What is a Half Wave Rectifier?
A rectifier is a device that converts alternating current (AC) to direct current (DC). It is done by using a diode or a group of diodes. Half wave rectifiers use one diode, while a full wave rectifier uses multiple diodes.
The working of a half wave rectifier takes advantage of the fact that diodes only allow current to flow in one direction.
Half Wave Rectifier Theory
A half wave rectifier is the simplest form of rectifier available. We will look at a complete half wave rectifier circuit later – but let’s first understand exactly what this type of rectifier is doing.
The diagram below illustrates the basic principle of a half-wave rectifier. When a standard AC waveform is passed through a half-wave rectifier, only half of the AC waveform remains. Half-wave rectifiers only allow one half-cycle (positive or negative half-cycle) of the AC voltage through and will block the other half-cycle on the DC side, as seen below.
Only one diode is required to construct a half-wave rectifier. In essence, this is all that the half-wave rectifier is doing.
Since DC systems are designed to have current flowing in a single direction (and constant voltage – which we’ll describe later), putting an AC waveform with positive and negative cycles through a DC device can have destructive (and dangerous) consequences. So we use half-wave rectifiers to convert the AC input power into DC output power.
But the diode is only part of it – a complete half-wave rectifier circuit consists of 3 main parts:
- A transformer
- A resistive load
- A diode
A half wave rectifier circuit diagram looks like this:
We’ll now go through the process of how a half-wave rectifier converts an AC voltage to a DC output.
First, a high AC voltage is applied to the to the primary side of the step-down transformer and we will get a low voltage at the secondary winding which will be applied to the diode.
During the positive half-cycle of the AC voltage, the diode becomes forward biased, allowing current to flow through. Conversely, in the negative half-cycle, it is reverse biased, which blocks the current. The resulting DC output waveform from this process is displayed in figure 3.
This can be confusing on first glance – so let’s dig into the theory of this a bit more.
We’ll focus on the secondary side of the circuit. If we replace the secondary transformer coils with a source voltage, we can simplify the circuit diagram of the half-wave rectifier as:
Now we don’t have the transformer part of the circuit distracting us.
For the positive half cycle of the AC source voltage, the equivalent circuit effectively becomes:
This is because the diode is forward biased, and is hence allowing current to pass through. So we have a closed circuit.
But for the negative half cycle of the AC source voltage, the equivalent circuit becomes:
Because the diode is now in reverse bias mode, no current is able to pass through it. As such, we now have an open circuit. Since current can not flow through to the load during this time, the output voltage is equal to zero.
This process occurs rapidly, as the AC waveform oscillates between positive and negative multiple times per second, influenced by the frequency.
Here’s what the half wave rectifier waveform looks like on the input side (Vin), and what it looks like on the output side (Vout) after rectification (i.e. conversion from AC to DC):
The graph above displays a positive half wave rectifier, which only permits the positive half-cycles to pass through the diode while blocking the negative ones.
The voltage waveform before and after a positive half wave rectifier is shown in figure 4 below.
Conversely, a negative half-wave rectifier will only allow negative half-cycles through the diode and will block the positive half-cycle. The only difference between a posive and negative half wave rectifier is the direction of the diode.
As you can see in figure 5 below, the diode is now in the opposite direction. Hence the diode will now be forward biased only when the AC waveform is in its negative half cycle.
