Rectifiers

Diagrams comparing half wave and full wave rectificationMost electronic devices receive power from an AC wall plug. These supply power in the form of alternating current, but what if a machine needs direct current? Actually, almost every electronic machine and home appliance operates on direct current even though alternating current is what is most readily available. Is there an easy way to turn alternating current into direct current? Well, yes, there is.

Rectifiers are electrical devices that convert alternating current into direct current through a process called rectification. How does rectification work? Rectification works by passing the supplied alternating current through a diode or series of diodes that only allow unidirectional current flow. As alternating current passes through the diodes, a new current waveform is produced that no longer has negative current flow and acts as direct current. Because DC (direct current) power sources are limited and uncommon, rectifiers are used in almost every electronic device and are by far the most commonly used circuit.

Construction

Rectifiers use a series of semiconductor diodes to convert alternating current into direct current. These rectifier diodes can handle high currents as compared to normal diodes. Diodes are like one-way streets, they only allow current to flow in one direction.

How many diodes are used in a rectifier? There can be up to 4 diodes in a rectifier. The number of diodes depends on the desired DC waveform. The two basic types of rectifiers are full-wave and half-wave, a full-wave rectifier requires 4 diodes while a half-wave rectifier only requires one.

Half wave rectifier diagramHalf-wave Rectifiers

The simplest type of rectifier is a half-wave rectifier. These rectifiers work by eliminating the negative part of the AC source, allowing only positive pulsating current to pass through. As shown in the picture, if you were to use a single diode rectifier you would get a waveform that looks like half of the original alternating current waveform, hence the name half-wave rectifier. This form of rectification is simple and cost efficient because it requires only one diode, however, this conversion causes a significant reduction in power output because half of the AC wave is sacrificed. Half-wave rectification produces spaced out bursts of direct current which has many drawbacks but can be utilized in certain low power applications

Full-wave Rectifiers

Full Wave Rectifier Diagram

Full-wave rectifiers, otherwise known as bridge rectifiers, use four diodes as opposed to one to make the entire AC waveform positive rather than simply eliminating the negative. When 4 diodes are used and set up as shown in this image, the resultant waveform looks like the absolute value of the original alternating current waveform. Unlike the half-wave rectifier, the flow of current does not stop for half the time and is always changing, hence the name full-wave rectifier. But how does adding more diodes to the rectifier configuration change the waveform so significantly? Adding more diodes allows them to be set up in a way that redirects the negative flow of current from the AC source rather than eliminating it. The diodes are connected in a closed loop that only allows current to flow through two diodes at a time depending on if the AC is in a positive or negative phase. The sets of diodes direct the current in a way that both negative and positive half cycles create an output DC signal with the same polarity. The DC still oscillates from zero to peak value as it does in a half-wave rectifier, but, as shown in the image, it doesn’t cut out for half the time, producing twice as much output power as a half-wave rectifier.

Filtering

Although rectifiers are effective in converting alternating current to direct current, the resulting direct current is not steady enough to effectively act as DC. The resulting current has a ripple that rises and falls in sync with the AC voltage fed into the rectifier. Additional filtering is required to smooth out the rectified DC to produce a smooth direct current suitable for even the most sensitive of circuits. Capacitors are effective in filtering the rectified DC current because they resist changes in voltage and cause the current ripple to be less extreme.

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