of shunts, selected by a range switch.
The resistors are placed in parallel to give different current ranges. Switch S (multiposition
switch) protects the meter movement from being damage during range changing.
a.
Individual Shunts
I
IM
ISH
RM
RSH1
S
RSH2
RSH3
RSH4
Figure 2.12 : Individual Shunt circuit.
&
L O
(Equation 2.12)
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b.
DC Meter / 17
Ayrton Shunt
Julat
10mA
R2
RSHT
RM = 50Ω
IM = 2 mA
Julat
100mA
R1
RSH1
Figure 2.13 : Individual Ayrton Shunt circuit.
Total shunt resistor, RSHT = R2 + R1
(Equation 2.13)
Total resistor, RT = RSHT + RM
(Equation 2.14)
# To calculate the total shunt resistor, determine from the lowest range.
RSHT = (….. + Rn + R2 + R1) =
&
O
(Equation 2.15)
# To calculate another RSH, start from the highest range.
L P&MNQ )& R
(Equation 2.16)
Exercise 6
Refer the circuit above, calculate shunt resistor ( R1 and R2) when using range - 10mA and
100mA.
Solution
To find total shunt resistor use the lowest range – 10mA.
RSHT = (R2 + R1) =
+Ω
+O
= 12.5Ω.
For shunt resistor at 100mA (highest range)
L -!P- E R
!
$Ω
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DC Meter / 18
L -Ω -Ω
$ $Ω
2.4
OHMMETER
The PMMC can change to be ohmmeter with connected voltage source and limited current
resistor in series. The type of Ohmmeter is series ohmmeter and parallel ohmmeter. The
purpose of an ohmmeter is to measure the resistance placed between its leads. This resistance
reading is indicated through a mechanical meter movement which operates on electric current.
2.4.1
Series Ohmmeter
R1
R1 = Limited Current Resistor
R2 = Zero Adjust Resistor
Rx = unknown Resistance
Rm = Meter Resistance
Rm
A
Rx
V
R2
B
Figure 2.14 : Individual Series Ohmmeter circuit.
Operation of Series Ohmmeter
When Rx = 0 ( AB terminal short), the current in circuit is maximum and the pointer shown the
full reading. Adjust the R2 until the full scale, IM. The pointer at full scale is mark as 0 ohm.
When Rx = infinity (AB terminal open), the current in circuit is 0. The unknown resistance must
connect series with basic meter movement. This circuit use to measure higher resistance and the
pointer is mark as infinity.
0Ω
AB Terminal Short
∞Ω
AB Terminal open
Figure 2.15 : The pointer location
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DC Meter / 19
When Rx connected;
& &
S
( )& )&T )&'
(Equation 2.17)
U P E E R
(Equation 2.18)
Exercise 7
Given PMMC with resistance 100Ω was using in series ohmmeter. R1 = 500Ω, R2 = 400Ω and
supply voltage = 10V. When connected with Rx, the reading shows 0.5mA. Find the value of Rx.
Solution
R1 = 500Ω
A
Rm = 100Ω
10V
Rx
R2 = 400Ω
B
U P E E R
U P E E "R
!
V Ω
2.4.2 Shunt Resistor in Series Type Ohmmeter
R1 = Limited Current Resistor
R2 = Zero Adjust Resistor
Rx = unknown Resistance
RM = Meter Resistance
RM
R1
A
Rx
V
R2
B
Figure 2.15 : Individual Shunt resistor in series type ohmmeter circuit.
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0
W E - E X
-
YC ZM
(Equation 2.19)
X
, the ratio between current reading when Rx is connected and full scale
current.
Req
IFS when RX = 0 ( AB short)
A
[
Vin
[
B
\]
&Q
where E - 0X
- EX
0
E -E X
-
X
(Equation 2.20)
IFS when RX is connected
RT
A
[
12
E U
U 12
[
U 12
0
E [
E Rx
Vin
B
(Equation 2.21)
Exercise 8
R1
A
Movement meter have 100µA FSD.
Assume R1 + R2 + Rm = 15KΩ
Rm
1.5V
Rx
R2
B
a)
b)
State the pointer location when Rx = 0.
State the location for ohmmeter scale at ½ FSD, ¼ FSD and ¾ FSD.
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Solution
a)
&
Q
&
( )&' )&T )&
,^)+^
__`abc
b) At ½ FSD
= ½ X 100µA
= 50µA.
U P E E R
+d At ¼ FSD
= ¼ X 100µA
= 25µA.
U P E E R
d "
At ¾ FSD
= ¾ X 100µA
= 75µA.
U P E E R
ed
EE 101 MEASUREMENT
2.4.3
DC Meter / 22
Parallel (Shunt) Ohmmeter
R2
R1
Ix A
I
Im
V
Rx
B
Figure 2.16 : Individual Shunt Ohmmeter circuit.
U
U U
U &
(Equation 2.22)
O
Operation of Shunt Ohmmeter
R1
I
A
IM
+
Vin
-
Rx
RM
S1
B
Figure 2.17 : Operation of Shunt Ohmmeter.
S1 is using for cut-off the battery (Vin) when not using the circuit.
When Rx = 0 ( AB terminal short), no current in circuit and the pointer is mark as 0 ohm.
When Rx = infinity (AB terminal open), the current (IM) in circuit is maximum. Adjust R1 until
the meter movement is full scale, and the pointer is mark as ∞Ω (Infinity).
∞Ω
AB Terminal Open
0Ω
AB Terminal short
Figure 2.18 : The pointer location
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Example 9
R1 = 4.5KΩ
Ix A
I = 5µA
State the ohmmeter scale when the
current is 0A, ½ FSD, ¼ FSD and FSD
Im
V
Rx
Rm = 500Ω
B
Solution
At 0A scale, U &
U O
+P++^R
PO+Rd
At ½ FSD, U
dP++^R
PORd
At ¼ FSD, U
dP++^R
PORd
GGF
At FSD, U
dP++^R
PORd
f
2.4.4
Function
i.
Current limiting resistance
A resistor inserted in an electrical circuit to limit the flow of current to some
predetermined value. It is used chiefly to protect tubes and other components
during warm-up.
ii.
Zero adjusts resistance.
A resistor inserted in an electrical circuit to adjusts the value of resistance to zero.
iii.
Meter resistance
A resistance of the meter's armature coil.
EE 101 MEASUREMENT
iv.
2.4.5
DC Meter / 24
Unknown resistance
A resistance that unknown value in a circuit.
Differentiate typical scale in series and shunt type ohmmeter.
Scale in Series Type Ohmmeter
Scale starts with infinity at left
side and zero at right side
Scale in Shunt Type Ohmmeter
Scale starts with zero at left side
and infinity at right side
ϖ
0
R1
0
ϖ
A
R1
A
-
R
m
When point A short B, Rx=0 (Maximum current
flows in circuit) so R2 is adjusted for pointer
pointing at Rm zero position (which means
current only flow through Rm). Normally it
known as 'Zero Adjusted'.
R2
E
E
Open
circuit
R2
E
B
When point A short B, Rx=0 (No current flows
in meter) so R1 is adjusted to pointing at zero
position. Normally it known as 'Zero
Adjusted'.
R1
A
R
m
B
R
m
E
A
R
m
Open
circuit
B
When point A-B open, Rx=infinity (No current When point A-B open, Rx=infinity (Maximum
flows in circuit) so pointer is pointing to infinity current flows in meter) so pointer is pointing
position.
to infinity position.
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R1
R
m
R1
A
Rx
R2
E
E
B
A
B
R
m
B
When point A-B connecting to unknown Rx to
-When point A-B connecting to unknown Rx
measure resistance value, pointer will point to a
to measure resistance value, pointer will point
certain value proportional to resistance value.
to a certain value proportional to resistance
value.
2.5
ANALOGUE AND DIGITAL MULTIMETER
An instrument designed to measure electrical quantities. A typical multimeter can measure
alternating- and direct-current potential differences (voltages), current, and resistance, with
several full-scale ranges provided for each quantity.
2.5.1 Analogue Multimeter
Figure 2.17 : : Schematic circuit analog multimeter
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DC Meter / 26
Range of Ohm, volt and ampere in analogue multimeter
a) DC Voltage: 0.5V, 2.5V, 10V, 50V, 250V, 1000V.
b) AC Voltage: 10V, 50V, 250V, 1000V.
c) DC Current: 50µA, 2.5mA, 25mA, 250mA.
A high current range is often missing from this type of meter.
d) AC Current: None. (You are unlikely to need to measure this).
e) Resistance: 20, 200, 2k, 20k, 200k.
These resistance values are in the middle of the scale for each range.
Figure 2.18 : Multimeter Range
Scale of Ohm, volt and ampere in analogue multimeter
Figure 2.19 : Multimeter Scale.
Check the setting of the range switch and choose an appropriate scale.
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Sensitivity of meter
he accuracy with which a meter can measure a voltage, current,
Sensitivity is define as the
resistance, or other quantity.
Multimeters must have a high sensitivity of at least 20k /V otherwise their resistance on
DC voltage ranges may be too low to avoid u
upsetting
psetting the circuit under test and giving an
incorrect reading. To obtain valid readings the meter resistance should be at least 10 times the
circuit resistance (take this to be the highest resistor value near where the meter is connected).
If you are buying an analogue multimeter make sure it has a high sensitivity of 20k
/V or greater on DC voltage ranges, anything less is not suitable for electronics. The sensitivity
is normally marked in a corner of the scale, ignore the lower AC value (sensitivity on AC ranges
is less important), the higher DC value is the critical one. Beware of cheap analogue multimeters
sold for electrical work on cars because their sensitivity is likely to be too low.
Analogue
nalogue meters take a little power from the circuit under tes
testt to operate their pointer.
They must have a high sensitivity of at least 20k /V or they may upset the circuit under test
and give an incorrect reading.
Analogue meter resistance
Analogue meter resistance is refers to resistance in coil winding armature and it can only carry
very small currents.
Step to do following measurement using multimeter
1.
Measure Resistance using analogue multimeter
• To measure the resistance of a component it must not be connected in a circuit. If
you try to measure resistance of components in a circuit you will obtain false
readings (even if the supply is disconnected) and you may damage the multimeter.
• The resistance scale on an analogue meter is normally at the top, it is an unusual
scale because it reads backwards and is not linear (evenly spaced). This is
unfortunate, but it is due to the way the meter works.
• Procedure
i. Set the meter to a suitable resistance range.
Choose a range so that the resistance you expect will be near the middle of the
scale. For example: with the scale shown below and an expected resistance of
about 50k choose the × 1k range.
ii. Hold the meter probes together and adjust the control on the front of the meter
which is usually labeled "0 ADJ" until the pointer reads zero (on the RIGHT
remember!). If you can't adjust it to read zero, the battery inside the meter needs
replacing.
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iii. Put the probes across the component.
Avoid touching more than one contact at a time or your resistance will upset the
reading!
2. Measuring voltage and current with a multimeter
i.
ii.
iii.
2.5.2
Select a range with a maximum greater than you expect the reading to be.
Connect the meter, making sure the leads are the correct way round.
Digital meters can be safely connected in reverse, but an analogue meter may be
damaged.
If the reading goes off the scale: immediately disconnect and select a higher
range.
DIGITAL MULTIMETERS
Figure 2.6. Digital Multimeter
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DC Meter / 29
SELECTOR SWITCH
Figure 2.6.1.—Model 8000A block diagram Digital Multimeter
Function of each block
Note that the block diagram divides the instrument into three major sections:
1)
The SIGNAL CONDITIONING section consist of input divider (resistors connected in
series), current shunt (resistor connected in parallel), selector switch, AC Convertor, active
filter ( filter AC signal to DC signal.
2)
The ANALOGUE-TO- DIGITAL CONVERTER section consist of Analog IC and Digital
IC, convert analog input to digital output.
3)
The DISPLAY section consist of four LEDs, analog control, decoder driver. The output
from Digital IC are in binary number, will past through BCD (Binary Coded Decimal). and
decoder driver where the measured value displayed decimal value.
Measuring using digital multimeter
1. Measuring voltage using digital multimeter
The steps are the same as analogue multimeter, but Digital meters can be safely connected
in reverse, but an analogue meter may be damaged.
2. Measuring current using digital multimeter . The steps are the same as analogue
multimeter, but Digital meters can be safely connected in reverse, but an analogue meter
may be damaged.
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3. Measuring resistance with a DIGITAL multimeter
i.
Set the meter to a resistance range greater than you expect the resistance to be.
Notice that the meter display shows "off the scale" (usually blank except for a 1 on
the left). Don't worry, this is not a fault, it is correct - the resistance of air is very
high!
2.6.
ii.
Touch the meter probes together and check that the meter reads zero.
If it doesn't read zero, turn the switch to 'Set Zero' if your meter has this and try
again.
iii.
Put the probes across the component.
Avoid touching more than one contact at a time or your resistance will upset the
reading!
DIFFERENTIATE BETWEEN ANALOG AND DIGITAL MULTIMETER
1. An
n analogue multimeter used scale on moving coil meter to indicate the measured
value while a digital multimeter used LEDs Display to display the measured value.
n analogue multimeter used resistors (shunt and multiplier) while a digital
2. An
multimeter used ICs, LEDs, Convertors, decoder driver.
2.7
COMPARE ADVANTANGES AND DISADVANTAGES
1.
The sensitivity of analogue multimeter depends on the voltage ranges. All digital
meters contain a battery to power the display so they use virtua
virtually
lly no power from
the circuit under test. This means that on their DC voltage ranges they have a very
high resistance (usually called input impedance) of 1M or more, usually 10M ,
and they are very unlikely to affect the circuit under test.
2.
When measuring voltage using digital multimeter. The steps are the same as
analogue multimeter, but Digital meters can be safely connected in reverse, but an
analogue meter may be damaged.
3.
When measuring current using digital multimeter. The steps are the same as
analogue multimeter, but Digital meters can be safely connected in reverse, but an
analogue meter may be damaged.
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2.5.8
DC Meter / 31
Multimeter safety precaution
As with other meters, the incorrect use of a multimeter could cause injury or damage.
The following safety precautions are the MINIMUM for using a multimeter.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Deenergize and discharge the circuit completely before connecting or
disconnecting a multimeter.
Never apply power to the circuit while measuring resistance with a multimeter.
Connect the multimeter in series with the circuit for current measurements, and
in parallel for voltage measurements.
Be certain the multimeter is switched to ac before attempting to measure ac
circuits.
Observe proper dc polarity when measuring dc.
When you are finished with a multimeter, switch it to the OFF position, if
available. If there is no OFF position, switch the multimeter to the highest ac
voltage position.
Always start with the highest voltage or current range.
Select a final range that allows a reading near the middle of the scale.
Adjust the "0 ohms" reading after changing resistance ranges and before making
a resistance measurement.
Be certain to read ac measurements on the ac scale of a multimeter.
Observe the general safety precautions for electrical and electronic devices.