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Rectification and Filtering

AC voltage is used as the power source for most homes. In car audio amplifiers, transformers are used to increase the available voltage and the output of the transformers is an AC voltage. This means that, for the great majority of devices we use, AC voltage is involved. AC voltage is great for power distribution and can be used directly non-critical tasks like powering electric motors and heaters but it's not great for more critical/sensitive circuits. For sensitive circuits, like those used for audio and video, the AC voltage is converted to DC voltage. Earlier, diodes, capacitors and transformers were covered. In this section you will see how they are used together to convert A.C. to D.C.

In the diagram below, the two diodes are connected so that they only allow the positive pulses through to the capacitor. You should notice that the output at point 'A' is inverted with respect to output 'B'. When we have a center tapped transformer as above, the output is considerably smoother than when the there is only one secondary output (output 'B' wouldn't exist). The combined output is pulsed D.C. It is not A.C. because the voltage doesn't cross the reference (black dashed) line. In this circuit, the solid black circuit line would be the reference point and would be considered to be 'ground' (in reality, it would be connected to ground).

In the flash demo below, you can see the AC waveform, the pulsed DC and two 'filtered' output diagrams. The left filtered diagram shows the effects of the smaller (lower value) capacitor. The right-most filtered diagram shows the effects of the larger capacitor. The capacitors serve to fill in the gaps between the pulsed DC to provide a smoothed DC output. When you vary the current draw via the slider, the current flow from the output of the power supply will increase or decrease. When the current flow increases, the ripple increases. Notice how the ripple (roughness) with the larger capacitor is less significant at maximum current draw. When you connect a load on a DC power supply (that's derived from an AC source), this ripple shows up on the supply. The severity of the ripple depends on the transformer's output capability, the DC current being drawn from the supply , the size of the capacitor and the frequency of operation (60Hz for the AC mains in the US, ~25,000Hz for switching power supplies in amplifiers).

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Please note that, in the real world, the capacitors would work together to act as one larger capacitor. The ripple would be the same at all points after the diodes. The ripple as shown above is as if each capacitor were connected to the circuit independently.

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