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In a power supply using and AC waveform input and diode rectifiers, this raw rectified output is normally smoothed using a reservoir capacitor
Aluminium electrolytic capacitors are ideal for the job as many electrolytics are able to have a sufficiently high capacitance and ripple current rating to supply the required current to smooth the waveform.
Capacitor smoothing basics
The raw DC supplied by a rectifier on its own would consist of a series of half sine waves with the voltage varying between zero and √2 times the RMS voltage (ignoring any diode and other losses). A supply of this nature would not be of any use for powering circuits because any analogue circuits would have the huge level of ripple superimposed on the output, and any digital circuits would not function because the power would be removed every half cycle.
To smooth the output of the rectifier a reservoir capacitor is used - placed across the output of the reciter and in parallel with the load.. This capacitor charges up when the voltage from the rectifier rises above that of the capacitor and then as the rectifier voltage falls, the capacitor provides the required current from its stored charge.
It should be remembered that the only way discharge path for the capacitor, apart from internal leakage is through the load to the rectifier / smoothing system. The diodes prevent backflow through the transformer, etc..
Smoothing capacitor value
The choice of the capacitor value needs to fulfil a number of requirements. In the first case the value must be chosen so that its time constant is very much longer than the time interval between the successive peaks of the rectified waveform:
Rload = the overall resistance of the load for the supply
C = value of capacitor in Farads
f = the ripple frequency - this will be twice the line frequency a full wave rectifier is used.
Smoothing capacitor ripple voltage
As there will always be some ripple on the output of a rectifier using a smoothing capacitor circuit, it is necessary to be able to estimate the approximate value. Over-specifying a capacitor too much will add extra cost, size and weight - under-specifying it will lead to poor performance.
The diagram above shows the ripple for a full wave rectifier with capacitor smoothing. If a half wave rectifier was used, then half the peaks would be missing and the ripple would be approximately twice the voltage.
For cases where the ripple is small compared to the supply voltage - which is almost always the case - it is possible to calculate the ripple from a knowledge of the circuit conditions:
Full wave rectifier
Half wave rectifier
These equations provide more than sufficient accuracy. Although the capacitor discharge for a purely resistive load is exponential, the inaccuracy introduced by the linear approximation is very small for low values of ripple.
It is also worth remembering that the input to a voltage regulator is not a purely resistive load but a constant current load. Finally, the tolerances of electrolytic capacitors used for rectifier smoothing circuits are large - ±20% at the very best, and this will mask any inaccuracies introduced by the assumptions in the equations.
Two of the major specifications of a capacitor are its capacitance and working voltage. However for applications where large levels of current may flow, as in the case of a rectifier smoothing capacitor, a third parameter is of importance - its maximum ripple current.
The ripple current is not just equal to the supply current. There are two scenarios:
- Capacitor discharge current: On the discharge cycle, the maximum current supplied by the capacitor occurs as the output from the rectifier circuit falls to zero. At this point all the current from the circuit is supplied by the capacitor. This is equal to the full current of the circuit.
- Capacitor charging current: On the charge cycle of the smoothing capacitor, the capacitor needs to replace all the lost charge, but it can only achieve this when the voltage from the rectifier exceeds that from the smoothing capacitor. This only occurs over a short period of the cycle. Consequently the current during this period is much higher. The large the capacitor, the better it reduces the ripple and the shorter the charge period.
When selecting a reservoir capacitor for smoothing applications in a power supply, not only is the value in terms of capacitance important to give the required ripple voltage reduction, but it is also very important to ensure that the capacitor ripple current rating is not exceeded. If too much current is drawn, the capacitor will heat up and its life expectancy reduced, or in extreme cases it could fail.
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