I understand current flow in the full wave rectifier. How does the negative voltage become positive?
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Full-wave rectification is a process of converting alternating current (AC) electrical power to a direct current (DC) form of electrical power. AC constantly cycles back and forth from its positive to its negative voltage limits in a form called a wave. Full-wave rectification converts both the positive and negative portions of the AC wave to a positive DC electrical signal, or its equivalent, using devices called diodes.
As AC power cycles, it takes the form of curves resembling a row of the letter "S" with each "S" laid on its side and end-to-end. Horizontally through the center is a reference point that represents zero in voltage. As the AC signal passes back and forth over the zero line, it resembles a series of humps above the line, which are positive, and an opposite series of humps below the line, which are negative. The goal of all rectifiers is to convert that continuously alternating signal to a constant, or direct, positive voltage for use by certain types of electrical equipment.
Rectifiers use diodes to perform rectification, or the conversion of AC to DC. Diodes will only conduct electrical power in one direction. As a result, if an AC signal is passes through a diode, only the half of the AC signal that is above the zero-voltage line is allowed to pass through. The resulting signal appears to be a series of humps with gaps between them where the negative halves of the AC signal were blocked. This arrangement that only converts half of the AC wave to DC is called a half-wave rectifier.
By placing four diodes in positions that allow two to be opposite in polarity to the other two, a device called a bridge rectifier is created that performs full-wave rectification. The positive half of the AC signal is allowed to pass by two of the diodes as it does in a half-wave rectifier. The diodes in opposing polarity then allow the negative half of the AC signal to pass; however, the negative half of the AC is passed as a positive voltage. The resulting output is a signal built from both halves of the AC wave that resembles humps connected end-to-end on the positive side of the zero voltage line without the gaps present in a half-wave rectifier.
While the electrical signals created from full-wave rectification are not true DC voltages, because they do not maintain a constant level, they are constant enough for many types of DC-powered equipment. If more rectification is needed to create a smoother DC voltage, devices called voltage regulators can then be used. These regulators average out the voltages of the humps created in full-wave rectification to constant “true” DC signals.