Transcript Capacitance - benchmark

CAPACITANCE
Benchmark Companies Inc
PO Box 473768
Aurora CO 80047
DEFINED
Capacitance is the measure of
how much electrical charge a
device can store.
Capacitance is measured in Farads (F)
The device used in electronics for
storing this charge is called the
capacitor.
DEFINED
C1
Non-polarized
C2
Electrolytic
Schematic Symbols
C3
Variable
CAPACITOR CODE
Determine the value of the capacitor by utilizing this general
method
The number
you obtain is
in picofarads.
100,000 ρF
or
.1uF
Ceramic Disc
Capacitors have a code
that shows their value
THEORY
Charging
The capacitor is made from two metal
plates with an insulator type material
in the center called a dielectric.
When power is applied to the circuit
the capacitor will attract electrons to
plate A.
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---
THEORY
Charging
The electrons can not pass through
the dielectric so the plate starts storing
electrons. The electrons on plate B
are repelled from plate A and attracted
to the power source leaving the plate
positively charged. This process lasts
until the capacitor has been fully
charged.
+++
---
THEORY
Discharging
Once the capacitor is charged
it may now be used to power a
consuming device. There is
one limitation. The device can
only be powered as long as
the charge is in the capacitor.
The electrons on plate A will
attract to plate B once there is
a path for them to flow. Once
all the electrons have been
neutralized, the capacitor has
lost all of its charge.
+++
---
RATING CAPACITORS
Capacitors are rated in Farads and Volts
- One farad is equal to 1amp being charged by an EMF of 1volt per
second. The formula is shown below.
C=
I avg
ΔE / Δt
C = Capacitance in Farads
I = Current in amps
E=Voltage in volts
t=time in seconds
-Typically capacitors are in Microfarads (µF) or Picofarads (pF).
-The voltage rating is a working rating the capacitor can handle
before being damaged.
CAPACITORS IN SERIES
The total capacitance in a series circuit adds up inversely
just as resisters do in parallel.
1
1
1
1



 ...
C T C1 C 2 C 3
CAPACITORS IN PARALLEL
The capacitance of capacitors in parallel work like resistors
in series. The capacitance of each of the capacitors add up
to the total capacitance of the circuit.
CT  C1  C2  C3  ...
CAPACITOR TYPES
Ceramic Disc
Electrolytic
Polyester Film
Radial
Lead
Axial
Lead
Variable Capacitor
CERAMIC DISC CAPACITORS
Capacitors are named for the
material that make up the
dielectric.
Ceramic Disc capacitors are
rounded in shape and are usually
a light brown color. The
advantage to the Ceramic Disc
capacitor is that it can work at
small capacitance and high
voltages.
Ceramic Disc
C1
ELECTROLYTIC CAPACITORS
Electrolytic Capacitors are capacitors that have
their plates polarized. One plate is positive and
one negative. It is very dangerous to reverse the
polarity of an electrolytic capacitor and should not
be done. Electrolytic capacitors can handle very
large capacitances for their size.
C2
Electrolytic
Radial
Lead
Axial
Lead
ELECTROLYTIC CAPACITORS
These capacitors look like tin cans and their leads
can be radial or axial. The rating of these devices
is easily read on the device itself - no code. The
rectangular shape in the lightly shaded area on
the capacitor indicates which lead is negative. On
the axial capacitor an arrow points to the negative
lead.
C2
Electrolytic
Radial
Lead
Axial
Lead
VARIABLE CAPACITORS
C3
Variable Capacitor
Variable Capacitors are used
in tuning circuits.
The capacitor can be used as
a filter in order to tune in a
specific frequency.
OTHER CAPACITORS
There are many other types of capacitors. Their names come
from the material that makes up the dielectric. The ratings for
these capacitors are usually labeled on the device.
Tantalum
Chip
Capacitor
C1
Mylar
Polypropylene
Polyester Film
Film
RC TIME CONSTANT
The RC Time Constant in the time it takes, in a series
resistor capacitor circuit, for voltage to rise to 63.2% or
fall to 36.8% of the peak voltage value of the circuit.
When five of these time constants occur, the capacitor
will be fully charged. The formula below can be used to
predict this value.
t = RC
t = Time in seconds
R = Resistance in Ohms
C = Capacitance in Farads
RC TIME CONSTANT
example
With a 1kΩ resistor and a 1μF capacitor placed in
series, what is the time constant of the circuit and
how long will it take to fully charge the capacitor?
Time Constant Calculation
t =RC
t=1kΩ x 1μF
t =1000Ω x .000001F
t=1ms
Capacitor Fully Charged
Full Charge Time = 1ms x 5
Full Charge Time = 5ms
RC TIME CONSTANT GRAPHIC
REPRESENTATION The first cursor proves that at
1ms the voltage is 6.32V
The second
cursor is
showing that
after 5 time
constants the
capacitor is
fully charged.
USING A MULTIMETER
Some Multimeters can read the capacitance
of capacitors. Place the capacitor in the
holes as shown in the figure to the right.
Set dial for the F(Farads) setting
then read the screen. Change
range if necessary. The
capacitor below is .02uF.
End of Presentation