Class notes
Inductance ,capacitance ,impedance
- Course
- ATAT105
Notes on inductance, inductive reactance ,capacitors ,capacitive reactance ,impedance ,time constants.
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Inductance● Inductance is the behavior of a coil of wire in resisting any change of current through the
coil
● Symbol is L
● Measured in henrys
● 1 henry of inductance generates 1 volt of induced voltage when current changes at 1
amp per second
● All conductors have inductance because they all generate back voltage
● Can be connected in the same way as resistors
● Coils are wound on a spool or bobbin,wire has very thin enamel insulation
● If different windings need insulation between them (like a transformer) an insulating
film/normex is used and core is added afterwards
● Inductors in series and parallel are calculated how resistors are for their respective
circuits
● If changing magnetic fields do not affect each other the total inductance is the sum of the
individual inductors
● If inductors are connected in parallel such that the changing magnetic field doest effect
each other, the total inductance is calculated like resistors in parallel (reciprocal)
Time constant of inductors
● Current doesn't rise instantly,it finds a minimum of opposition and then starts to flow
● The change in current from 0 to maximum induces a minimum EMF that opposes current
flow
● The time required for current to rise to 63% of its peak value is the TC of the circuit
● TC is determined by the value of inductance and resistance in the circuit
● Formula is L / R
● It takes 2 TC to rise to 86%
● It takes 3 TC to rise to 95%
● It takes 5 TC to rise to 100% (maximum)
● Back EMF kicks in when current decreases quick and the process is slowed because the
current is affected by the opposition of the back EMF
Capacitors
● Capacitance is the ability of a body to store electrical charge
● capacitors/condenser stores energy in the electrical field that exists between 2
conductors (plates) separated by a dielectric(insulator)
● Measured in farads
● Symbol is C
, ● 1 farad is the capacity required to hold 1 coulomb of electricity under 1v
● Mostly measured in micro or pico farads
● Formula is Q / E where Q is change in coulombs and E is voltage in volts
● 1 plate attached to positive side and one attached to negative side, electrons are drawn
from positive to negative
● There is no current flow across an insulator but the plates accumulate excess electrons
which results in plates being charged
● Current flow stops when capacitor becomes fully charged
● Used for timing,filtering and isolating AC or DC
● Capacitance is affected by 3 variables
● Area of plates,separation distance between plates and composition of the dielectric
● Large plate = more electrons can be stored ,small plate = less electrons can be stored
● Close = stronger , further = weaker
● More electrons stored on the negative plate
● If plates become too close the field cna become so strong that it passes through the
insulator and breaks down the dielectric causing a short circuit or damage (dielectric
breakdown)
● Due to this all capacitors are rated at least 10% higher than the highest approved
voltage in the circuit (DC rating)
● In an RC circuit current leads voltage by 90 degrees
● In an RL circuit voltage leads current by 90 degrees
● 2 ways capacitors store energy are electrostatic attraction across the dielectric and
distortion of electron orbits of atoms in the dielectric
● Dielectric stress is when electrons are attracted to the positive plate and protons are
attracted to the negative plate
● Electrolytic capacitors: polarized,only to be used in DC circuits,low working voltage,large
amounts of capacity
● Non electrolytic (paper): low value of capacitance needed,plates are made of paper with
metal foil and wax paper coiled, may be encapsulated in plastic,found in aircraft
magnetos
● Mica capacitor: small capacity required but high working voltage,stacks of foil
sandwiched between sheets of mica and encapsulated in plastic
● ceramic capacitors : high voltage, low capacitance,leads are attached to silver plating
and the entire unit is covered with protective insulation
● High voltage capacitor: high voltage application,paper capacitor in a metal container
filled with oil, oil flows when voltage surge breaks through the insulator and will restore it
by acting as the insulator,self healing
● Variable capacitor: changes its capacity
● TC is how long the capacitor takes to charge and discharge
● Capacitors in series use the inverse rule
● Capacitors in parallel just get added
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