spiritual – temporal
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ECEN 301 Discussion #6 – Things Practical 1
Spiritual – Temporal1 Nephi 15:3232 And it came to pass that I said unto them that it was a
representation of things both temporal and spiritual; for the day should come that they must be judged of their works, yea, even the works which were done by the temporal body in their days of probation.
Mosiah 2:4141 And moreover, I would desire that ye should consider on the
blessed and happy state of those that keep the commandments of God. For behold, they are blessed in all things, both temporal and spiritual; and if they hold out faithful to the end they are received into heaven, that thereby they may dwell with God in a state of never-ending happiness. O remember, remember that these things are true; for the Lord God hath spoken it.
ECEN 301 Discussion #6 – Things Practical 2
Lecture 6 – The Wheatstone Bridge
An Applications of Things Electrical
ECEN 301 Discussion #6 – Things Practical 3
Wheatstone BridgeA circuit used to find:• An unknown resistance Rx
• The unknown voltage vab • Another unknown value (such as a force)
vs+_
R2
R1 R3
Rx
bava vb
c
d
c R1
vs+_
R3
R2 Rx
d
a bva vb
ECEN 301 Discussion #6 – Things Practical 4
Wheatstone Bridge• The circuit consists of the parallel combination of 3
subcircuits with the same voltage:a) The voltage sourceb) Series combination of R1 and R2
c) Series combination of R3 and Rx• Voltage divider between:
a) R1 and R2• v2 = vad
b) R3 and Rx• vx = vbd 21
2
RRRvv sad
vs+_
R2
R1 R3
Rx
bava vb
c
d x
xsbd RRRvv
3
ECEN 301 Discussion #6 – Things Practical 5
Wheatstone Bridge• KVL around the bottom loop:
x
xs
bdadab
RRR
RRRv
vvv
321
2
vs+_
R2
R1 R3
Rx
bava vb
c
d
ECEN 301 Discussion #6 – Things Practical 6
Wheatstone Bridge• Example1: when is vab = 0?
vs+_
R2
R1 R3
Rx
bava vb
c
d
ECEN 301 Discussion #6 – Things Practical 7
Wheatstone Bridge• Example1: when is vab = 0?
31
2
21
3
132
21232
2132
321
2
321
2
321
2
0
RR
RR
RR
RR
RRRRRRRRRRRRRRRRRR
RRR
RRRRR
RRR
Rv
RRR
RRRvv
x
x
x
xxx
xx
x
x
x
xs
x
xsab
vs+_
R2
R1 R3
Rx
bava vb
c
d
ECEN 301 Discussion #6 – Things Practical 8
Wheatstone Bridge• Find Rx:
s
ab
s
ab
x
x
xsab
vv
RRR
vv
RRRR
R
RRR
RRRvv
21
2
21
23
321
2
1
vs
+_
R2
R1 R3
Rx
bava vb
c
d
ECEN 301 Discussion #6 – Things Practical 9
Wheatstone Bridge• Example2: find Rx
• R1 = R2 = R3 = 1kΩ, vs = 12V, vab = 12mV
s
ab
s
ab
x
vv
RRR
vv
RRRR
R
21
2
21
23
1vs
+_
R2
R1 R3
Rx
bava vb
c
d
ECEN 301 Discussion #6 – Things Practical 10
Wheatstone Bridge• Example2: find Rx
• R1 = R2 = R3 = 1kΩ, vs = 12V, vab = 12mV
996501.0
499
10211
102110
121012
102101
121012
1021010
1
3
33
3
3
3
3
3
33
21
2
21
23
s
ab
s
ab
x
vv
RRR
vv
RRRR
R
vs+_
R2
R1 R3
Rx
bava vb
c
d
ECEN 301 Discussion #6 – Things Practical 11
Resistance Strain Gauges• Strain gauge: device bonded to the surface of an object and
whose resistance varies as a function of surface strain• Used to perform measurements of:
• Strain• Stress,• Force• Torque• Pressure
• NB: cylindrical resistance:
• Compression/elongation will change resistance
ALR
Compression lower resistance Stretch higher resistance
L – length of cylindrical resistorσ – conductivity of the resistorA – resistor cross-sectional area
ECEN 301 Discussion #6 – Things Practical 12
Resistance Strain Gauges• Gauge factor (GF): the relationship between change in
resistance and change in length • value of about 2 is common
LL
• Strain (ε): the fractional change in length of an object• Max strain that can be measured is about 0.4 – 0.5 percent
• i.e. ε = 0.004 to 0.005• For a 120Ω resistor: +/– 1.2 Ω
LLRRGF
// 0
R0 – the zero strain resistance
ECEN 301 Discussion #6 – Things Practical 13
Resistance Strain Gauges• Change in resistance due to strain:
GFRR 0
RGCircuit symbol for a strain gauge
ECEN 301 Discussion #6 – Things Practical 14
Wheatstone Bridge• Wheatstone bridge commonly used to measure force
using strain gauge resistors• Example: force applied to a cantilever beam
• Two strain gauges (R1 and R4) on top• Two strain gauges (R2 and R3) on bottom• Under no strain: R1 = R2 = R3 = R4 = R0
R1
vs+_
R3
R2 R4
d
a bva vb
ia ib
c
R4
R1
R2 & R3
FL
w
h
ECEN 301 Discussion #6 – Things Practical 15
Wheatstone Bridge• Under the strain of force F we have:
R4
R1
R2 & R3
FL
w
hvs
+_
R0 + ∆R
bava vb
c
d
R0 + ∆R
R0 - ∆R
R0 - ∆R
ia ib
tension
tension
compression
compression
RRRR 041
RRRR 032
ECEN 301 Discussion #6 – Things Practical 16
Wheatstone Bridge• From elementary statics it can be shown that:
R4
R1
R2 & R3
FL
w
hvs
+_
R0 + ∆R
bava vb
c
d
R0 + ∆R
R0 - ∆R
R0 - ∆R
ia ib
YwhLF
2
6 Y is the beam’s modulus of elasticity
ECEN 301 Discussion #6 – Things Practical 17
Wheatstone Bridge• Using Ohm’s Law:
vs+_
R0 + ∆R
bava vb
c
d
R0 + ∆R
R0 - ∆R
R0 - ∆R
ia ib
43 RRvi s
b
21 RRvi s
a
R1
vs+_
R3
R2 R4
d
a bva vb
ia ib
c
ECEN 301 Discussion #6 – Things Practical 18
Wheatstone Bridge• Using Ohm’s Law:
vs+_
R0 + ∆R
bava vb
c
d
R0 + ∆R
R0 - ∆R
R0 - ∆R
ia ib
GFvRRv
RRRRRRv
RRRRRRv
RRRv
RRRv
RiRivvvvvv
s
s
ss
ss
ab
adbdabbaout
0
00
0
00
0
21
2
43
4
24
ECEN 301 Discussion #6 – Things Practical 19
Wheatstone Bridge• Find vo in terms of force F:
kF
FYwhGFLv
YwhLFGFv
GFvv
s
s
so
2
2
6
6
k is a calibration constant
vs+_
R0 + ∆R
bava vb
c
d
R0 + ∆R
R0 - ∆R
R0 - ∆R
ia ib
ECEN 301 Discussion #6 – Things Practical 20
Wheatstone Bridge• Example3: using the Wheatstone bridge as a strain measurement tool find the
maximum vo• the bridge measures forces ranging from 0 to 500 N• L = 0.3m, w = 0.05m, h = 0.01m, GF = 2, Y = 69x109N/m2, vs = 12V
NmVFNVF
F
FYwhGFLv
kFv
s
o
/125.0/1025.1
1069)01.0)(05.0()3.0)(2)(12(6
6
4
92
2
R4
R1
R2 & R3
FL
w
h
ECEN 301 Discussion #6 – Things Practical 21
Wheatstone Bridge• Example3: using the Wheatstone bridge as a strain measurement tool find
the maximum vo• the bridge measures forces ranging from 0 to 500 N• L = 0.3m, w = 0.05m, h = 0.01m, GF = 2, Y = 69x109N/m2, vs = 12V
mV
NmVFvo
5.62)500(1025.1
/125.0: voltageimumMax
4
R4
R1
R2 & R3
FL
w
h
ECEN 301 Discussion #6 – Things Practical 22
Practical Sources
ECEN 301 Discussion #6 – Things Practical 23
Ideal Sources• Ideal current source
• 3A source
012345678
0 1 2 3 4 5 6 7 8
v
i
• Ideal voltage source• 6V source
012345678
0 1 2 3 4 5 6 7 8
v
i
Provides a prescribed voltage across its terminals irrespective of the current flowing through it.
Provides a prescribed current irrespective of the voltage across it.
ECEN 301 Discussion #6 – Things Practical 24
Practical Sources• Actual voltage sources have limitations
• There is a limit to the number of total electrons any battery can motivate through a circuit
• How to measure limitations?• Total number of electrons? (huge number)• Use coulombs? (also too huge)• amp-hour – unit invented for this purpose
• 1 amp-hour = 1 amp for 1 hour = 2 amps for ½ hour
= 1/3 amp for 3 hours
• Batteries have ratings indicating their current limitations• Car battery – 12V, 70 amp-hours (A-h) @ 3.5 A (for 20 hours)• D – cell (1.5V) carbon-zinc battery – 4.6 amp-hours @ 100mA (for 46 hours)• 9 – volt carbon-zinc battery – 400 mA-hours @ 8mA (for 50 hours)
1 amp = 1 coulombs/second1 amp-hour = 3600 coulombs
ECEN 301 Discussion #6 – Things Practical 25
Practical Sources• Actual voltage sources have limitations
• As the load resistance (RL) decreases, the voltage source (vs) is required to provide increasing amounts of current (i) in order to maintain vs
+_vs v
+
_
i
Load (RL)
L
s
Rvi
As RL 0, vs has to provide an infinite amount of current!
ECEN 301 Discussion #6 – Things Practical 26
Practical Sources• Actual voltage sources have limitations
A series resistance rs poses a limit to the maximum current the voltage source can provide
Ls
ss Rr
vi
+_vs
+
vL
–
is
rs
Practical voltage source
Load (RL)
ECEN 301 Discussion #6 – Things Practical 27
Practical Sources• Actual voltage sources have limitations
A series resistance rs poses a limit to the maximum current the voltage source can provide
+_vs
+
vL
–
is
rs
Practical voltage source
Load (RL)Ls
Ls
LsL
RrRv
Riv
NB: vs = vL
ECEN 301 Discussion #6 – Things Practical 28
Practical Sources• Actual voltage sources have limitations
+_vs
+
vL
–
is
rs
s
sS
s
ssR
Ls
ss
rvi
rvi
Rrvi
L
max
0lim
Practical voltage source
A series resistance rs poses a limit to the maximum current the voltage source can provide
ECEN 301 Discussion #6 – Things Practical 29
Practical Sources• Actual current sources have limitations
• As the load resistance (RL) increases, the current source (is) is required to provide increasing amounts of current (v) in order to maintain is
is
+
vs
–
Load (RL)
Ls Riv
As RL ∞, is has to provide an infinite amount of voltage!
ECEN 301 Discussion #6 – Things Practical 30
Practical Sources• Actual current sources have limitations
A series resistance rs poses a limit to the maximum voltage the current source can provide
)||( LssS Rriv
is
+
vs
–
rs
Practical current source
Load (RL)
ECEN 301 Discussion #6 – Things Practical 31
Practical Sources• Actual current sources have limitations
ssS
sssR
LssS
riv
rivRriv
L
max
lim)||(
A series resistance rs poses a limit to the maximum voltage the current source can provide
is
+
vs
–
rs
Practical current source
ECEN 301 Discussion #6 – Things Practical 32
Practical Sources
012345678
0 1 2 3 4 5 6 7 8
v
i
012345678
0 1 2 3 4 5 6 7 8
v
i
• Actual current and voltage sources have limitations
Ideal 3A current source Ideal 6V votage source
ECEN 301 Discussion #6 – Things Practical 33
Practical Sources
012345678
0 1 2 3 4 5 6 7 8
v
i
012345678
0 1 2 3 4 5 6 7 8
v
i
• Actual current and voltage sources have limitations
Practical 3A current source Practical 6V voltage source
ECEN 301 Discussion #6 – Things Practical 34
Measuring Devices
ECEN 301 Discussion #6 – Things Practical 35
Ohmmeter• Ohmmeter: measures the resistance of a circuit
elementNB: the resistance of an element can only be measured when the element is disconnected from all other circuit elements
Ω Ω ROhmmeter symbol
Ohmmeter connection setup for circuit element resistance measurement
ECEN 301 Discussion #6 – Things Practical 36
Ammeter• Ammeter: a device that can measure the
current flowing though a circuit element when connected in series with that circuit elementNB: 1. the ammeter must be connected in series with the circuit element 2. the ammeter should not restrict the flow of current (i.e. cause a voltage drop) – an ideal ammeter has zero resistance
A Ammeter symbol
iR2vs
+_
R1 R1
iR2
vs+_
A
ECEN 301 Discussion #6 – Things Practical 37
Voltmeter• Voltmeter: a device that can measure the
voltage across a circuit element when connected in parallel with that circuit element
NB: 1. the voltmeter must be connected in parallel with the circuit element 2. the voltmeter should not draw any current away from the element – an ideal voltmeter has infinite resistance
V Voltmeter symbol vs
i
+R2
–
+_
R1 R1
i
+R2
–vs
+_ V+v2
–
ECEN 301 Discussion #6 – Things Practical 38
Wattmeter• Wattmeter: a device that can measure the
power dissipated by a circuit element
NB: 1. the wattmeter must be connected in parallel with the circuit element, but also in series with the circuit. – a wattmeter is simply the combination of a voltmeter and an ammeter
W Wattmeter symbol vs
i
+R2
–
+_
R1 R1
i
+R2
–vs
+_
W
ECEN 301 Discussion #6 – Things Practical 39
Practical Voltmeters and Ammeters• In reality, voltmeters and ammeters have internal
resistances• Practical ammeters will always add some resistance• Practical voltmeters will always draw some current
V
IdealVoltmeter
Vrm
PracticalVoltmeter
A
IdealAmmeter
PracticalAmmeter
A
rm