Alternating current (AC) power is responsible for running many types of electrical appliances. But what if you need direct current (DC) power to operate a computer or LED? Converting AC to DC uses a configuration of diodes—and in some cases a transformer—known as a linear rectifier, or just a rectifier.
What is a Rectifier?
Rectifiers come in two basic types: full-wave and half-wave. Full-wave rectifiers turn an entire AC waveform into a series of single-polarity DC pulses, while half-wave rectifiers simply cut off half the electrical output of an AC signal, leaving pulses of DC. Historically, we’ve also seen several other interesting devices accomplish voltage rectification, which we’ll touch on later in the article.
Half-Wave Rectifier
Since an AC circuit varies its voltage between a positive and negative value—for our example, we’ll use the 60Hz 120VAC value seen in the US— eliminating the negative or positive half of this electrical wave would leave you with a somewhat choppy source of DC power. You can accomplish this transfer method using a single diode, which only allows current flow in only one direction.
Let’s dig deeper into this AC to DC conversion.
- Notice that “120VAC” is the root mean squared (RMS) power value, which is a more practical reading of the sinusoidally-varying mains power source’s 170 and -170-volt peaks.
- If you cut off the max -170V part of the waveform, you’re left with power that builds up to +170VDC, decreases to 0V, then stays there for 1/120th of a second (since a full AC power cycle takes 1/60th of a second) before building up to 170 again.
This type of conversion causes a significant reduction in power output. Theoretically, this works out to be 40.6 percent of the AC input. In reality, this number would be lower due to the inevitable loss of efficiency that comes with conversion.
Besides lower average power, the potential drawback to this type of conversion is that the converted electricity comes in intermittent pulses. One interesting application of this conversion method takes advantage of this limitation: a simple AC light bulb dimmer. The light can stay illuminated but appears dimmer to human eyes because of the short gaps in electricity flow.
How Does a Full-Wave Bridge Rectifier Work?
A single diode can transform AC power into an intermittent DC flow, but a bridge rectifier uses four diodes to reverse the direction of both sides of the AC pulse. With a bridge rectifier, the DC still oscillates from zero to a peak value, but it doesn’t cut out half the time. This method feeds twice the power to a DC output as half-wave, which works out to a theoretical 81.2 percent power conversion ratio (lower in the real world, again).
This kind of rectified power also filters more easily to give an acceptably clean DC output. The non-instantaneous gaps typical of half-wave rectified DC don’t exist, even if the output varies from zero to a maximum in a sinusoidal pattern.
You can see how the full-wave bridge rectifier operates in the circuit diagram above:
- Either side of the AC line connects to a node between the non-conductive side of one diode and the conductive side of another.
- Each diode allows current to pass to the positive side of the load when that AC line is in a positive state.
- The negative side of the load connects to the opposite node in the 4-diode chain, allowing current to pass on either end when in the correct phase, resulting in a properly rectified current flow.
Center-Tapped Transformer vs Bridge Rectifier
A third rectifier circuit variation uses only two diodes, but it produces a fully rectified DC signal using a center-tapped transformer. We’ve illustrated the conversion process in the above image:
- Both transformer outputs feed through a diode that allows positive current only to pass through to the positive side of the load.
- The DC load’s negative side connects to the center tap of the transformer’s secondary winding, forming a zero-voltage reference.
- As a result, each side of the AC wave outputs as positive DC.
- You can reverse the diodes (or two more added in parallel) to form a negative version of this wave as well if needed.
As with bridge rectifiers, DC still oscillates in a sinusoidal pattern and requires filtering in most cases. One disadvantage of this type of setup is that you’ll need to procure a transformer for this conversion. The diode setup we used in the other two methods may be a much more cost-effective solution.
Understanding 3-Phase Power and Beyond
In some situations, you may need to convert 3-phase power (or more) into a DC circuit. The good news is that it’s easy to expand a bridge rectifier’s functionality by adding more diodes. Increasing your number of diodes will direct even more pulsing power to the positive and negative inputs on a load. A 3-phase source will need six diodes, while a 6-phase current source will need 12. One advantage of a multi-phase source is that the AC phases overlap, resulting in a comparatively smooth DC output.
Other AC to DC Conversion Methods
People have been rectifying current from AC into DC long before diodes made from semiconducting materials entered the market. Here are a few fascinating methods of yesteryear:
- Mercury-arc rectifiers used a gas-filled tube to resolve electrical power into DC.
- An AC motor drove a DC generator, resolving power electromechanically.
- One early practice used a switched-mode power supply, a method that we still use today.
When you’re specifying components, knowing what’s available can be much of the struggle. If you need inspiration for a unique application, turning to historical makers and inventors can help you find creative solutions. It might even keep you from having to re-invent the AC-to-DC flywheel.