7.4 A.C. /D.C. Rectification

The electricity supplied to you house by your regional power plant is in alternating current (A.C.) so that it can be transformed for easy transportation, with minimum power loss along power lines. However, in the home, we often want direct current (D.C.), for use in electronic systems, such as a stereo. For this reason, we want to be able to convert A.C. to D.C. fairly easily. This can be done by connecting a few diodes together.

Here is the graph of a normal A.C. current (against time)flowing to the light bulb in this circuit:

This can be converted to D.C:

in only a few steps. As you can see, half of the time, the alternating current in a circuit is negative, which means the conventional current flows in the opposite direction through the circuit (i.e. if it had been flowing clockwise, it would change to flowing counter-clockwise for half the time).

The "negative" A.C. in a circuit can be eliminated by placing a diode in the system, which blocks any current that flows against it. The diode acts as a current valve. The current can flow when its direction is with the diode (the direction the "arrow" part of the diode points):

but whenever the current tries to flow opposite the direction of the diode, it is blocked, and instead no current flows for that time.

(Remember that current flows through every point in the circuit at the same time, so if one part is blocked, all are). A Current-time graph for this situation of half wave rectified current looks like this:

The next step in A.C. D.C. rectification is to replace the missing current with a current reading of the same sign as the rest (i.e. no relatively negative readings). This can be done using another system of diodes. This setup of diodes is called a bridge rectifier. No matter which way the current flows from the power source, it still reaches the resistor (which has replaced the light bulb) in the same direction, and flows out of it from the same direction as well.

The current-time reading at the resistor would now look something like this:

which, you may notice, looks a lot like the varying or "lumpy" D.C. created by a simple D.C. generator. This shape is due to the ‘negative’ half of the A.C. waveform that is effectively "flipped" by the bridge rectifier.

Finally, in order to have the smooth D.C necessary for most electronics systems, we introduce a capacitor in to the system. The capacitor stores charge which it then gives out as the current drops again to its minimum. The capacitor gets charged whenever the current reaches its peak value. The resulting Current-time graph is now:

Often there may be small bumps in the current output followed by a decline in current (until the next bump), but with a large enough capacitor, these are not noticeable