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| Capacitors |
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Definition
Capacitor Types
Value Identification
RC Timing
Decoupling
AC Coupling
Other Capacitor Configurations
Definition:
From the more technical side, capacitors store energy in the form of an electric
field. A simple capacitor consists of two conductive plates seperated by an insulating "dielectric". Even more techincal,
capacitance is directly proportional to the surface area of these plates and inversely proportional to the separation between
them.
From a more practical perspective, I tend to think of capacitors in one of two ways depending on the application.
In situations where the ability to store energy is being directly exploited, such as power supply decoupling and RC timing
circuits, it is useful to think of a capacitor as it is described above. As a low capacity and inefficient rechargable cell.
In
applications where a capacitor is interacting with an AC signal it can be more useful to think of a capacitor as a device
which passes AC signals of frequncies proportional to the capacitance value while impeding DC signals. More on this below....
Capacitor Types:
The image below contains the 5 basic types of capacitor; Ceramic, Polyester, MKT,
Tantalum and Electrolytic. Value ranges given below are approximate.
1. Disc Ceramic: While they are limited to quite small values, disc cermaics boast a small and solid
construction with comparatively high voltage ratings. They range from 1pF to 0.47µF and are not polarised. This type can often
be used to replace a polyester capacitor of the same value.
2. Polyester "Green Caps": Ranging from 0.01µF to
5µF polyester capacitors have similar properties to disc ceramics with some larger values and a slightly larger construction.
They are not polarised.
3. MKT Polyester: A varation of polyester capacitors used where price matters less than
performance. High temperature stability and accuracy land MKT capacitors in higher end audio circuits and power supplies.
They range from 1nF to about 10µF. (values over 1µF are quite expensive) MKTs are not polarised.
4. Tantalum:
Tantalum capacitors pack a large capacity into a relatively small and tough package compared to electrolytics, but pay for
this in voltage ratings. The device pictured above is 100µF (like the electrolytic next to it) but is rated at 3.6V, compared
to 16V. They are often polarised and range from 0.1µF to 100µF.
5. Radial Electrolytic: Used for all values
above 0.1µF. Electrolytics have lower accuracy and temperature stability than most other types and are almost always polarised.
It's usually best to only use an electrolytic when no other type can be used, or for all values over 100µF. Cheap electrolytics
are usually made from plastic and rubber and therefore melt easily during soldering.
6. Axial Electrolytic:
The same as other electrolytics but the leads emerge from each end, rather than the same end as in the radial types.
Value Identification:
Most capacitors use a 4 character code similar to the four band color
code on resistors. The first two characters are numeric and represent the two most significant figures of the value (in pF),
the third is the multiplier or the number of zeros, and the fourth (if it exists) is a letter representing a tolerance. For
example, "473K" works out to 47000pF, 47nF or 0.047µF at 10% tolerance. This is called the IEC code. IEC tolerance characters
are shown in the table below.
| M |
20% |
| K |
10% |
| J |
5% |
| C |
±0.25pF |
Electrolytics almost always have their value explicitly stated in µF along with their
voltage rating. Disc ceramics with values under 100pF often simply have their value stated in pF, but if the marking has three
numeric characters it should be read as IEC instead. Some MKT capacitors will have a value followed by or including a metric
abbrieviation. (eg. 47n = 47nF) If the metric abbvieviation is included it acts as a decimal point. (eg. 4n7 = 4.7nF)
When
no voltage rating is specified for a disc ceramic, polyester or MKT capacitor it is usually safe to assume an arbitary
figure like 50V.
RC Timing:
A capacitor is "charged" by applying electrical energy (DC)
to the dielectric in the form of a potential difference between the components leads. The capacitor will draw current as it
charges, inducing a measurable voltage drop across the component. The closer to fully charged the capacitor comes, the larger
the voltage across it and the smaller the current drawn. This phenomenon, along with the influence the value of the capacitor
has on the charging time can be used to create reasonably accurate time delays.
While changing the value of C in the above circuit will influence the on-time, (time when pin 3 of the 555 is
at Vcc) large capacitors are not known for a high degree of accuracy. Furthermore, an adjustable delay is not exactly practical
from a this approach. This is where R comes in by limiting the current by which the capacitor is able to charge and therefore,
the on-time. High accuracy resistors are available, (and are relatively cheap) as well as variable types.
By choosing
an appropriate value for C and adjusting R as required, the above circuit can acheive on-times from a few microseconds to
a year or more. (long term stability is more due to the 555 than the resistor and capacitor) Choosing values for C which allow
R to be over a few KΩ will help keep current draw low.
RC timing is by no means limited to the 555 Timer IC, but can be used in a number of ways. The classic "multi-vibrator" circuit is an example.
Decoupling:
Placing
a relatively large (greater than 100µF) capacitor across a power supply helps to smooth out noise, this is well known. The
practice is called "decoupling", and is most easily understood if the capacitor is thought of as a storage cell. (my first
definition) When a decoupled power supply is switched on, the capacitor across it will charge to Vcc and proceed to do nothing
except draw a small amount of current to compensate for it's own leakage. However, if Vcc falls briefly under load the capacitor
will present a potential higher than Vcc and the stored energy will flow into the load, helping to maintain the supply voltage.
Should Vcc "spike", the capacitor will begin to charge as there is a now a potential difference across the dielectric, effectively
absorbing some of the spike's energy.
Power supply decoupling has a few drawbacks:
Large electrolytic capacitors are physically large, taking up PCB space and money.
By the very means they help keep Vcc stable, decoupling capacitors will have a significant effect on Vcc rise
and fall times during normal power on and shutdown.
A word of warning; If Vcc is shorted to ground (ie. 0Ω) large
amounts of current are demanded from the power supply. Ohms Law will come up with current as mathematically undefined, ie. infinate current. Power supplies are usually designed to cope
with overload in a variety of ways, the simpliest of all being a fuse, but capacitors are not. Irregardless of how the power
supply reacts the capacitor will deliver all it's energy to the load very briefly, in the order of tens of amps. (similar
to shorted car battery, but very brief) Regulator circuitry is more prone to damage due to the low impedence of regulator
IC's presenting an alternate path to ground, the circuit itself is ironically protected by the short circuit.
Worrying
about this is really just paranoia, but I like to call it food for thought.
AC Coupling:
The
ability of the capacitor to pass AC signals while blocking DC signals is most often used in audio circuits, in order to allow
opamps to work of a single supply potential. (or just to protect against DC inputs, which can be noisy)
Obviously, an operational amplifier cannot produce an output voltage outsite Vcc or Vss, which is why audio
equipment often requires "dual power supplies" to give the opamps a power supply with which they can amplify the negative
half of a signal. Unfortunately, many situations make dual power supplies impractical or even impossible, (eg. battery powered
equipment) so a work around is required.
 The red part of the signal will not be present on the output.
The image above shows an input signal which
is unbiased, or centred around 0V. With the single positive supply potential between Vcc and Vss connected to the opamp
in the circuit above the negative side of the input signal will not appear on the output. (red) However, the AC coupling capacitor(s)
and voltage divider present a solution.
The capacitor on the input will remove any DC offset from the input signal, (a precaution which
is not neccessary if the input is known to be free of DC bias) and the voltage dividor will provide its own DC offset of half
Vcc, resulting in the second waveform in the above image. This waveform is inside the supply voltages and will not be clipped.
The output from the opamp is then passed through another AC coupling capacitor, (this one is always neccessary) to remove
the DC offset from the voltage divider.
Other Capacitor Configurations:
Like other
passives, capacitors can be combined in parallel or series to create more interesting values. The maths is also somewhat similar:
Note that the result for series capacitance is also a reciprocal and must be reciprocated to obtain the correct
result. The need for unusual capacitor values is rare for the average hobbyist, but knowledge of the above rules can be of
use. If you are prototyping something and need a large capacitor you don't have, it's useful to know a bunch of smaller ones
in parallel will deliver similar results.
Neither series or parallel layouts have any effect on the voltage rating
of the capacitors used.
If you have any comments or questions please don't hesitate to contact me.
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| Last Updated: 01/08/2004 |
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