A2 PHYSICS: Electronics
Capacitors
A2 PHYSICS: Electronics
What is a capacitor?
• Electronic component• Two conducting surfaces separated by an insulating material• Stores charge• Uses
– Time delays– Filters– Tuned circuits
A2 PHYSICS: Electronics
Capacitor construction
• Two metal plates• Separated by insulating
material• ‘Sandwich’ construction• ‘Swiss roll’ structure• Capacitance set by...
dAC
A2 PHYSICS: Electronics
Defining capacitance
• ‘Good’ capacitors store a lot of charge…• …when only a small voltage is applied• Capacitance is charge stored per volt• Capacitance is measured in farads F
– Big unit so nF, mF and F are used
VQC
A2 PHYSICS: Electronics
Graphical representation
Equating to the equation of a straight line
mxyCVQVQC
Q
V
Gradient term is the capacitance of the capacitor
Charge stored is directly proportional to the applied voltage
A2 PHYSICS: Electronics
Energy stored by a capacitor
• By general definition E=QV– product of charge and voltage
• By graphical consideration...
QVE21
Area term is the energy stored in the capacitor
Q
V
A2 PHYSICS: Electronics
Other expressions for energy
• By substitution of Q=CV
CQE
CVE
QVE
2
2
212121
A2 PHYSICS: Electronics
Charging a capacitor
• Current flow• Initially
– High• Finally
– Zero• Exponential model• Charging factors
– Capacitance– Resistance
I
t
A2 PHYSICS: Electronics
Discharging a capacitor
• Current flow• Initially
– High– Opposite to charging
• Finally– Zero
• Exponential model• Discharging factors
– Capacitance– Resistance
I
t
A2 PHYSICS: Electronics
V or Q
t
V or Q
t
Voltage and charge characteristics
• Charging Discharging
RCt
eQQ
0)1(0 RCt
eVV
A2 PHYSICS: Electronics• Product of
– Capacitance of the capacitor being charged– Resistance of the charging circuit– CR
• Symbol ‘Tau’• Unit seconds
Time constant
tCRtQ
VVQCR
A2 PHYSICS: Electronics
When t equals tau during discharge
• At t = tau the capacitor has fallen to 37% of its original value.
• By a similar analysis tau can be considered to be the time taken for the capacitor to reach 63% of full charge.
37.00
10
0
0
QQeQQ
eQQ
eQQ
RCRC
RCt
A2 PHYSICS: Electronics
Graphical determination of tau
• V at 37%• Q at 37%• Compared to initial
maximum discharge
V orQ
tRtC
RCtt
A2 PHYSICS: Electronics
Logarithmic discharge analysis
• Mathematical consideration of discharge
• Exponential relationship • Taking natural logs equates
expression to ‘y=mx+c’• Gradient is -1/Tau
0
0
0
0
ln1ln
lnln
VtRC
V
RCtVV
eVV
eVV
RCt
RCt
A2 PHYSICS: Electronics
Logarithmic discharge graph
lnV
t
Gradient term is the -1/Tau
A2 PHYSICS: Electronics
Presented By…..
130170111092 Rathor Vijendrasingh
130170111095 Rudra Pavan
130170111098 Shah Nidhi
EC- J1 5th Semester, 2015
Thank You
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