The process of conversion of ac electrical power into dc power is called Rectification. The essential component of a rectifier circuit is semiconductor devices like a diode, thyristor, etc. These semiconductor devices will be used as switches for the conversion of ac power to dc power.

Generally, the output of a rectifier circuit is not pure dc instead it will be a pulsating dc. Thus in order to obtain pure dc, filters are used at the output side of the rectifier. Depending upon the switching devices used in ac to dc power conversion, rectifier circuits are divided into two types,

• Uncontrolled Rectifiers
• Controlled Rectifiers.

What is an Uncontrolled Rectifier?

An uncontrolled rectifier is a type of ac to dc converter whose output voltage is fixed i.e., the output voltage is constant and cannot be varied. Generally, semiconductor diodes are used in uncontrolled rectifiers that convert alternating current into direct current.

Since there is no gate terminal for a diode, the conduction and commutation of the diode depend upon the circuit condition. Whenever the diode comes into the forward-biased condition, it conducts. When the diode gets reverse-biased, commutation takes place. The output voltage and load current in an uncontrolled rectifier depend upon the input ac voltage and load (like R, RL, RC, RLC, etc.).

Types of Uncontrolled Rectifiers :

Uncontrolled rectifiers are divided into single-phase and three-phase. The below shows the classification of uncontrolled rectifiers or converters.

• Single-phase Uncontrolled rectifier
  • Half-wave uncontrolled rectifier
  • Full-wave uncontrolled rectifier
    • Center tapped rectifier
    • Bridge rectifier
• Three-phase Uncontrolled rectifier
  • Half-wave uncontrolled rectifier
  • Full-wave uncontrolled rectifier
    • Center tapped rectifier
    • Bridge rectifier
  • Double-star uncontrolled rectifier

Single-Phase Half-Wave Uncontrolled Rectifier :

A single-phase half-wave uncontrolled rectifier is the simplest-type ac to dc converter. The below shows the circuit diagram of a single-phase half-wave uncontrolled rectifier with resistive load. The circuit uses only one diode which rectifies only half cycle of ac input i.e., the diode only converts half-cycle of the ac input into dc.

During the positive half-cycle of ac input, terminal A is positive with respect to B. Thus the diode gets forward biased and therefore load current starts flowing in the circuit. Now once the negative half-cycle starts, terminal B becomes positive with respect to A. Due to this diode gets reverse-biased (non-conducting state) and thus no current flows and no voltage is developed across the load.

Therefore, only half cycle of ac input will reach the load in the form of dc that is why the converter is said to be called a half-wave rectifier. The half cycle of the ac input converted into dc can be either positive or negative depending upon the connection of the diode in the circuit. Later in order to vary the voltage level transformer is used at the output side.

Single-Phase Full-Wave Uncontrolled Rectifier :

A single-phase full-wave uncontrolled rectifier uses more than one diode and converts both positive and negative half cycles of ac input into dc. There are two circuit configurations used for single-phase full-wave rectification, center-tapped, and bridge-type full-wave rectifiers.

In a center-tapped full-wave uncontrolled rectifier, a center-tap transformer is used which supplies ac power to the circuit. The circuit is made with the parallel combination of two half-wave rectifier circuits with the common load connected to the middle point of the transformer’s secondary winding as shown below.

In the above circuit, one half-wave circuit handles one half-cycle of the ac input i.e., the diode D₁ converts the positive half-cycle of ac into dc while the diode D₂ converts the negative half-cycle of ac into dc.

The circuit arrangement of a full-wave bridge rectifier is shown below. The circuit consists of four diodes connected in the form of a closed-loop bridge. Compared to a center-tapped rectifier, this type of circuit connection has the advantage of a reduction in the peak inverse voltage of each diode.

During the positive half-cycle of ac, point A is positive with respect to B. Due to these, diodes D₁ and D₂ get forward-biased and thus conduct load current. When the negative half-cyle of ac input starts, point B is made positive with respect to A. In this case, diodes D₃ and D₄ get forward-biased, and thus conduction takes place through D₃ and D₄.

Therefore, in each half-cycle, pair of diodes conduct that makes alternating current flow in one direction through the load, and hence an output voltage in the same direction is developed across the load for each half-cycle.

The above same classification of uncontrolled rectifiers can be seen in the three-phase system except that the number of pulses is three for a half-wave circuit and six for a full-wave circuit. Therefore we can see that the diode starts conducting when it is forward-biased, there is no control over the diode and dc output voltage. Hence the rectifier circuit is called an Uncontrolled rectifier.

Advantages of Uncontrolled Rectifier :

• In an uncontrolled rectifier, only diodes are used. Due to this cost of an uncontrolled rectifier circuit is less compared to SCR-based rectifiers.
• No triggering or firing circuit is needed for an uncontrolled rectifier.
• The circuit design of an uncontrolled rectifier is simple compared to a controlled rectifier.

Disadvantages of Uncontrolled Rectifier :

• Less control over the conduction angle compared to SCR used in controlled rectifiers.
• We cannot control the output voltage.
• In order to vary output voltage additional tap-changing transformer or autotransformer is required.