Rectifiers are circuits that turn an alternating current (AC) into a direct current (DC). Rectifiers are one of the most useful applications of diodes, and are incredibly useful in the field of electronics because most electronic devices use DC, but the power grid (mains electricity) supplies AC.
The three most common types of rectifiers are half-wave, full-wave, and bridge rectifiers. Each of these offers advantages and disadvantages. Bridge rectifiers tend to be the most commonly used because they combine the best features of full-wave rectifiers at only a slightly higher cost and larger size than half-wave rectifiers.
Polyphase rectifiers are used for multiple phases, such as the three phase power produced by an AC generator.
Rectifier Comparison Chart
The following table provides a comparison of each type of rectifier.
|Type||Number of Diodes||Transformer Type||Output|
|Full-Wave Rectifier||2||Center Tapped||Full-wave|
Rectifier Technical Comparison Chart
|Parameter||Half-Wave Rectifier||Full-Wave Rectifier||Bridge Rectifier|
|Number of Diodes||1||2||4|
|DC Current (IDC)||Im/π||2Im/π||2Im/π|
|Ripple Factor (γ)||1.21||0.483||0.483|
|Transformer Utilization Factor (TUF)||0.286||0.57||0.81|
|Peak Inverse Voltage (PIV)||Vm||2Vm||Vm|
Types of Rectifiers
Half-wave rectifiers are the simplest type of rectifier, and are the perfect starting point for learning about rectifiers and other diode circuits.
A half-wave rectifier is a circuit that allows only one half of an alternating current (AC) waveform to pass, turning an AC signal into a pulsed direct current (DC) signal with large time gaps between the pulses.
Half-wave rectifiers use only one single diode, and are the simplest way to convert AC into DC.
However, we can immediately see that half-wave rectifiers present some significant limitations in terms of both efficiency and output quality. By using only half of the input wave, half-wave rectifiers are very inefficient, with major losses that can’t be reduced without choosing a different type of rectifier.
The output of the half-wave rectifier is also of very low quality because the pulses are so far apart.
Full-wave rectifiers use two diodes and a center-tapped transformer to convert an entire AC waveform into a series of continuous DC pulses.
This has the advantage that the rectifier is twice as efficient as a half-wave rectifier, and also produces a much higher quality output waveform.
The main disadvantage of full-wave rectifiers is the requirement of using a center-tapped transformer. In order to achieve the same output voltage of a half-wave rectifier, a center-tapped transformer must have twice the number of secondary windings as a standard transformer. Large and expensive, it significantly adds to the total cost of the full-wave rectifier.
Bridge rectifiers are the most common type of rectifier because they output a full wave while only requiring a standard transformer.
The only drawback is that they use four diodes instead of one or two. Given the extremely low cost of diodes, this makes them the choice for nearly all applications.
Bridge rectifiers are slightly more complex than either half-wave or full-wave, and make use of an ingenious construction that uses the four diodes to ensure the current passes through the load in the same direction.
Polyphase rectifiers are used in industrial applications where more than one phase must be rectified.
They are used most commonly in high power circuits, where the phases can be combined to form a much smoother output, thereby increasing efficiency and reducing the need for large and expensive filter circuits.
Is The Output of a Rectifier AC or DC?
The answer to the question ‘is the output of a rectifier AC or DC?’ is yes. The best way to think about rectifier output is that it has both AC and DC components.
In fact, the total output current can be described mathematically as being the sum of the DC and AC components. This can be expressed through the relation:
I (output current) = IDC (DC component) + I’ (AC Component)
The output current is the pulsating waveform, which is expressed as the absolute value of a sine wave.
The DC component is expressed as the average value of the output current.
The AC component can then be calculated by finding the difference between the output current and DC component:
I’ (AC component) = I (output current) – IDC (DC component)
This AC component is often expressed with regards to the ripple factor.
The ripple factor is the ratio of the root mean square (RMS) of the AC component to the average output value.
It is essentially an indication of how ‘rippled’ the output is. Since an ideal DC output would be flat, a lower ripple factor is usually better and indicates a higher quality, more-ideal DC output signal.
We’ll learn how to calculate the ripple factor of each rectifier we encounter but the most important thing to know that a higher ripple factor means that there is a larger AC component within the output DC waveform. We want the ripple factor to be as low as possible.
Efficiency so important in the field of electronics that a large part of product development is usually spent trying to increase the efficiency and reduce power consumption as much as possible.
Nowhere is this more true than in power conversion, where small gains in efficiency have enormous impacts on the total efficiency of the system as a whole.
This is why full-wave rectifiers are chosen over half-wave rectifiers, and why bridge rectifiers are chosen above the other two. The added cost of the extra diodes is so small compared with the cost of inefficiency that choosing a lower efficiency design is never a real option.
The output of any rectifier, including a bridge rectifier, is still far from the ideal flat DC waveform.
Filter circuits are used to improve the output quality.
A simple, common, and relatively effective filter is a simple capacitor filter. A capacitor in parallel with the load will charge while the voltage is increasing and then discharge while the voltage is decreasing, supplying voltage to the load so that it experiences a much cleaner output.