DC electronics

Resistance and Capacitance

Resistance

• Because electrons have mass and are held in place by polarity “bonds” – energy is consumed to dislodge electrons

• Resistance is affected by – Composition of material– Length of material– Cross-sectional area of material– Temperature of material

Resistance

• Composition– Resistance is the opposite of conductivity so

resistance is lowest in silver, gold, copper, etc.• Length

– Resistance increases as length increases– Causes voltage drop in long cables

• Cross-sectional area– Resistance increases as cross section decreases– Causes heating of under-sized cables

Resistance

• Cross-sectional area– Wire diameter is described by system of “gauges”

called American Wire Gauge (AWG)– Bigger number equals smaller diameter

• 28 is used for telephone, 12 or 14 is for household current, 00 is for entrance cable, etc

• Temperature– As temperature increases, so does resistance– “Super conductors” are cooled to absolute zero

AC versus DC+120 Volts

-120 Volts

0Volts

+12Volts

-12Volts

0Volts

Capacitance

• There is a field of force surrounding the electrons known as electrostatic

• Current can be caused to flow even when there is a “break” in a circuit

• When enough electrons, or negative electrostatic force, build up – adjacent electrons can be caused to move

Capacitance

• Capacitors are constructed of two plates in close proximity

• Plates are separated by an insulator called a dielectric

• Current can flow through a capacitor even though the plates are separated by an insulator

Capacitance

Plate Plate

Dielectric

_

Capacitance

Plate Plate

Dielectric

_ _

__

_

___

_

_

_

_

Capacitors

• Capacitors have the ability to store electrons• The quantity of electrons in a capacitor is

measured in farads• Farad is the measure of capacitance• 1 farad = 6.28 X 10• Size of plates and the dielectric constant

determine capacitance

18

Capacitors

• Dielectric may be made of– Wax paper– Ceramic– Mylar– Mica– Electrolyte– Air (variable)

Capacitors

• One application of capacitors is to convert AC to DC

• During each phase of AC when electrons are pushed onto the “in” plate – some current flows off the “out” plate

• During the opposite phase when no electrons are moving into capacitor – no current flows out

Capacitors

Output current from a capacitor

+

_

Capacitors

Resistance

• Resistance is a measure of the reluctance of the electrons to leave their orbits

• Resistors are devices that limit current flow • Resistors convert electrical energy into heat• If current flow is too high, and the resistor

cannot dissipate sufficient heat – it will burn up

Resistors

• Common types of resistors– Carbon composition– Carbon film– Metal film– Wire wound– Foil – Grid– Variable

Resistors

Variable resistor

As the wiper is moved further from the current input, the resistance is

increased and the volume level is decreased.

Variable resistors

Fade down – more resistance – less volume

Circuits

• Components may be connected– In Parallel– In Series– In Series/Parallel

Parallel

Current divides itself and flows along parallel branches

Series

Calculating resistance

• Series = additive R + R + R = R• Parallel = combined resistance is always lower than the

lowest value

1 + 1 + 1

1

R R R1 2 n

R R

R R

1 2

21

X

+

1 2 n t

To solve for a pair of resistors in parallel

Values • Often values may be very small or very large• Use of Greek prefixes allows those expressions

– Pico = one-trillionth (one-millionth millionth)– Micro = one-millionth– Milli = one-thousandth– Kilo = thousand– Mega = million– Giga = billion– Tera = trillion