8.2. CAPACITORS
ELECTRICAL CHARACTERISTICS
A) capacitors at dc
EFFECT: If we connect a capacitor to a dc voltagesource there
will flow a considerable high current in the first instant. But this current
will decrease fast, to lower values and it will be 0 after a relatively short
time.
REASON: When first connected to the dc-voltage, the charge on
the capacitor is 0. Therefore the voltage source will start to charge the
capacitor. The charges brought to the capacitor are electrons pushed to the
negative plate, and electrons sucked out of the positive one. As soon as the
plates have a charge big enough to stand for the same voltage as it has the
source, there is no more potential difference and therefore the current in the
circuit must be 0 again.
fig. 57
B) capacitors at ac
EFFECT: For easier understanding let us first imagine not a
sinusoidal alternating voltage but a FLAT-TOPPED AC-VOLTAGE one. (This means
nothing else than a dc voltage whose polarity is changed over after a certain
period of time). In this case we can easily imagine, that there will be a charge
current at each change of polarity.
RESULT: There flows a current always if there is a change of
voltage.
fig. 58
Now let us observe a SINUSOIDAL AC VOLTAGE:
- After having had a closer look to that ac-voltage
we will find, that it is changing all the time except of the two instants at
the peaks.
- Applying the results of the findings at the flat-topped
ac-voltage we can foresee that the current will now flow all the time because
the voltage changes constantly.
- If we have understood that the current flowing in this circuit
is depending on the change of the applied voltage we can easily predict, that
the amount of current flowing will depend on the velocity of voltage change. (as
faster the voltage will change as higher will be the current flowing).
-Applying these modified findings, we can conclude, that the
current will have its maximum when the voltage is changing at its fastest rate -
and this is the fact at the noughtpoints of the ac-voltage.
Of course between the four points found with the considerations
above there are values of the current which form altogether a sinewaveagain.
These considerations enable us to explain now two characteristics of an
ac-current flowing through a capacitor.
fig. 59
PHASE RELATION
As we see the current is always flowing “earlier” than
the voltage is arising.
The current is phase-shifted in relation to the voltage. The
current is LEADING
The biggest phases-hift possible is a quarter of a period (or 90
degrees).
fig. 60
FREQUENCY RESPONSE
As we found - the current depends on the change of voltage.
fig. 61
If we compare now two different frequencies with the same
amplitude of voltage we can see, that at a higher frequency the change of the
voltage must be around the noughtpoint higher than at a lower frequency.
This observation makes it clear that the current at a higher
frequency will be higher and therefore we can derive that the ac-resistance
which is called IMPEDANCE OF A CAPACITOR IS AS LOWER AS HIGHER THE FREQUENCY
CONNECTED IS.
IMPEDANCE/CAPACITIVE REACTANCE
The impedance of a capacitor can be calculated by the formula:
Whereby R is the OHMIC RESISTANCE which is causing
“losses” and X is the so called CAPACITIVE RACTANCE which is to be
calculated by the formula:
MAIN FUNCTIONS OF CAPACITORS
1. To smoothen the pulsating currents in power
supplies. You can also say to “short circuit” ac-components within
pulsating dc-voltage. SMOOTHING CAPACITORS
fig. 62a
2. To block dc-voltage and to let ac-curents flow from amplifier
to amplifier stage. COUPLING CAPACITORS.
fig. 62b
3. Combined with resistors we find them in so called PASSES
which let only pass special frequency ranges.
fig. 62c
4. in combination with inductors for TUNED CIRCUITS, which
filter out special frequencies from a certain mixture of signals.
fig. 62d
KINDS OF CAPACITORS
POLYESTER CAPACITORS have almost replaced paper capacitors. They
are made in values of 0.01 mikro Farad up to 10 mikro Farad. They are for
general purpose use.
MICA CAPACITORS are used in RF circuits and are made in values
up to 0.01 mikro Farad.
CERAMIC CAPACITORS have an extremly constant capacity. They are
consisting of a ceramic chig which has a layer of metla on both sides.
ELOCTROLYTIC CAPACITOR are made by putting an oxide layer on the
surface of an aluminium foil. The other plate of the capacitor is formed by an
electrolyte in which the foil is emersed after having been rolled. The oxide is
the dieelectric. They are polarized and may exclusively be connected in the
fitting direction otheriwse they might explode.
VARIABLE AIR DIEELECTRIC CAPACITORS consist of tow groups of
plates made from aluminium sheets. One of the groups is fixed the other one is
movable. They can be moved in and out and so change the capacity of the
capacitor. They are used only for tuned circuits.
CHECKING AND HANDLING OF CAPACITORS IN RADIO SETS.
Big capacitors are almost always smoothing capacitors and
therefore it is possible to measure the voltage at them. It should be under
normal conditions near to the supply voltage.
With smaller capacitors it is not possible to measure the
voltage, there you can only measure if the capacitors has a high resistance for
dc.
If you have to replace a capacitor you have to observe two
values:
1. the voltage rating: capacitors are
limitted in voltage applicable to them. If there is no fitting replacement. You
can connect them in series
2. the capacitor If you don't have a fitting one you can
arrange one by connecting several in parallel but keep in mind the voltage
rating.
To find the values of a special capacitor you will find either
the specifications printed on them, or you find the colour code system, whereby
the value found out is in piko Farad.
CHECK YOURSELF:
1. How is a capacitor behaving at dc or ac?
2.
How is the phase relation between voltage and current at ac?
3. How is the
influence of the frequency on the impedance?
4. What does the term impedance
mean?
5. What does the term reactance mean?
6. Which different functions
can capacitors be used for in radios?
7. Which different kinds of capacitors
for you know
8. What to do in order to check a capacitor in an electronic
device?
9. What is necessary to be kept in mind if you want to replace a
capacitor.