Transcript tcom 308-2-capacitors and resistors

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
0
Volts
-120
Volts
+12
Volts
0
Volts
-12
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
Dielectric
Plate
Plate
Capacitance
Dielectric
Plate
Plate
_
_
__
_
_
__
_ _
_
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 1018
• Size of plates and the dielectric constant
determine capacitance
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
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 R1+ R2+ Rn = Rt
• Parallel = combined resistance is always lower
than the lowest value
1
1 +1+1
R1 R 2 Rn
To solve for a pair of resistors in parallel
R1 X
R2
+
R2
R1
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