Capacitors

Capacitors

revision

What is a capacitor?

• Electronic component• Two conducting surfaces separated by an insulating material• Stores charge• Uses

– Time delays– Filters– Tuned circuits

Capacitor construction

• Two metal plates• Separated by insulating

material• ‘Sandwich’ construction• ‘Swiss roll’ structure• Capacitance set by...

d

AC

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

V

QC

Graphical representation

As the capacitor is charged to higher and higher P.d. ……..

mxy

CVQV

QC

Q

V

Gradient term is the capacitance of the capacitor

Charge stored is directly proportional to the applied voltage

Energy stored by a capacitor

• By general definition E=QV– product of charge and voltage

• By graphical consideration...

QVE2

1

Area term is the energy stored in the capacitor

Q

V

Other expressions for energy

• By substitution of Q=CV

C

QE

CVE

QVE

2

2

2

1

2

12

1

Charging a capacitor

• Current flow• Initially

– High• Finally

– Zero• Exponential model

– Because rate of charge flow depends on ‘how much charge is on plate'

• Charging factors– Capacitance– Resistance in outer

circuit

I

t

Discharging a capacitor• Current flow• Initially

– High– Opposite to charging

• Finally– Zero

• Exponential model– Because rate of charge flow

depends on how much charge is on plate

• Discharging factors– Capacitance– Resistance in outer

circuit

I

t

V or Q

t

V or Q

t

Voltage and charge characteristics

• Charging Discharging

RCt

eQQ

0)1(0RCt

eVV

• Product of– Capacitance of the capacitor being charged– Resistance of the charging circuit– CR

• Symbol ‘Tau’• Unit seconds

• To show units of tau are seconds, use Ohm’s Law to substitute for R and Capacitance definition to substitute for C

Time constant

tCR

tQ

V

V

QCR

When t equals tau during discharge

• Discharging: At t = tau the capacitor has fallen to 37% of its original value.

• Charging :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

QQ

eQQ

eQQ

eQQ

RCRC

RCt

Graphical determination of tau

• V at 37%• Q at 37%• Compared to initial

maximum discharge

V

or

Q

t

RtC

RCt

t

Logarithmic discharge analysis

• Mathematical consideration of discharge

• Exponential relationship • Taking natural logs produces

expression of form ‘y=mx+c’• Gradient is -1/Tau

0

0

0

0

ln1

ln

lnln

VtRC

V

RCtVV

eV

V

eVV

RCt

RCt

Logarithmic discharge graph

lnV

t

Gradient term is the -1/Tau