Download lecture guide – unit 19

ELIZABETHTOWN TECHNICAL COLLEGE
BEX 100 BASIC ELECTRICITY FOR NON MAJORS
LECTURE GUIDE – UNIT 19
Instructor: Jerry Brown
Reference: Delmar’s Standard Textbook of Electricity, Second Edition
CAPACITORS
Objectives
Upon completing this unit, you should
1. List the three factors that determine the capacitance of a capacitor
2. Discuss the electrostatic charge
3. Discuss differences between nonpolarized and polarized capacitors
4. Compute values for series and parallel connections of capacitors
5. Compute a RC time constant and capacitor charging time
Introduction
Capacitors perform many useful functions
Power factor correction
Storing electrical charge to produce large current flow
Timing circuit
Electronic filter
Capacitors are devices that oppose voltage change
Capacitors can be:
Polarized – polarity sensitive – can only be used in DC circuits
Nonploarized – sometimes called “AC capacitors” – can be used in AC and
DC circuits
Capacitors
DO NOT HANDLE OR PLAY WITH CHARGED CAPACITORS. Can cause heart fibrillation
Simple Capacitor
dielectric)
–
metal
plates
separated
by
an
insulator
(called
the
Three factors determine Capacitance:
Area of the plates
Distance between the plates
Dielectric material
Charging a capacitor
Current flows only during charging or discharging
Capacitor will store applied voltage after current stops
Should remained charged – except for leakage through dielectric
Electrostatic charge – electric charge that is stationary
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Dielectric stress – atoms of insulator distorted as their electrons are
attracted by positive charged plate and repelled by negative charged plate
If dielectric breaks down and allows current to flow, capacitor has been
shorted (failed) – All capacitors have a voltage rating
Military study – On average, capacitors operated at ½ rated voltage, life
is 8X the average of those operated at rated voltage
When capacitors discharge through conductor between plates, very high current
is produced – used for electronic flash on cameras:
Switch
1.5 V
1.5 V
Oscillator
(Square Wave AC)
500 V
Diode
Capacitor
Xenon
Tube
Gunpowder example (Work  Time = Power)
Dielectric Constant- Ratio of a material’s
quality” in a capacitor to air, Air = 1
electrical
“insulating
Capacitor Ratings – A “farad” is the basic unit of capacitance named
after Michael Faraday, a famous scientist
FARAD (F) – the charge produced by the movement of one coulomb of
electrons between the plates with the application of one volt
Such a large amount of capacitance that it is seldom used in the
real world. More common:
F
“Microfarad” = 1/1,000,000 F
nF
“Nanofarad” = 1/1,000,000,000 F
pF
“Picofarad” = 1/1,000,000,000,000 F
Capacitors in parallel – Has the same effect as adding plates to one capacitor
or increasing the area of the plates:
CT = C1 + C2 + C3 + .. CN
Capacitors in series – Has the same effect as increasing the distance between
the plates or reducing the total capacitance
CT =
1
2
1/ C1 + 1/ C1 + 1/ C1 + .. 1/ CN
RC Time Constants ()– Capacitors charge and discharge at an exponential rate –
during each unit of time (called the “Time Constant”), charge will change 63.5%
of its ultimate value
In 5 time constants, capacitor will reach 99.4% of its ultimate charge.
Calculating time constants
 = R x C
where:
 = one time constant in seconds
R = Resistance in Ohms
C = Capacitance in farads
Two basic groups of capacitors – POLARIZED AND NON-POLARIZED
Non-Polarized Capacitors
Not sensitive to polarity connection
Often referred to as “AC” capacitors
May be used in both AC and DC circuits
Common type called Paper or Oil Filled Capacitor
Name comes from dielectric used (thin
dielectric oil)
paper
soaked
in
Polarized Capacitors
Also called “Electrolytic Capacitors”
Polarity sensitive – generally may only be used in DC circuits
Two basic types: Wet & Dry
Wet – Positive plate is aluminum foil; negative plate
actually an electrolyte made from borax solution. Second
piece of foil in contact with electrolyte becomes negative
terminal. Electrolyte is very thin – reverse current will
dissolve, but correcting current direction can restore film.
Dry – similar to wet, except borax solution held in gauze to
prevent capacitor from leaking. If shorted, cannot be
restored by reversing current flow, so will be destroyed.
AC Electrolytic Capacitors
Used principal of wet polarized capacitors
Made by connecting two wet type electrolytic capacitors together
inside the same case
The two capacitors will have their negative (or positive) terminals
connected together – AC current applied to the leads will short one
capacitor and form the other. When the current reverses during the
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next half cycle, the first capacitor will form and the other will
be shorted.
Variable Capacitors
Can be adjusted to change capacitance over a certain range
Generally have a set of stationary and a set of movable plates
Plates are interleaved to change capacitance value
Uses air as dielectric and has small plates, so ratings
picofarads
Trimmer Capacitor
movable plate
–
Special
type
with
one
usually in
stationary
and
one
Uses screw adjustment to move the plate away from the stationary
plate to change capacitance
Testing capacitors
Use ohmmeter for testing shorts
Test for leakage using variable voltage supply and micro ammeter – apply
rated voltage and current should be zero if no leakage
AC capacitors are tested for leakage using a HIPOT because of its ability
to produce a high voltage needed for testing dielectric.
RMS = 354 volts (AC) x 0.707
RMS = 250.3 Volts
Factoring Peak Values
Peak = RMS x 1.414
Example:
Peak = 250 Volts x 1.414
Peak = 354 Volts (AC)
SUMMARY
1. Capacitors are devices that oppose a change a in voltage
2. Three
a.
b.
c.
factors that determine capacitance of a capacitor are:
The surface area of the plates
The distance between the plates
The type of dielectric
3. A capacitor stores energy in an electrostatic field
4. Current can
discharging
flow
only
during
the
time
a
capacitor
is
charging
or
5. Capacitors charge and discharge at an exponential rate
6. The basic unit of capacitance is the farad
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7. Capacitors are generally rated in microfarads, nanofarads, or picofarads
8. When capacitors are connected in parallel their capacitance values add
9. When capacitors are connected in series, the reciprocal of the total
capacitance is equal to the sum of the reciprocals of all the capacitors
10. The charge and discharge times of a capacitor are proportional to the
amount of capacitance and resistance in the circuit
11. Five time constants are required to charge or discharge a capacitor
12. Nonpolarized capacitors are often called AC capacitors
13. Nonpolarized capacitors
current circuits
can
be
connected
to
direct
or
alternating
14. Polarized capacitors are often referred to as electrolytic capacitors
15. Polarized capacitors can be connected to direct current circuits only
16. There are two basic types of electrolytic capacitors, the wet type and
the dry type
17. Wet type electrolytic capacitors can be reformed if reconnected to the
correct polarity
18. Dry type electrolytic capacitors
connected to the incorrect polarity
will
be
permanently
damaged
if
19. Capacitors are often marked with color codes or with numbers and letters
20. To test a capacitor for leakage, a micro ammeter should be connected in
series with the capacitor and rated voltage applied to the circuit
REVIEW QUESTIONS
1. What is the dielectric?
2. List three factors that determine the capacitance of a capacitor:
3. A capacitor uses air as a dielectric and has a capacitance of 3 F. A
dielectric material is inserted between the plates without changing the
spacing, and the capacitance becomes 15 F. What is the dielectric
constant of this material?
4. In what form is the energy of a capacitor stored?
5. Four capacitors having values of 20 F, 50 F, 40 F, and 60 F are
connected in parallel. What is the total capacitance of this circuit?
6. If the four capacitors in question 5 were to be connected in series, what
would be the total capacitance of the circuit?
7. A 22-F capacitor is connected in series with a 90-k resistor. How long
will to take this capacitor to charge?
8. A 450-F capacitor has a total charge time of 0.5 s. How much resistance
is connected in series with the capacitor?
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9. Can a nonpolarized capacitor be connected to a direct current circuit?
10. Explain how an AC electrolytic capacitor is constructed.
11. What type of electrolytic capacitor will be permanently damaged if
connected to the incorrect polarity?
12. A 500-F capacitor is connected to a 300-k resistor. What is the total
charge time of this capacitor?
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