Transcript Class I, II, and III Dielectric Capacitor Codes

Introduction to Capacitors
Chapter 20
Capacitance Vocab.
 Capacitance – the property of a dielectric to
store electric charge.
 Dielectric – an insulator used to store electric
charge in a capacitor
 Electrode - A collector or emitter of electric
charge (not necessarily metallic)
 A capacitor is made up of 2 electrodes and a
dielectric.
 Permittivity - how well a dielectric material can
establish electrostatic lines of force. (Like
permeability but for capacitors)
Inside a capacitor
Electrodes or
Top 8 Capacitor Concepts
 Do not use the property of magnetism like inductors do!
 Capacitors store energy in an electrostatic field instead
 The number of electrons it can store in this field determines its
capacitance size
 The units of Capacitance are measured in Farads [F] and the
symbol for Capacitance is C.
 Capacitors act like springs
 Opposes DC but passes high frequency AC
 Easy to flow during change but is open when DC
 Some capacitors are polarity sensitive (For DC circuits) Signal
to ground
 Current doesn’t actually flow through a capacitor, rather it
accumulates charge.
Capacitor Schematic Symbol
In the presence of an electric field,
electrons are attracted towards the
more positive side.
Factors affecting size of capacitance
 Laser analogy
 Looking at the capacitor
on the left, which facts do
you think affect the size of
the capacitor?
1. Area of the plates
2. Distance between the
plates
3. Dielectric constant – the
material the dielectric is
made of.
The equation given for capacitance
size:
K = Dielectric Constant (or Permittivity)
From Table 20.1
A = Area of plate (measured in meters2)
D = Distance between plates (measured in meters)
Everything is referenced to vacuum as K = 1, because a
vacuum is a very weak dielectric
Determining Capacitance Size
Dielectric Material - Air
A = .004m2
D = .001m
Find the size of the Capacitance given the above info.
Another example
Dielectric Material – Aluminum Oxide
A = 4cm x 6cm
D = .0005m
Find the size of the Capacitance given the above info.
Another example
Dielectric Material – Tantalum Oxide
A = 3cm x 3cm
D = 4um
Find the size of the Capacitance given the above info.
The typical range for a capacitor is
from .5F to 1 pF.
More generally for our class, between
1mF and 10nF
RVOTD
http://www.youtube.com/watch?v=Ej
OvI0TOx98&NR=1
All these examples beg the question:
What is a Farad?
For any given capacitor, the ratio of the charge on one plate to
the potential difference across the plates is a constant. This
constant is the property called capacitance.
A capacitor has a capacitance of 1 Farad if 1 Coulomb of charge
must be deposited to raise the potential difference by 1V across
the capacitor.
1 Farad = 1 coulomb/1 volt
Determining capacitance
Determine the capacitance when 50uC of charge deposited on
the plates raises the potential difference by 2V.
Determine the Charge required to raise a 10nF capacitor by 15V.
So how does current, voltage and capacitance relate?
If you recall, what does Current equal in terms of Charge?
A constant current of 5mA charges a 20uF capacitor in 80ms.
What is the voltage across the capacitor?
… but t is always a change in time, not just a particular
instant. It’s also a change in charge Q, which causes there to
be a change in voltage. (See 20.1.5)
This equation should look very similar to one we’ve already
worked with.
Determine the capacitor current when the voltage across a 4uF
capacitor changes from 16V to 24V in 2ms.
Another Example
Determine the capacitor current when the voltage across a 100nF
capacitor changes from 0V to 5V in 2us.
Read page 143 on your own.
The starting up of voltage across a capacitor is similar to the
starting up of current across an inductor.
The starting up of current across a capacitor is similar to the
starting up of voltage across an inductor.
There are many types of capacitors,
each one useful for different
applications.
Mica Capacitor
(See table 155 for typical applications and pro’s/con’s)
Notice the Voltage Rating.
Is this capacitor polar sensitive?
Ceramic Capacitor
Plastic Film Capacitor
Aluminum Oxide Capacitor
When installing this type of
capacitor, you must put the side
with the negative dashes on the
most negative side of the
circuit.
This is an Electrolytic Capacitor
which means it is polar
sensitive. This kind is typically
used in DC circuits since AC
circuits are not polarized.
Tantalum Oxide Capacitor
Capacitor Codes
What do all the letters and numbers mean on a capacitor?
They symbols on a capacitor refer to both the capacitance
size and the temperature affect on the capacitance size.
The capacitors in your kit have a tolerance of +/-20%
How do you determine the size of a capacitor?
Determining capacitance size in some cases is not as
straight forward as determining resistance size.
Temperature Coefficient Codes
 There are 3 classes of capacitors – Class I, II,
and III
 Class I dielectrics display the most stable
characteristics.
 The most common Class I dielectric is the COG
designation, which is 0ppm/°C ±30ppm/°C,
which is equivalent to the NPO (zero
temperature coefficients) code defined by MIL
specifications
Table 20.4; Temperature Coefficient of
Class I Dielectrics
Determine the temp. coefficient of a
capacitance with the marking R2G.
-220 ppm/ºC ±30ppm/ºC
 Class I dielectrics display the most stable
characteristics.
 The most common Class I dielectric is the COG
designation, which is 0ppm/°C ±30ppm/°C,
which is equivalent to the NPO (zero
temperature coefficients) code defined by MIL
specifications
 Skip the next 3 slides
Code Practice!
 Using the chart in section 20.4.1, table 20.4
determine the temperature coefficient of a
ceramic capacitor marked with the following:
 U4M, P6G, T0G, R7K
 -75000ppm/°C ±1000ppm/°C
 15ppm/°C ±30ppm/°C
 -47ppm/°C ±30ppm/°C
 220ppm/°C ±250ppm/°C
Class II
 Offer much higher constants than Class I, but with
less stable properties to changes in temperature,
and voltage.
 They have a maximum capacitance change of ±15%
or less over an operating temperature range of 55°C to +125°C.
 They are called general-purpose capacitors.
Table 20.5 Max Change in Capacitance
 Determine:
 Y5V
 X7R
 Y5V= +22%, -82%
maximum change from
+25°C, over a -30°C to
+85°C temperature
range.
Determining
Capacitance
Size
Capacitance Size – Through Hole
If the capacitor is physically large enough, it will say the
capacitor size right on it.
Otherwise, whole number values on capacitors usually
indicate a value in picofarads.
Ie: 102 = 1 0 with 2 more 0’s. So 1000 picofarads or 1nF.
See table 20.6 for value interpretations
What is the value and tolerance of a cap that says 223J?
22nF+/-5% = .022uF
What is the value and tolerance of a cap that says 334K?
330nF +/-10% = .330uF
What is the value of a cap that says 229?
2.2pF
Capacitance Size – Through Hole
If the value is 4 numbers long, then just read it directly in
pF
Capacitance Size – Surface Mount
Capacitance is always given in picoFarads
The first digit is typically a letter. This letter represents a
number. (See table 20.7) ie: U = 5.6
The second digit is the multiplier
Ex: What capacitance is T2?
T = 5.1 & 2 = x100…… So 5.1x100 = 510pF
What capacitance is X4?
75000pF = 75nF = 0.075uF
What capacitance is 39?
39pF (If there is no letter, the number is read directly in
pF.)
How to add capacitors in parallel and in series.

So why is this true? What is the reasoning behind this?
What is the total capacitance CT of
the circuit below?
What is the total capacitance CT of
the circuit below?
What is the total capacitance CT of
the circuit below?
What is Stray Capacitance?
Stray capacitance is more noticeable
at higher frequencies
Ways to reduce stray capacitance
1. Use low frequencies (not really an option)
2. Keep lead lengths short
3. Use lots of space between wires and traces
4. Mount components high off chassis
What would happen if you measured
resistance of a capacitor using a
VOM?
How to could you tell if a cap is shorted?
How to could you tell if a cap has an open?
Finish going over Capacitor/Inductor Comparison Handout
Go over Cap/Ind Lead Lag handout