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 0 Volts +12 Volts -12 Volts 0 Volts

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 Dielectric

_ Capacitance 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 R R R 1 2 n RR X 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