Ed's Radio (AA6ED) – Rectification

Half Wave Rectifiers

A rectifier circuit is used to convert AC voltage into DC voltage. The simplest type of a rectifier is a half wave rectifier. As shown below, it consists of an AC power source, a diode, and a load requiring DC voltage. The load is represented by resistor Rload in the circuit below. For this tutorial, we will assume the voltage source is 60 Hz household AC.

Half Wave RectifierHalf Wave Form

The AC power source provides an alternating voltage, but the diode only allows current flow when forward biased. Because of this, if you measured the voltage across Rload and compared it to the voltage across the AC power source, you would see only half the cycle is being used (see the wave form figures above). This is called 60 Hz pulsating DC.

A half wave rectifier circuit is very simple to build, but not very efficient. The highest theoretical efficiency for a half wave rectifier would be 50% since you are throwing away half of each AC cycle. In reality, things are even worse. Remember that a diode has a forward-threshold voltage that must be exceeded before the diode will begin conducting. Depending upon the makeup of the diode, this will be about 0.3 or 0.7 volts. As the AC cycle rises from zero to the forward-threshold voltage, no conduction occurs so this power is also wasted. Thus, a highly efficient half wave rectifier will only be about 40% efficient – not too good.

Full Wave Rectifiers

A more common type of AC rectifier is a full wave rectifier. This utilizes the full AC cycle and is much more efficient than a half wave rectifier. A typical full wave rectifier circuit is show below in figure 16.

Full Wave Circuit

A full wave rectifier, also known as a bridge rectifier, works in the following manner: When the AC power source is in the first half of its cycle, current flow will be in the direction indicated by the green line and arrows shown in figure 17. When the voltage reverses during the second half of its cycle, the current flow is shown by the red line and arrows in figure 18.

Full Wave Current AFull Wave Current B

As you can see from the figures above, current flows through RLoad during both half-cycles of the power source. Also, irregardless of which half-cycle the power source is in, the current flow through RLoad is always in the same direction. That's the beauty of a bridge rectifier. The resulting wave form across RLoad is shown below along with a comparison of the AC voltage cycle from the power source.

Full Wave Form

From the wave form figure above, the negative half of the AC wave has been flipped over to become a positive cycle. Thus, the resulting frequency is doubled and this is known as 120 Hz pulsating DC. While this is much better than a half wave rectifier, it still is not perfect DC. Pure DC would give a linear graph, while this still has AC ripples in it. Also, the same caveat regarding a diode's forward-threshold voltage still applies to the full wave bridge.

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