Physics - Converting AC electricity into DC electricity: smoothing
Converting AC electricity into DC electricity: smoothing
The half-wave rectified signal shown above now needs to be converted into
a steady DC voltage. At times there is a lot of current flowing in the
circuit and at other times there is no current flowing at all. A device is
needed that could store electricity during current flow and release it
later when the supplied current drops. The device capable of doing this is
A capacitor is added to the previous circuit by being placed in parallel
with the load as shown.
During the time when a positive voltage is supplied to the circuit, the
capacitor is able to charge, holding a maximum voltage equivalent to the
supply voltage at any instant. Note that during this stage, current is also
flowing in a forward direction through the load as required.
As the supply voltage begins to drop off, this is just like the situation
where a battery is disconnected from the capacitor plates. The capacitor is
allowed to discharge through the load in the circuit. The power supply is
no longer providing current to the load, but the capacitor providing
current instead. The capacitor is discharging during this process. It is
therefore important that the capacitor is able to supply enough voltage,
and therefore current, to the load. If the capacitor's voltage drops too
much, we are back at a situation like the previous one where the load is
not receiving a steady, smooth voltage. Larger capacitors are able to
provide more current with less drop in voltage occurring. They produce
smoother signals. Or to put it another way, a circuit with a large time
constant is desirable so the little drop in voltage occurs during the
switch off time of the input signal. If a large time constant exists, the
voltage across the capacitor would not have time to diminish by a great
amount before the next cycle of the input signal begins.
The graph below compares the half-wave rectified input signal to the
smoothed signal that is obtained once a capacitor is employed. Compare the
effect of using a small or a large capacitor.
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