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Full Wave Rectifiers and Transformers

Introduction

The national electrical grid provides an abundance of electricity to users in the form of alternating current (AC) at 110 to 230 volts and 50 or 60 Hz, depending on the region. However, what if someone requires a continuous supply of direct current (DC) voltage from this source? This course will cover full wave rectifiers, which are commonly used in the industry for converting AC to DC.

What is a full bridge rectifier?

In the last course we covered a half wave rectifier – just a simple diode. When AC voltage is applied to its anode, cathode will conduct only during the positive halfwave. Resulting waveform is far from DC, but it is always positive. If averaged the output, we would get a DC voltage source. This method has many significant drawbacks. Most of them originate from the fact, that diode is in conducting mode for only about half the time, but we won’t go into details. Finding a circuit, that will conduct voltage in positive direction would alleviate the problem. Conveniently such circuit exists and is depicted below. We call it a full wave rectifier.

A full wave rectifier is composed of two pairs of diodes that ensure voltage ‘flows’ in the correct direction. It converts AC voltage to DC voltage. To comprehend its operation, it is useful to examine what happens when the applied voltage is positive and negative. For the purpose of analysis, we can assume that diodes conduct electricity when the current flows from the anode to the cathode.

If we apply a signal to an ideal full wave rectifier, its output will be an equivalent of mathematical function out=abs(in). To illustrate that with a sine wave input where black is input and red is rectified output:

Written by Luka Pogačnik Edited by Andraž Pirc

This teaching material was created by Red Pitaya & Zavod 404 in the scope of the Smart4All innovation project.

Transformers

There are only a few devices that require rectified mains voltage to operate. Usually required voltage is a lot lower. A low cost solution is to use a transformer with an appropriate winding ratio. A transformer outputs voltage that is higher, lower, or equal to the input voltage based on how many turns input and output windings have. The exact relation is such:

U_{OUT}=\frac{N_OUT}{N_IN} U_IN

Aside from changing the amplitude, a transformer also galvanically disconnects input and output. ADALP2000 component kit, from which we select components for this course, has two transformers in it. Both are from Coilcraft’s Hexa-Path series. They have six individual windings, from which we can construct “any” transformer we want. Biggest voltage ratio we can construct is 5:1 (or 1:5). This is done by selecting one coil to be input/output, and wiring the remaining five in series, paying attention on polarity (marked with a dot next to the inductor symbol. The following diagram is from the transformer’s datasheet

Consumer electronics usually use a transformer to convert mains voltage into something lower, but we will be using it in reverse: to change RP’s +-1 V output to +-5V. Here is input to the transformer in such configuration (yellow) Vs. output (green).