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.