Controlled Rectifiers or SCR Rectifiers

 

Rectifiers are the electronic circuits or devices that convert AC voltage (sinusoidal waveform) into DC Voltage (unidirectional pulsating waveform). These are the most important circuits in electronics because DC supply is the fundamental need of almost every electronic circuit and device.


There are various types of rectifiers used in modern electronics, including half wave rectifiers, full wave rectifiers, bridge rectifiers and center tapped rectifiers. These rectifiers have different circuit configurations and operations. (discussed in separate articles)


In these rectifiers, we can obtain either the fixed DC output voltage from the fixed AC source or variable (desired) DC output DC voltage from the fixed AC source. Rectifier circuits that provide fixed DC output voltage from the fixed AC source are called uncontrolled rectifiers, whereas the rectifier circuit that provides variable DC output voltage from a fixed AC source is called a controlled rectifier.


In this article we will explore the important concepts of the controlled rectifiers such as what is a controlled rectifier? half wave-controlled rectifier, full wave-controlled rectifier, full wave bridge-controlled rectifier, full wave center tapped controlled rectifier. We will discuss their circuit diagrams, working, output waveforms and important formulas. Lastly, we conclude the discussion and understand how we can control the output of these rectifiers.




What is Controlled Rectifier?


Rectifier circuits that provide a variable DC output voltage from a fixed AC voltage supply are called controlled rectifiers. In this type of rectifier circuits, we have full control over the DC output voltage that's why they are often referred to as fully controlled rectifiers or full converters. By full control, we mean that we can obtain DC output voltage of either positive or negative polarities, although the direction of current flow remains the same throughout the operation. Therefore, these rectifiers can also work as inverters while charging the battery.


In this type of rectifier circuits, we employ only controlled switches, specifically those in which we can control its on-state or conduction state such as SCRs, MOSFETs etc. so that we can alter the pulsating DC output waveform by controlling the conduction of these switches.

 

To make it easier to understand, we will refer to the rectifier circuit using SCR and a resistive load.

 

So, before moving further it is recommended to first go through the working of SCR so that you can easily grasp the concept discussed in this article. 

 

Key points about SCR that are used throughout this article.

 

  • SCR stands for Silicon Controlled Rectifier. As its name implies, it is a controlled rectifier in which we can control its conduction by controlling the gate current. 

  • SCR only conducts in forward direction and blocks the conduction in reverse direction.

  • SCR only starts conduction after applying any triggering method (gate signal); otherwise, it remains in forward blocking mode.



Types of Controlled Rectifier


As we discussed in the types of rectifiers, rectifiers are classified as half wave rectifiers and full wave rectifiers. Further full wave rectifiers use two types of circuit configuration: bridge rectifier and center tapped rectifier. Similarly, controlled rectifiers are classified as half wave-controlled rectifiers and full wave-controlled rectifiers and full wave-controlled rectifiers are further classified as full wave bridge-controlled rectifiers and full wave center tapped controlled rectifiers.


Let’s discuss the working and circuit diagram of all the controlled rectifiers mentioned above separately.



Half Wave Controlled Rectifier


In a half wave-controlled rectifier circuit, only one SCR is used. The SCR is connected in such a way that it only conducts for the desired half of the AC input supply. The circuit diagram for a half wave-controlled rectifier using SCR is shown in the given figure.

Half Wave Controlled Rectifier



Working and Output Waveform


During the positive half of the AC signal, the anode (A) of the SCR is positive and the cathode (K) is negative. In this condition, SCR is in forward blocking mode and does not conduct current. However, after applying gate signal to SCR, it will transition to forward conduction mode from forward blocking mode and start conducting. In this condition, the output voltage follows the input voltage.

half wave controlled rectifier


During the negative half of the AC signal anode (A) of the SCR is negative and cathode (K) is positive. In this condition, SCR is in reverse blocking mode and does not conduct current. Therefore, the output voltage is zero.

half wave controlled rectifier


The output waveform of the half wave-controlled rectifier can be visualized with the help of the given figure.

half wave controlled rectifier


Full Wave Controlled Rectifier


Full wave rectifiers use two types of circuit configuration that are bridge rectifier circuit and center tapped rectifier circuit.



Full Controlled Bridge Rectifier


In a controlled bridge full wave rectifier circuit, four SCRs are used. These SCRs are connected in a bridge-like arrangement. The circuit diagram for bridge rectifiers using SCR is shown in figure.

Full Controlled Bridge Rectifier

Working and Output Waveform


During the positive half of the AC signal, SCR 1 and SCR 3 are forward biased and SCR 2 & SCR 4 are reverse biased. Initially, SCR 1 & SCR 3 are in forward blocking mode and do not conduct the current, but after applying the gate current they will immediately become in forward conduction mode from forward blocking mode and start conduction. In this condition the output voltage follows the input voltage.


Full Controlled Bridge Rectifier


Similarly, during the negative half of the AC signal, SCR 2 & SCR 4 are forward biased, and SCR 1 & and SCR 3 reverse biased. As SCR 2 & and SCR 4 are forward biased, once the gate signal is applied, they will conduct the current. In this condition, the voltage across the load is the negative of the input voltage.

full controlled bridge rectifier


The output waveform of the controlled bridge full wave rectifier is visualized with the help of the given figure.




Centre Tapped Controlled Rectifier


In the center tapped controlled rectifier, two SCRs and a transformer with center tapped secondary are used. In this rectifier, these two SCRs are connected to the both terminals (A & B) of the secondary of the transformer, and the other terminal of the SCRs are connected to load and the center tapping of the secondary of the transformer. The circuit diagram for full wave center tapped rectifier using SCRs is shown in figure.

Center Tapped Controlled Rectifier


Working and Output Waveform


During the positive half of the AC signal, the terminal A of the secondary is positive, and the terminal B of the secondary of the transformer is negative. Therefore, SCR 1 becomes forward biased and starts conduction immediately after applying the gate current and SCR 2 becomes reverse biased and it does not support conduction of current. 


Center Tapped Controlled Rectifier


During the negative half of the AC signal, the terminal B of the secondary is positive, and the terminal A of the secondary of the transformer is negative. Therefore, SCR 2 becomes forward biased and starts conduction immediately after applying the gate current, while SCR 1 becomes reverse biased and it does not support conduction of current. 

Center Tapped Controlled Rectifier

The output waveform of the full wave center tapped controlled rectifier can be visualized with the help of the given figure.

Center Tapped Controlled Rectifier


Important Formulas of Controlled Rectifier

 

Formula for calculating the average value of the output waveform of voltage and current.


For Half Wave Rectifier






For Full Wave Rectifier





Formula for calculating the RMS Value of the output Waveform of voltage and Current.

 

For Half Wave Rectifier






For Full Wave Rectifier








How to control the output voltage of a controlled rectifier?

We clearly see from the above equations that the average and the rms value of the output waveforms of the controlled rectifiers depend on the firing angle (α) of the SCR used in their respective circuits. So, with the help of these mathematical equations, we can say, by changing the firing angle of the SCR used in the rectifier circuit we can change their output values. 

Although, we also observe this in the working and output waveforms of rectifiers above, that α is the delay in the output waveform with respect to input. So, by altering the firing angle α, we alter the delay in output waveforms, consequently, the effective waveform of the output changes.

 

No comments:

Post a Comment

Please feel free to provide feedback and suggestions, and also don't hesitate to ask your questions.

Featured Post

Popular Feed