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Early capacitors or “condensers” were known as Leyden Jars.
They were named after the university where they were developed.
Today’s capacitors look more like these
Start
Run
Surface Mount
Ceramic
Variable
Electrolytic
Noise Suppression
FUNCTION
Capacitors store electrical charge.
They are used in timing circuits because it takes time
for a capacitor to fill with charge.
They are used to smooth varying DC supplies by
acting as a reservoir of charge.
They are used in filter circuits because capacitors
easily pass AC (changing) signals but they block DC
(constant) signals.
Capacitance
This is a measure of a capacitor's ability to store charge.
A large capacitance means that more charge can be stored.
Capacitance is measured in farads, symbol F. However 1F
is very large, so prefixes are used to show the smaller values.
Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):
•µ means 10-6 (millionth), so 1000000µF = 1F
•n means 10-9 (thousand-millionth), so 1000nF = 1µF
•p means 10-12 (million-millionth), so 1000pF = 1nF
AIR
Electrolytic Capacitors
Electrolytic capacitors are polarized.
They must be connected correctly.
At least one lead will be marked + or -.
There are two designs of electrolytic capacitors:
Axial, where the leads are attached to each end.
Radial, where both leads are at the same end.
ELECTROLYTIC
Usually polarized (has a + and – terminal)
DC use only
Cheap, not very accurate
Good storage capability, wide range of voltage & µF
Used in ripple filters, timing circuits & other,
not good for high frequency
De-rate voltage by 100%
(for 12 volt circuit use 24 volt capacitor)
CAN EXPLODE if rated voltage is exceeded
or polarity reversed!
ELECTROLYTIC FAILURE MODES
What to look for
TANTALUM
They are electrolytic so watch for the '+' and '-' indicators
Excellent temperature and frequency characteristics
Small size, reliable, common values readily available
Expensive, easily damaged by spikes, large values exists but hard to find
Super Capacitors
Despite the large capacitance value, its physical
dimensions are relatively small. It has a diameter of 21
mm (almost an inch) and a height of 11 mm (1/2 inch).
Like other electrolytics the super capacitor is also
polarized so exercise caution in regards to the breakdown voltage. Care must be taken when using this
capacitor. It has such large capacitance that, without
precautions, it would destroy part of a powersupply
such as the bridge rectifier, volt regulators, or whatever
because of the huge inrush current at charge. For a
brief moment, this capacitor acts like a short circuit
when the capacitor is charged. Protection circuitry is a
must for this type.
Variable Capacitors
Used in radio tuning circuits
Sometimes called 'tuning capacitors'.
They have very small capacitance values,
typically between 100pF and 500pF
(100pF = 0.0001µF).
Trimmer Capacitors
Trimmer capacitors (trimmers) are miniature variable capacitors.
They are designed to be mounted directly onto the circuit board and
adjusted only when the circuit is built.
PARALLEL CAPACITORS
Capacitors connected in parallel have their capacitance added together,
which is just the opposite of parallel resistors. It is an excellent way of
increasing the total storage capacity of an electric charge:
Ctotal = C1 + C2 + C3
SERIES CAPACITORS
Opposite of calculating resistors in series.
Capacitors in series will have total
capacitance lower than the lowest value.
pF, nF, & µF
A capacitor marked 105 means 10+5zeros = 10 + 00000 = 1.000.000pF =
1000 nF = 1 µF. Value is in pF (PicoFarads).
The letters added to the value is the tolerance and in some cases a
second letter is the temperature coefficient mostly only used in military
applications.
A ceramic capacitor with 474J printed on it it means: 47+4zeros =
470,000pF or 470nF or 0.47µF and J=5% tolerance.
Capacitor Number Code
A number code is often used on small capacitors where printing is difficult:
• the 1st number is the 1st digit,
• the 2nd number is the 2nd digit,
• the 3rd number is the number of zeros to give the capacitance in pF.
• letters indicate tolerance and voltage rating.
For example: 102 means 1000pF = 1nF (not 102pF!)
For example: 472J means 4700pF = 4.7nF (J means 5% tolerance).
CONVERSION VALUES
microFarads (µF)
nanoFarads (nF)
picoFarads (pF)
0.000001µF
=
0.001nF
=
1pF
0.00001µF
=
0.01nF
=
10pF
0.0001µF
=
0.1nF
=
100pF
0.001µF
=
1nF
=
1000pF
0.01µF
=
10nF
=
10,000pF
0.1µF
=
100nF
=
100,000pF
1µF
=
1000nF
=
1,000,000pF
10µF
=
10,000nF
=
10,000,000pF
100µF
=
100,000nF
=
100,000,000pF