This article will help you understand the basics of wave rectifier circuits.
While dealing with electronics we will come across two types of currents; AC(alternative Current) and DC (Direct Current). AC is mostly used for bigger electric circuits, such as industrial machines, while DC is far more useful when it comes to the smaller, household devices. For that, we need to convert AC into DC.
Wave rectification is a method used in electronic circuits to convert AC into DC. The electric circuits designed to convert AC into DC are called rectifiers. You can think of your phone charger as an example to a rectifier.
There are two types of rectifier circuits; half wave rectifier and full wave rectifier.
1. Half Wave Rectifiers
Think of a full sine wave. Half of it will be on the positive side of the coordinate system while the other half is on the negative side, right? Well, a rectifier will either cut off the negative half or cut off the positive half while turning the negative half upside down in order to create a rectified output.
Notice: When working with half ware rectifiers we convert the signal of a sine wave. A sinusoidal voltage source is shown as below, in order to resemble the sinus wave.
A sinusoidal voltage source will have a positive end and a negative end just like with any voltage source. However, unlike the DC voltage sources, the current will flow from the positive end to the negative end half the time and do the exact opposite half the time. So let’s say that the time it takes for a full wave to form (or to complete a cycle) with a sinusoidal voltage source is T. The current will flow through the circuit clockwise during T/2, and counter-clockwise during the rest of the time T/2. This is called an alternative current. In this context, in order to convert the alternating current into a direct current, we will simply need to place a diode within the circuit that uses this kind of a voltage source.
The diode you place can either be an ideal diode or a real one. If you are working with an ideal diode, considering there are no voltage consuming components in between the voltage source and the diode, there will be two outcomes. When the current flows in the same direction as the diode works (T/2 of the time), the output voltage Vout will be equal to the input voltage that the sinusoidal voltage source feeds the circuit with, Vin. When the current flows in the opposite direction of the diode (T/2 of the time), since the circuit will work like an open circuit, the output voltage Vout will be 0V(zero volt). Now if you draw the output voltage graph and compare it to the original sinusoidal wave graph, you will see the diagram below:
where the output voltage graph won’t have any negative half cycles but instead, zero values. Now that makes the difference between half wave rectifiers and full wave rectifiers: keep in mind that a full wave rectifier won’t simply isolate the negative half cycle waves but instead, turn them upside down. More about this in the following sections of this article.
You will find that in real life, there are no ideal diodes that can allow the input voltage to be equal to the output voltage. Real diodes will have a threshold voltage, and in order for the current to flow through the diode even if it’s the same direction, the voltage feeding the circuit will have to be greater than that threshold. And when it is greater than the threshold, the diode will consume some of the input voltage. If we name the threshold value of the diode Vd, Vout will be equal to Vin- Vd; therefore Vin will be greater than Vout.
In this case, the output voltage will form only when the input voltage feed exceeds the threshold voltage Vd. In the sinus wave, there are periods where the input voltage goes from 0 to up to the Vd and goes down from Vd to 0. During these periods of time, there will be no output voltage and therefore when drawing the output voltage graph, make sure to mark these periods as 0V. And since the Vout will at max be equal to Vin-Vd, Vmax in the output voltage graph should be marked equal to not Vin as we did with an idea diode, but equal to Vin-Vd as shown below.
Use the formula below to calculate the average DC voltage (Vdc) for ideal and non ideal diodes within half wave rectifiers:
- Ideal diodes: Vdc=0.318*Vmax
- Non-ideal diodes: Vdc=0.318*(Vmax-Vd)
2. Full Wave Rectifiers
A full wave rectifier will fill the 0V parts you will come across when working with a half wave rectifier and grant you a more direct, more stable, clean current. There are two types of a full wave rectifier: a bridge rectifier and a center tapped transformer.
a) Bridge Rectifier
A bridge rectifier is made with four diodes, and sometimes, a resistance between each couple.
Let’s work on the example above. The given bridge rectifier is fed from two paths above and below, and since the voltage source will give an alternating current, the current will flow through both paths periodically. Now since this is a working circuit, if the current enters from the path above, you will have to make sure that it leaves from the path below and vice versa. Therefore the current can not go back to the path it used to enter the circuit. Then, at every node the current encounters, it will flow through the most appropriate path. If the diode doesn’t allow the flow, for example, the current will use one of the other paths that allows it to flow until it finally leaves the circuit.
If the diodes are not ideal, the exact same thing will happen except there will a a loss of Vd each time the current flows through a diode.
b) Center Tapped Transformer
The sinusoidal voltage source will give two signals, T/2 of the time from the positive end and T/2 of the time from the negative end as usual. This type of rectifiers will work with a transistor and two diodes. When the current flows from positive end to negative end of the source during the first half of the time, it will enter the transformer through the positive end of the transformer to complete a mesh as the diodes allow. In the opposite direction during the second half of the time, the exact opposite will happen.
In both cases of Full Wave Rectification, since none of the diodes will absorb the voltage at any given time but only turn it upside down, the output voltage Vout graph will be as shown below:
Use the formula below to calculate the average DC voltage (Vdc) within full wave rectifiers:
Notice: The coefficent is twice the coefficient we used with the half wave rectifiers, since now we have twice the wave.