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Rectification Explained | Electronics for novices

A guide for novices and newcomers, we take a look at the inner workings of rectification and the many applications of a bridge rectifier.

By: Frederick Chua

What is rectification?

Rectification is the process of converting alternating current (AC) to direct current (DC). Rectification is mainly needed in power supplies and as envelope detectors of radio signals. In its simplest form, a rectifier (the device required for rectification) is the same as a solid state diode, but most rectifier units contain a specific arrangement of diodes to better convert AC to DC. 

This opens up a whole host of other important issues when considering rectification, such as the type of AC supply, waveform and smoothing. The idea of a diode bridge (to become a bridge rectifier) was first patented in 1896 by the Polish electrotechnician Karol Pollack, but has since evolved and become more sophisticated. 

Single-phase rectifiers 

Most low power rectifiers (such as for domestic equipment) are single-phase. Single-phase rectifiers require a single diode which blocks either the positive or negative half of the input waveform, thus meaning a reduction by half in output efficiency and a lowered voltage; this is known as half-wave rectification, as only half the waveform is carried on through. Full-wave rectification means that both polarities are translated from input to the output and thus yield a higher average voltage. This requires the use of a center-tapped transformer.

Three-phase rectifiers

Otherwise known as multi-phase, three-phase rectifiers similarly use half-wave and full-wave circuits, but replace diodes with thyristors which can regulate the output voltage. Because of this added regulation, three-phase rectifiers tend to be used in high-power and industrial equipment. Depending on capability, you can find a wide range of bridge rectifiers configured in various ways from major electrical suppliers, such as RS Components

Bridge rectifier

A bridge rectifier (diode bridge) has a specific arrangement of four or more diodes if a simple transformer without a center tap is used. This is a full wave rectifier which, as the name suggests, converts both the positive and negative halves of the input waveform to a single polarity. 

A diagram of a four diode bridge rectifier

Rectifier Output smoothing

Seeing as both full-wave and half-wave rectification produce pulsed voltage, a filter or smoothing circuit are needed to make the output voltage DC constant. At its most basic level, a reservoir capacitor can be placed across the DC output of the rectifier. The reservoir capacitor is so named as it discharges stored energy in between pulsations in the AC power source. In this short amount of time, the released energy combines with the pulsed power to gradually move to a state of smoothed current. 

This diagram shows a bridge rectifier (D1) in action. The smoothing capacitor (C1) stores and discharges energy in between pulsations. The discharged energy from C1 can be interpreted as the red line in the waveform graph on the right. 

What is it used for?

Virtually all electronics require direct current, and so bridge rectifiers are used primarily in power supplies for phones, laptops, washing machines et cetera. They are also extensively used in the automobile industry, and are found particularly in alternators. With the advent of the modern car and its more sophisticated electronics. In response, more complex and efficient rectifiers have been produced to put up with the demand for the various electrical systems around the car. 

The future of rectification

From the simple patent of 1896 to its presence in some of the most sophisticated technology in the world, what’s next for the rectification? As with most scientific advances, electrotechnicians are turning their attention to the environment and communications. In terms of environmental technology, this would see solar cells replaced with tiny antenna, nantenna, that can directly rectify light waves to produce DC electric power. This would make solar energy far more efficient. In terms of communications, the bridge rectifier is being developed to rectify higher frequencies in the terahertz and light frequencies. This is particularly useful in the world of fibre optics, and more specifically in optical heterodyne detection

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