|Radio and Electronics (DED Philippinen, 66 p.)|
|10. BLOCKS OF RADIOS / -1- / POWER SUPPLIES|
|10.4. SMOOTHING AND FILTER CIRCUITS|
The output of rectifiers - although unidirectional - is not suitable to supply a radio because it contains still a RIPPLE - an ac partition of the voltage. These ripples cause distortion to the radio, supplied.
Therefore there are means needed which reduce these ripples and convert the output voltage of the rectifier to a steady dc-supply.
The effect of the circuits doing this job is called SMOOTHING. The simplest form of smoothing is obtained by connecting a capacitor in parallel to the output of the rectifier terminals as shown in figure 93.
During the first half of the ripple when the output voltage of the rectifier is increasing - the capacitor will be charged.
Therefore during this period the capacitor stands for an additional load which means the overall voltage at the output will be smaller than without the capacitor (depending on the internal resistance of the voltage source). This effect will go on till the voltage has reached its peak.
During the second half of the ripple - when the voltage drops again - the capacitor is discharged again. The capacitor stands now for a second energy source parallel to the original voltage source (the rectifier). So the voltage at the output terminals will be higher than without the capacitor because the current drawn from the rectifier is diminished. Summing up: The capacitor will tend to fill the valleys between the ripples.
It acts like a store (reservoir) which stores energy during the time of surplus energy coming form the rectifier and which gives out this stored energy during the time of shortage of energy. Therefore this capacitor is called a RESERVOIR CAPACITOR too.
The capacitance, necessary for this capacitor depends on:
a) the load current expected (As bigger the load current as bigger must be Cres).
b) the kind of rectification used before - With one-way rectification the Cres must be bigger than with two-way rectification.
But the capacity of the reservoir capacitor will be limitted by the maximum current permissible for the diodes in the rectifier. This is because in the moment of the first charging (when switching on the supply) it will act similarly like a short circuit for the diodes.
Anyway, we will find always a ripple left on top of the output voltage of such a dc-supply with reservoir capacitor and therefore in most of the cases the output voltage of such a kind of circuit will not be satisfying for the supply of a radio. Additional SMOOTHING is necessary.
The output voltage of Cres is still a dc voltage with an additional ac-component - even though the ac-component is diminished.
The alternating component is not really sinusoidal but this makes little difference to the general principle.
The frequency of the ac-component - the so called RIPPLE is obviously depending on the mains frequency (in most of the cases we will find either 50 or 60 Hz) and the type of rectification used.
If one-way rectification is applied and the supply frequency is 50 Hz we will find a ripple of 50 Hz.
And if two-way rectification is used we will find a ripple of 100 Hz.