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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 Figure 2.3. : A movable coil in a magnetic field (with current). 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. EE 101 MEASUREMENT DC Meter / 4
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, T d = BINA (Equation 2.1)
Where B = flux density in Wb/m 2 (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 6m 2. The number of winding around coil is 50 and flux 0.12 wb/m 2. If 1mA current through the coil, calculate the deflection torque.
Solution
Td = BINA = (0.12 wb/m 2)( 1mA)(50)(6m 2) = 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 EE 101 MEASUREMENT DC Meter / 5
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. Periodic, in which the pointer oscillates about the final position before coming to rest. 2. 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 over- damped 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 EE 101 MEASUREMENT DC Meter / 6
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 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. EE 101 MEASUREMENT DC Meter / 8
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 = full scale deflection current of the movement (I fsd )
RM = internal resistance of the movement
RS = multiplier resistance V = full range voltage of the instrument
Current in series;
IS = I M (Equation 2.2)
From Ohm Law;
V = IM (R S + R M)
= IM RS + I MRM (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:
= 500 Ω
= 50 uA
ˢ ˞ .˞
ŵŴˢ ˞ . ŹŴŴé ŹŴ˓
ͭé
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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.
R 1 R 2
Rm V V2 1 + 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 0- 50V.
Solution i. Range 0 – 15V
˞# ˞# . ˞ ˞# ˞ ŵŹ . ŵŴŴ #' # ˞ = 15KΩ # # ͭΩ
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ii. Range 0 – 50V
˞$ ˞$ ˞$ . ˞ .˞#
˞$ ŹŴ .ŵŴŴ.ŵŸ# ŹŴˢ ˞$ ŹŴ Ω ŵ˭˓ $ % ͭΩ
2.2.7 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 = 200K Ω
200 R2 = 50K Ω 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 R 2,
R2 &' ˢ$ ˲ˢ V2 = ------X V &()&'
R1 + R 2 '",Ω ˲ŶŴŴˢ '",Ω)$"",Ω
= 40 V
Analysis voltmeter with sensitivity 1000Ω/V across R 2.
Impendent voltmeter, R in = 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 = 200K Ω
200V R2 = 50K Ω V R = 50K Ω in
Req = ------
R2 selari dengan R in
The circuit can be simplify as show in figure below
R = 200K Ω 1 ˞$0˞12 ˞./ ˞$ 3 ˞12
ŹŴ 0ŹŴ ˞ 200V Req = 25K Ω ./ ŹŴ 3 ŹŴ
˞./ ŶŹ Ω
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So, the reading at voltmeter is
&45 Voltage across R eq , ˢ$ ˲ˢ &45 )&(
$', ˲ŶŴŴˢ $',)$"",
$$ $$ V
ˢ$˱˩ˮ˨˰?ˬˮ˭˥ˮ˥A ˟˥8˩ˮ˩˰˩ˮ˳ ˲ŵŴŴC ˢ$˱˩ˮ˨?˯ˮ˰?ˬˮ˭˥ˮ˥A ŶŶŶŶˢ ˲ŵŴŴC ŸŴˢ
C
˗AA?A ŵŴŴC . ˟˥8˩ˮ˩˰˩ˮ˳
ŵŴŴC . ŹŹŹŹC
ŸŸŸŹC ŸŸŸŹC
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 R 2.
Impendent voltmeter, R in = 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 = 200K Ω
200V R2 = 50K Ω V Rin = 1M Ω
R2 selari dengan R in
The circuit can be simplify as show in figure below
R1 = 200K Ω
˞$0˞12 ˞./ ˞$ - ˞12 200V R = 47.62K Ω eq ŹŴ 0ŵŴŴŴ ˞ ./ ŹŴ - ŵŴŴŴ
˞./ ŸŻźŶ Ω So, the reading at voltmeter is
&45 Voltage across R eq , ˢ$ ˲ˢ &45 )&(
ŸŻźŶ ˲ŶŴŴˢ ŸŻźŶ - ŶŴŴ
%H I V
ˢ$˱˩ˮ˨˰?ˬˮ˭˥ˮ˥A ˟˥8˩ˮ˩˰˩ˮ˳ ˲ŵŴŴC ˢ$˱˩ˮ˨?˯ˮ˰?ˬˮ˭˥ˮ˥A ŷKŸźˢ ˲ŵŴŴC ŸŴˢ
I C
˗AA?A ŵŴŴC . ˟˥8˩ˮ˩˰˩ˮ˳
˗AA?A ŵŴŴC . #źŵŹC
= ŷKŹC EE 101 MEASUREMENT DC Meter / 15
2.3 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:
I R I M M
I SH R SH
Figure 2.11 : DC Ammeter circuit.
RM = internal resistance of the movement
RSH = shunt resistance
ISH =shunt current
IM = full scale deflection current of the movement I = full scale current of the ammeter + shunt (i.e. total current)
* R SH is smaller than RM
ˢ& L ˢ& (Equation 2.8)
& L ˞ L ˞ (Equation 2.9)
& ˞ L (Equation 2.10) MN
From Ohm’s Laws
- L
L .
Therefore
& ˞ L (Equation 2.11) O
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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 .
ŵ˭˓ŵŴŴ Ω ˞ L ŵŴŴ˭˓ . ŵ˭˓
= 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 RM
ISH R SH1
S RSH2
RSH3
RSH4
Figure 2.12 : Individual Shunt circuit.
& ˞ L (Equation 2.12) O
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b. Ayrton Shunt
Julat
10mA R2 RSHT
RM = 50Ω IM = 2 mA Julat R1 RSH1 100mA
Figure 2.13 : Individual Ayrton Shunt circuit.
Total shunt resistor, R SHT = R 2 + R 1 (Equation 2.13)
Total resistor, R T = R SHT + R M (Equation 2.14)
# To calculate the total shunt resistor, determine from the lowest range.
& RSHT = (….. + R n + R 2 + R 1) = (Equation 2.15) O
# To calculate another R SH , start from the highest range.
P& )& R ˞ MNQ (Equation 2.16) L
Exercise 6
Refer the circuit above, calculate shunt resistor ( R 1 and R 2) when using range - 10mA and 100mA.
Solution To find total shunt resistor use the lowest range – 10mA.
$ '" Ω RSHT = (R 2 + R 1) = #" O$ = 12.5Ω.
For shunt resistor at 100mA (highest range)
Ŷ˭˓PŵŶŹ - ŹŴR ˞ ˞ L# # ŵŴŴ˭˓
$ Ω
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˞$ ˞ L .˞#
˞$ ŵŶŹΩ . ŵŶŹ 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
Rm R1 A
R1 = Limited Current Resistor R2 = Zero Adjust Resistor V Rx Rx = unknown Resistance R2
Rm = Meter Resistance B Figure 2.14 : Individual Series Ohmmeter circuit.
Operation of Series Ohmmeter
When R x = 0 ( AB terminal short), the current in circuit is maximum and the pointer shown the full reading. Adjust the R 2 until the full scale, I M. The pointer at full scale is mark as 0 ohm.
When R x = 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 ∞Ω
AB Terminal Short AB Terminal open
Figure 2.15 : The pointer location
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When R x connected;
&S &()&)&T)&' (Equation 2.17) ˞ .P˞ -˞ -˞ R U # $ (Equation 2.18)
Exercise 7
Given PMMC with resistance 100Ω was using in series ohmmeter. R 1 = 500Ω, R 2 = 400Ω and supply voltage = 10V. When connected with R x, the reading shows 0.5mA. Find the value of R x.
Solution
R1 = 500 Ω A
Rm = 100 Ω 10V Rx
R2 = 400 Ω B
ˢ ˞ .P˞ -˞ -˞ R U # $
ŵŴˢ ˞ .PŹŴŴ - ŵŴŴ- ŸŴŴR U ŴŹ˭
ʹ ͭ Ω
2.4.2 Shunt Resistor in Series Type Ohmmeter
R1 = Limited Current Resistor RM R1 R2 = Zero Adjust Resistor A
Rx = unknown Resistance
RM = Meter Resistance V Rx R2
B
Figure 2.15 : Individual Shunt resistor in series type ohmmeter circuit. EE 101 MEASUREMENT DC Meter / 20
˞Ŷ0˞X ˞ˠ ˞ŵ - (Equation 2.19) ˞Ŷ-˞X
˖C , the ratio between current reading when R x is connected and full scale ZM current.
Req A IFS when R X = 0 ( AB short)
\] ˞Ŷ0˞X [ where ˞ ˞ŵ - &Q ˞Ŷ-˞X Vin ˢ [ ˞Ŷ0˞X ˞ŵ - B ˞Ŷ -˞X
(Equation 2.20)
R IFS when R X is connected T A ˢ12 [ ˞ -˞U V Rx in ˢ12 ˞U .˞ [ B ˢ12 ˞$0˞ ˞U .˞# - [ ˞$ -˞ (Equation 2.21)
Exercise 8
R1 A Movement meter have 100µA FSD.
Rm Assume R1 + R2 + Rm = 15KΩ 1.5V Rx
R2 B a) State the pointer location when Rx = 0. b) State the location for ohmmeter scale at ½ FSD, ¼ FSD and ¾ FSD. EE 101 MEASUREMENT DC Meter / 21
Solution
a) &Q
&()&')&T)&
#' #',Z)"Z
__`ͨbͦ
b) At ½ FSD = ½ X 100µA = 50µA.
˞ .P˞ -˞ -˞ R U # $
#' . ŵŹ é '"d
ͭé At ¼ FSD = ¼ X 100µA = 25µA.
˞ .P˞ -˞ -˞ R U # $
#' . ŵŹ é $'d
Ÿͭé
At ¾ FSD = ¾ X 100µA = 75µA.
˞ .P˞ -˞ -˞ R U # $
#' . ŵŹ é e'd
ͭé
EE 101 MEASUREMENT DC Meter / 22
2.4.3 Parallel (Shunt) Ohmmeter
R2 R1 A I Ix
Im V R x
B Figure 2.16 : Individual Shunt Ohmmeter circuit.
ˢ ˢU
˞ U˞U
& ˞U (Equation 2.22) O
Operation of Shunt Ohmmeter
R1 I A
+ IM V R in x
S1 RM -
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 (I M) in circuit is maximum. Adjust R1 until the meter movement is full scale, and the pointer is mark as ∞Ω (Infinity).
∞Ω 0Ω
AB Terminal Open AB Terminal short
Figure 2.18 : The pointer location EE 101 MEASUREMENT DC Meter / 23
Example 9
R1 = 4.5KΩ A Ix I = 5 µA State the ohmmeter scale when the Im current is 0A, ½ FSD, ¼ FSD and FSD R V R = 500 Ω x m
B Solution
& At 0A scale, ˞U O
"P'""ZR ˞ U P'O"Rd
Ŵé
$'d P'""ZR At ½ FSD, ˞ U P'O$'Rd
ŹŴŴé
#$'d P'""ZR At ¼ FSD, ˞ U P'O#$'Rd
ŵźźŻé
'd P'""ZR At FSD, ˞ U P'O'Rd
fé
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.
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iv. Unknown resistance A resistance that unknown value in a circuit.
2.4.5 Differentiate typical scale in series and shunt type ohmmeter.
Scale in Series Type Ohmmeter Scale in Shunt Type Ohmmeter
Scale starts with infinity at left Scale starts with zero at left side side and zero at right side and infinity at right side
ϖ 0 R1 0 ϖ
A R1 A - R R2 E R When point A short B, Rx=0 (Maximum currentm m flows in circuit) so R2 is adjusted for pointer E B B pointing at Rm zero position (which means current only flow through Rm). Normally it When point A short B, Rx=0 (No current flows known as 'Zero Adjusted'. in meter) so R1 is adjusted to pointing at zero position. Normally it known as 'Zero Adjusted'.
R1 A A
Open Open R R2 E R circuit circuit m m
E 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 A R1 A
Rx R R2 E R B m m
B B E
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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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
Sensitivity is define as t he accuracy with which a meter can measure a voltage, current, 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 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 meters take a little power from the circuit under tes t 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. EE 101 MEASUREMENT DC Meter / 28
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. Select a range with a maximum greater than you expect the reading to be. ii. 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. iii. If the reading goes off the scale: immediately disconnect and select a higher range.
2.5.2 DIGITAL MULTIMETERS
Figure 2.6. Digital Multimeter
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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!
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!
2.6. DIFFERENTIATE BETWEEN ANALOG AND DIGITAL MULTIMETER
1. An 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.
2. An analogue multimeter used resistors (shunt and multiplier) while a digital 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 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 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. Deenergize and discharge the circuit completely before connecting or disconnecting a multimeter. 2. Never apply power to the circuit while measuring resistance with a multimeter. 3. Connect the multimeter in series with the circuit for current measurements, and in parallel for voltage measurements. 4. Be certain the multimeter is switched to ac before attempting to measure ac circuits. 5. Observe proper dc polarity when measuring dc. 6. 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. 7. Always start with the highest voltage or current range. 8. Select a final range that allows a reading near the middle of the scale. 9. Adjust the "0 ohms" reading after changing resistance ranges and before making a resistance measurement. 10. Be certain to read ac measurements on the ac scale of a multimeter. 11. Observe the general safety precautions for electrical and electronic devices.