When scientists and engineers were first formulating plans for distributing electric power, they had no practicable systems for increasing or decreasing DC transmission voltages. AC transmission voltages, by contrast, were easily stepped up or stepped down by means of a transformer. As a result, AC became the worldwide standard for electric power transmission.
AC voltage is perfectly adequate for light bulbs and motors, the two types of load that were of primary concern back in the early days of electric power. I suspect that George Westinghouse, perhaps history’s most influential proponent of AC distribution, could never have imagined how much inconvenience AC voltages would create for engineers working in the era of digital electronics.
We still need light bulbs and motors, but our lives are now also filled with devices that require stable DC supply voltages. Meanwhile, despite the introduction of high-voltage DC (HVDC) power systems, AC transmission still predominates. AC-to-DC conversion—also known as rectification—is therefore a fundamental task of electrical engineering.
The circuits that perform rectification are, by extension, fundamental components of electrical engineering. In this article, we’ll discuss the most widely used of these components: the full-wave bridge rectifier, also known as the full-bridge rectifier or simply as the bridge rectifier. To understand what makes this circuit so useful, we first need to understand how rectification—both full-wave and half-wave—works.
Half-Wave Rectification with a Diode
The term rectification derives from a Latin word meaning “to straighten.” Accordingly, a rectifier circuit takes current moving in two directions and “straightens” it so that it moves in only one direction. The defining characteristic of a diode is that it allows current to flow fairly freely in one direction (anode to cathode) and strongly opposes current flow in the other direction (cathode to anode), so it’s perhaps not surprising to hear that all semiconductor diodes belong to the broad category of rectifiers.
Rectifying AC voltage takes only a single diode, as we see in Figure 1. Note that the resistor on the figure’s right side represents the load circuitry.
Figure 1. Single-diode rectifier circuit. Image used courtesy of Robert Keim
When the rectifier circuit’s input voltage is positive, current flows through the diode and creates a voltage across the load resistor. When the input voltage is negative, requiring current flow in the opposite direction, the diode functions like an open circuit—since no current is flowing through the resistor, no voltage is generated, and both of the resistor’s terminals are at the 0 V ground potential. The result is the orange DC waveform in Figure 2.
Figure 2. A single diode converts the green AC waveform to the orange DC waveform. Image used courtesy of Robert Keim
It may seem rather generous to describe the rectified waveform as “DC,” and it’s true that we would never want to use it as a DC supply voltage for electronic circuitry. Strictly speaking, however, it is indeed a DC waveform. Despite its drastic fluctuations, the voltage never changes polarity, meaning that the current produced by the waveform will always move in the same direction.
Though simple and at least somewhat effective, the single-diode approach has a conspicuous shortcoming—the positive half of the source waveform is retained, but the negative half is discarded. This is known as half-wave rectification, and it causes large gaps in the output waveform. It would be much better if we could find a way to rectify the input signal without wasting half of it, and that’s exactly what a full-wave rectifier does.
Full-Wave Rectification with Four Diodes
While it’s possible to achieve full-wave rectification using two diodes and a center-tapped transformer, such rectifiers tend to be bulkier and costlier than their full-bridge counterparts. The higher the power of the application, the larger and more expensive it is to include a transformer.
Instead of two diodes and a transformer, a full-bridge rectifier requires four diodes connected in such a way that both positive and negative input voltages will drive current through a load in the same direction. Figure 3 displays the full-bridge rectifier’s four diodes in a classic diamond configuration.
Figure 3. The diamond configuration is the classic representation of a full-wave bridge rectifier. Image used courtesy of Tony R. Kuphaldt
In this circuit, the source is connected to the nodes where a cathode meets an anode; the output is taken from the nodes that join two anodes or two cathodes. Both positive and negative source voltages will cause current to flow from the positive terminal of the load resistor to the negative terminal.
Using the LTspice implementation in Figure 4, let’s take a closer look at how the four-diode arrangement achieves full-wave rectification.
Figure 4. An LTspice implementation of a full-bridge rectifier. Image used courtesy of Robert Keim
As the schematic shows, the negative terminal of the input source is the simulator’s reference node, and the output voltage must be measured differentially from the “rectified+” node to the “rectified–” node.
Because the input source and the rectified output don’t share a common reference potential, the operation of this circuit can be a bit confusing if you focus on voltages. The key is to think in terms of current, which must originate from and return to the source while always flowing downward through the load resistor.
As long as current flows downward through the load, the voltage across the load—or, put a different way, the voltage supplied to the load—will be positive. The green arrows in Figure 5 illustrate the flow of current when the source voltage is positive.
Figure 5. Full-bridge rectifier current flow during the positive half-cycle of the input wave. Image used courtesy of Robert Keim
When the source voltage is negative, however, the lower terminal of the source symbol is higher in voltage than the upper terminal. The current thus begins at the lower terminal and flows toward the upper terminal, as we see in Figure 6.
Figure 6. Full-bridge-rectifier current flow during the negative half-cycle of the input wave. Image used courtesy of Robert Keim
The result, as we see in Figure 7’s voltage plot, is a rectified waveform that preserves the input wave’s positive half-cycle and inverts the negative half-cycle.
Figure 7. The full-bridge rectifier in Figure 4 converts the green AC waveform to the orange DC waveform. Image used courtesy of Robert Keim
After the full-bridge rectifier has done its work, we can produce a usable DC supply voltage by using a capacitor to smooth the fluctuating DC signal, then using a linear regulator to stabilize the smoothed signal.
Further Reading
We’ve now covered the basics of how this clever interconnection of four diodes performs AC-to-DC conversion. You can learn more about full-bridge rectifiers by reading—and, if you want, building—the full-wave bridge rectifier with output filtering project available on the All About Circuits website.
