Capacitors
Capacitors
A capacitor is made of two conductive plates separated by an insulator.
Any two conductive surfaces with an insulator between them and a voltage across
them will have a capacitance between the two plates. This even applies to clouds,
Earth's surface, the air between them and the static electric charge that builds up.
A charge can be stored between the clouds and Earth's surface. The magnitude of
the charge that can be stored depends on the surface area of the conductors, the
distance between the conductors and the material of the insulator. The larger the
conductors the more charge that can be built up. The closer together the
conductors the higher the potential charge, as long as the insulator holds up to
that voltage.
Capacitors are rated in Farads. Typical values we find are rated in much
less than 1 Farad. Micro-Farads, Nano-Farads and Pico-Farads are most common.
Different material is used to make different capacitor families, each with unique
qualities for specific purposes.
Ceramic Capacitors
The insulating material is a sheet of ceramic material. Usually the metal is
painted on the opposite sides of the ceramic making the plates of the capacitor.
This simple construction is what we find on Ceramic Disk capacitors, or
Monolythic Ceramic (MLC) capacitors. These may be sandwiched together to
make Multi-Layer Ceramic capacitors that have a higher capacitance per package
size. Typical values are below 1 Micro-Farad, but they can be found in the MicroFarad range.
Baring physical damage ceramic caps seldom fail, but it can happen.
Ceramic caps usually come in 20%, 10% or 5% parts. They are rated in
Capacitance by a three digit code in text, read like a resistor; two significant
digits and a third digit that represents the number of zeros that follow, measured
in Pico-Farads. 103 would be "1" "0" followed by three zeros "000", or 10,000 pF
= 10 nF = 0.01 μF.
Ceramic capacitors are also rated by temperature drift. This is a rating for
how much the capacitance (Farad rating) will drift with changes in temperature.
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"NPO" and "X7R" are best. "Z5U" is poor.
As temperature increases the breakdown voltage of the capacitor
decreases (derates). A capacitor rated for operation at 85° C will fail more
quickly than one rated for 105° C if ran in a hot environment.
They work well in AC or DC circuits but are usually rated in DC
maximum voltage. Realize that 50 V DC is a peak value. 50 V DC equals only
about 35 V AC RMS.
Plastic Film Capacitors
The insulating material is a plastic film of some sort. The conductors may
be a metal foil or just a metallic coating on the plastic (metallized polyester types
for example). Various plastic materials are used, some of the more popular being
Mylar, Polyester, Polypropylene and Polystyrene. Mica capacitors also fall in this
category. These caps have less lose and are specified for audio circuits most of
the time, where AC to RF signals are found. Typical values are below 1 MicroFarad, but they can be found in the Micro-Farad range, and down to pico-Farads.
Plastic Film capacitors seldom fail, but it can happen. Temperature is their
worst enemy. The breakdown voltage derates quickly as temperature increases.
Plastic Film caps usually come in 20%, 10% or 5% parts. They are also rated by
Capacitance by a three digit code in text, read like a resistor; two significant
digits and a third digit that represents the number of zeros that follow, measured
in Pico-Farads. 103 would be "1" "0" followed by three zeros "000", or 10,000 pF
= 10 nF = 0.01 μF.
They work well in AC or DC circuits but are usually rated in DC
maximum voltage. Realize that 50 V DC is peak. 50 V DC equals only about 35
V AC RMS.
Electrolytic Capacitors
These pack a higher capacitance in a smaller package. The insulator may
be only an oxidized layer on the metal surface and thus may be atomic layers thin.
This gives us capacitances that start around 1 μF on the low side and go up to
the Farad range on the high side. The metal used may be Aluminum (the most
popular), Tantalum (higher quality than Aluminum) or Niobium (high
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Capacitors
capacitance in a small package). Other exotic metals might be used for specific
applications.
Electrolytic capacitors are the most commonly failing component in
electronics. Temperature is their worst enemy. The breakdown voltage derates
quickly as temperature increases. Electrolytic caps usually come in 10%, 20% or
even "-20% +100%" parts. They are also rated by Capacitance in micro-farads or
Farads in clear text.
They DO NOT work well in AC. Most are stated for DC only circuits and
are polarized. Non-Polarized types can be found. They do this by putting two
caps back-to-back giving a much smaller capacitance for case size.
If you look at the manufacturer's specs you will find that most are rated
for 1,000 to 2,000 hours of operation at a given temperature. To the manufacturer
of a video monitor saving a few bucks by putting in 85° C rated parts saves a lot
of money when manufacturing thousands of monitors. To the tech doing repairs
the small added expense of putting in 105° C rated parts makes better sense. 105°
C rated parts are strongly suggested in all repairs. 85° C rated parts are okay for
test fixtures that will not be closed up in a game and get hot. 85° C rated parts are
okay for learning how capacitors work.
Electrolytic capacitors are usually marked for polarity. Usually the
negative side is marked. Be aware that this is not always the case. In history some
European manufacturers have marked the positive side of the caps, and the
positive side of the symbol on the board. Eastern manufacturers usually mark the
negative side of the capacitor and the board. PAY ATTENTION WHEN
REMOVING THE OLD CAPS. Note which side of the cap comes out of which
marking on the board.
What the stuff is a Farad?
One Farad is the capacitance (ability to store a charge) that gives a Time
Constant of one second when charged (or discharged) through a resistance of One
Ohm. One Time Constant is the time it takes for the charge to change 63.6% of
the total value it can change.
Note in Design 309, sheet 2, we have a Time Constant Curve. It is rated in
percent of change per second, not any specific voltage per second. Starting at T0
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(uncharged) the capacitor will charge up to 63.6% of the total value in the first
Time Constant (R x C). In the second time constant it will charge up 63.6% of the
difference between its present charge and the maximum charge. The curve is
therefore not linear (a straight line) but like a sine curve. The total charge after
two time constants is 86.75% of the maximum value. After three we have
95.18%. After four we have 98.24%. After five time constants we have 99.36%
and this is considered a full charge. We are within 1% of the final value. For parts
rated in 5% or 10% this is close enough to complete. Most circuits are designed to
work within the first time constant, or often less.
Note that the capacitor has an effective resistance that changes as the
capacitor charges. When completely discharged the capacitor has an effective
resistance of almost zero. This is not perfectly zero. The conductors that make up
the plates have a resistance. This plays part of one characteristic of a capacitor
called ESR (Effective Series Resistance) of the capacitor. As the capacitor
charges it effectively has a higher resistance. As part of a voltage divider we can
put a value on this resistance at any specific time by telling what voltage is
dropped across the capacitor versus the resistor. (See Schematic Design 309,
sheet 1).
Capacitor Exercise One
In the circuit of Drawing 1 we have a 1 M Ohm resistor and a 1 microFarad capacitor. R x C tells us we have a time constant of 1 second. At the first
instant we push SW1 and hold it the capacitor is initially uncharged, has no
resistance, and all the voltage is dropped across the resistor. As the capacitor
charges through the first time constant its effective resistance changes. At T1 (one
time constant, one second in this case) the capacitor has 3.18 Volts across it.
(63.6% of 5 Volts). The capacitor has an effective resistance of 1,747,000 Ohms.
After two time constants the capacitor has an effective resistance of 6,547,000
Ohms. After five time constants the capacitor has almost all the voltage dropped
across it and has an effective resistance of around 155 M Ohms.
When we push and hold SW2 the capacitor starts to discharge with the
opposite curve. It will discharge 63.6% of the total value within the first time
constant. By five time constants it is considered completely discharged.
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The calculation for this exercise can be found in Excel File named Design
309 R-C Time Constants.
We do not have to charge and discharge at the same rate. Drawing 2
shows how we can charge quickly through SW1 and R1, but discharge slowly
through SW2 and R2. This is important to us because in the next two drawings
we get to a practical application of these concepts. In Drawing 3 we have a
pulsating DC pulse being applied to the capacitor through a diode. Our charge
time is very short. If you did the Diode Exercise you will have some idea of how
small the resistance of the diode will be and the capacitor will charge up very
quickly as the incoming pulse is applied. We draw a surge of current through the
diode when the capacitor is charging.
When the incoming pulse drops below the charge on the capacitor the
diode becomes reverse biased and the capacitor only sees the value of R1 as its
path and will discharge through R1 at a relatively slower rate than it charged at. If
we supply incoming charges at a rate faster than the C1 x R1 time constant the
capacitor will retain a charge. We dump a charge onto the cap is short bursts of
high current and drain it off slowly and steady through R1.
In Drawing 4 we see this in a real world application. This is a basic
unregulated power supply. The AC coming out of the transformer is rectified and
becomes a series of positive pulses. Pulsating DC at a 60 Hz rate in our case. The
current through the transformer and diode are a string of high current pulses each
time the Reservoir Capacitor (popular term for this application) is charged. If we
output 12 Volt pulses and are connected to a load of 12 Ohms we will draw the
charge off of the capacitor at a rate of 1 Amp. We must choose a capacitor whose
capacitance will give us an R x C time constant sufficient to supply 1 Amp
constantly between charging pulses to give us a steady output voltage of 12 Volts.
We can observe some very important points at this time in our lecture. We
must test power supplies with a load. Using no load resistance if the capacitor is
leaky or going bad we will not see the problem because the capacitor charges but
does not get discharged. If we check the output voltage with only a meter it will
appear to be okay. But if we put a load on the circuit we discover that the
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capacitor will not hold a charge and we get serious ripple in our output voltage.
We might be able to see this with a meter, or might not, depending on the quality
of the meter and the frequency of our ripple. It is much better to check the output
of our power supply with an oscilloscope and with a load.
Herschel
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