Capacitance/Capacitors

Capacitance

• Capacitance is the ability of a dielectric (insulator) to store an electrical charge

• Charge = Coulomb = Q = 6.25 X 1018 electrons

Capacitors• A capacitor is a component that stores electrical

charge• Schematic

• Make up of a capacitor

• The unit of measurement for capacitance is the Farad

Note to teacher: charge up a 470 µF cap to 30v and then discharge

Dielectric (Insulator) 2 Conducting Plates

Non-Polarity Sensitive Polarity Sensitive+

-

Farad• 1 Farad: when 1 coulomb is stored in a dielectric

with a potential of 1 volt

• 1 Farad is a very large value• Typical values of capacitance are in:

– microfarads (μF) (10-6) – picofarads (pF) (10-12)

• Higher farad value = more stored charge– Note: Charge up 470 μF and 1000 μF to show difference

1 coulomb 1 VoltFarad=

CoulombVolt

C=QV

How Capacitors Work

• http://micro.magnet.fsu.edu/electromag/java/capacitor/index.html

Charging Storing a charge

Discharge

Factors affecting Capacitance: The ability of a capacitor to store electrical charge (capacitance size) depends upon

1. Size (surface area) of the plates– Larger plate area = more capacitance

2. Distance between the plates– Capacitance increases as the width decreases

3. Dielectric MaterialAir = 1Paper = 2-6Ceramics = 80 – 1200Mica = 3-8Mylar = 2-3Electrolytic = 7

• http://micro.magnet.fsu.edu/electromag/java/capacitance/index.html

100 μF 200 μF

200 μF100 μF

Three Important Facts When Choosing a Capacitor

1. Capacitance Value2. Voltage Rating3. Polarity

Capacitance ValueRules of thumb (as in 99% of the time)1. MF or mF on an old capacitor = µF .2. If a capacitor has a value of 1 µF or more it is usually an

electrolytic (canister) capacitor. Conversely, if the capacitance value is less than 1 µF it is usually a disc or chicklet variety.

Capacitance Values of Disc CapacitorsThe value of the disc capacitors is usually given in one

of two methods. 1. The capacitance value is sometimes printed on the

face of the disc capacitor like the following. If this is done then the value of the capacitance is given in micro farads (µF).

.1 Z .047 k

Capacitance Values of Disc Capacitors2. The capacitance value is sometimes printed in

code on the face of the disc as shown below. If this is the case then the capacitance value is given in picofarads (pF). The code given is the same as the code used to determine resistor values. For example: 102 would represent1= 1st digit of value, 0 = 2nd digit of value, 2= multiplier (how many zeros)

Therefore 102 = 1000 picofarads (pF) or .001 micro farads (µF)

Capacitors in Series and Parallel• Capacitors in Series

• Capacitors in Parallel

10 μF 10 μF

CT = 5 μF

10 μF 10 μF

CT = 20 μF

Notice that they are exactly opposite of resistors!!!!!!!!

In series the width of the dielectric is additive

In parallel the plate surface area is additive

Resistor/Capacitor Time Constants