What is a Half Wave Rectifier : Circuit & Its Characteristics

In the period 1880’s itself, the identification and uniqueness of rectifiers were started. The advancement of rectifiers has invented various approaches in the domain of power electronics. The initial diode which was employed in the rectifier was designed in the year 1883. With the evolution of vacuum diodes which was pioneered in the initial days of the 1900s, there happened limitations to rectifiers. Whereas with the modifications in mercury arc tubes, the usage of rectifiers was extended to various megawatt ranges. And the one type of rectifier is Half wave rectifier.

An enhancement in the vacuum diodes showed evolution for mercury arc tubes and these mercury arc tubes were termed as rectifier tubes. With the development of rectifiers, many other materials were pioneered. So, this is a brief explanation of how rectifiers were evolved and how they developed. Let us have a clear and detailed explanation of knowing what is a half wave rectifier, its circuit, working principle, and characteristics.

What is Half Wave Rectifier?

A rectifier is an electronic device that converts AC voltage into DC voltage. In other words, it converts alternating current to direct current. A rectifier is used in almost all electronic devices. Mostly it is used to convert the mains voltage into DC voltage in the power supply section. By using DC voltage supply electronic devices work. According to the period of conduction, rectifiers are classified into two categories: Half Wave Rectifier and Full Wave Rectifier

Construction

When compared with a full-wave rectifier, an HWR is the easiest rectifier for construction. Only with a single diode, the construction of the device can be done.

A half-wave rectifier consists of the below components:

• Alternating current source
• The resistor at the load section
• A diode
• A step-down transformer

AC Source

This current source supplies alternating current to the entire circuit. This AC current is generally represented as a sine signal.

Step-Down Transformer

In order to increase or decrease the AC voltage, a transformer is usually employed. As a step-down transformer is used here, it decreases the AC voltage while when a step-up transformer is used, it enhances the AC voltage from a minimal level to a high level. In an HWR, a mostly step-down transformer is employed where because the required voltage for a diode is very minimal. When a transformer is not used, then a large amount of AC voltage will cause damage to the diode. Whereas in a few situations, a step-up transformer can also be used.

In the step-down device, the secondary winding has minimal turns than that of the primary winding. Because of this, a step-down transformer decreases the voltage level from primary to the secondary winding.

Diode

Using diode in a half-wave rectifier allows the flow of current only in one direction whereas it stops the current flow in another path.

Resistor

This is the device that blocks the electric current flow only to a specified level.

This is the construction of half wave rectifier.

Working of Half Wave Rectifier

During the positive half cycle, the diode is under forwarding bias condition and it conducts current to RL (Load resistance). A voltage is developed across the load, which is the same as the input AC signal of the positive half cycle.

Alternatively, during the negative half cycle, the diode is under reverse bias condition and there is no current flow through the diode. Only the AC input voltage appears across the load and it is the net result which is possible during the positive half cycle. The output voltage pulsates the DC voltage.

Rectifier Circuits

Single-phase circuits or multi-phase circuit comes under the rectifier circuits. For domestic applications single-phase low power rectifier circuits are used and industrial HVDC applications require three-phase rectification. The most important application of a PN junction diode is rectification and it is the process of converting AC to DC.

Half-Wave Rectification

In a single-phase half-wave rectifier, either the negative or positive half of the AC voltage flows, while the other half of the AC voltage is blocked. Hence the output receives only one half of the AC wave. A single diode is required for a single-phase half-wave rectification and three diodes for a three-phase supply. Half wave rectifier produces more amount of ripple content than full-wave rectifiers and to eliminate the harmonics it requires much more filtering.

For a sinusoidal input voltage, the no-load output DC voltage for an ideal half-wave rectifier is

Vrms = Vpeak / 2

Vdc = Vpeak / ᴨ

Where

• Vdc, Vav – DC output voltage or average output voltage
• Vpeak – peak value of input phase voltage
• Vrms – the output voltage of root mean square value

Operation of Half-Wave Rectifier

PN junction diode conducts only during the forward bias condition. Half wave rectifier uses the same principle as PN junction diode and thus converts AC to DC. In a half-wave rectifier circuit, the load resistance is connected in series with the PN junction diode. Alternating current is the input of the half-wave rectifier. A step-down transformer takes an input voltage and the resulting output of the transformer is given to the load resistor and to the diode.

The operation of HWR is explained in two phases which are

• Positive half-wave process
• Negative half-wave process

Positive Half-Wave

When a frequency of 60 Hz as input AC voltage, a step-down transformer decreases this into minimal voltage. So, a minimum voltage is generated at the transformer’s secondary winding. This voltage at the secondary winding termed as secondary voltage (Vs). The minimal voltage is fed as the input voltage to the diode.

When the input voltage reaches to diode, at the time of the positive half cycle, the diode moves into forwarding bias condition and permit the flow of electric current, whereas, at the time of the negative half cycle, the diode moves into negative bias condition and obstructs the flow of electric current. The positive side of the input signal that is applied to the diode is the same as the forward DC voltage which is applied to the P-N diode. In the same way, the negative side of the input signal that is applied to the diode is the same as the reverse DC voltage which is applied to the P-N diode

So, it was known that diode conducts current in forwarding biased condition and obstructs the flow of current in reverse-biased condition. In the same way, in an AC circuit, the diode allows the flow of current for the duration of the +ve cycle and blocks the current flow at the time of the -ve cycle. Coming to +ve HWR, it will not entirely obstruct the -ve half-cycles, it allows few segments of -ve half-cycles or allows minimal negative current. This is the current generation because of minority charge carriers that are in the diode.

The generation of current through this minority charge carriers is very minimal and so it can be neglected. This minimal portion of -ve half cycles is not able to observe at the load section. In a practical diode, it is considered that the negative current is ‘0’.

The resistor at the load section utilizes the DC current which is produced by the diode. So, the resistor is termed as an electrical load resistor where the DC voltage/current is calculated across this resistor (R_L). The electrical output is considered as the circuit’s electrical factor which utilizes electric current. In an HWR, the resistor makes use of diode produced current. Because of this, the resistor is called a load resistor. The R_L in HWR’s is used for the restriction or limitation of additional DC current generated by the diode.

So, it was concluded that the output signal in a half wave rectifier is a continuous +ve half-cycles which are sinusoidal in form.

Negative Half-Wave

The operation and construction of half-wave rectifier in a negative way is almost identical to the positive half wave rectifier. The only scenario that will be changed here is the diode direction.

When a frequency of 60 Hz as input AC voltage, a step-down transformer decreases this into minimal voltage. So, a minimal voltage is generated at the transformer’s secondary winding. This voltage at the secondary winding termed as secondary voltage (Vs). The minimal voltage is fed as an input voltage to the diode.

When the input voltage reaches to diode, at the time of the negative half cycle, the diode moves into forwarding bias condition and permit the flow of electric current, whereas, at the time of the positive half cycle, the diode moves into negative bias condition and obstructs the flow of electric current. The negative side of the input signal that is applied to the diode is the same as the forward DC voltage which is applied to the P-N diode. In the same way, the positive side of the input signal that is applied to the diode is the same as the reverse DC voltage which is applied to the P-N diode

So, it was known that diode conducts current in reverse biased condition and obstructs the flow of current in forward-biased condition. In the same way, in an AC circuit, the diode allows the flow of current for the duration of -ve cycle and blocks the current flow at the time of the +ve cycle. Coming to -ve HWR, it will not entirely obstruct the +ve half-cycles, it allows few segments of +ve half-cycles or allows minimal positive current. This is the current generation because of minority charge carriers that are in the diode.

The generation of current through this minority charge carriers is very minimal and so it can be neglected. This minimal portion of +ve half cycles is not able to observe at the load section. In a practical diode, it is considered that a positive current is ‘0’.

The resistor at the load section utilizes the DC current which is produced by the diode. So, the resistor is termed as an electrical load resistor where the DC voltage/current is calculated across this resistor (R_L). The electrical output is considered as the circuit’s electrical factor which utilizes electric current. In an HWR, the resistor makes use of diode produced current. Because of this, the resistor is called a load resistor. The R_L in HWR’s is used for the restriction or limitation of additional DC current generated by the diode.

In an ideal diode, the +ve and -ve half-cycles at the output section appears to be similar to +ve and -ve half cycle But in practical scenarios, the +ve and -ve half cycles are somewhat different from the input cycles and this is negligible.

So, it was concluded that the output signal in a half-wave rectifier is a continuous -ve half-cycles which are sinusoidal in form. So, the output of the half-wave rectifier is continuous +ve and -ve sine signals, but not pure DC signal and in pulsating form.

This pulsating DC value gets changed through a short time period.

Working of a Half-Wave Rectifier

During the positive half cycle, when the secondary winding of the upper end is positive with respect to the lower end, the diode is under forwarding bias condition and it conducts current. During the positive half-cycles, the input voltage is applied directly to the load resistance when the forward resistance of the diode is assumed to be zero. The waveforms of output voltage and output current are the same as that of the AC input voltage.

During the negative half-cycle, when the secondary winding of the lower end is positive with respect to the upper end, the diode is under reverse bias condition and it does not conduct current. During the negative half-cycle, the voltage and current across the load remain zero. The magnitude of the reverse current is very small and it is neglected. So, no power is delivered during the negative half cycle.

A series of positive half cycles is the output voltage that is developed across the load resistance. The output is a pulsating DC wave and to make the smooth output wave filters, which should be across the load, are used. If the input wave is of half-cycle, then it is known as a half-wave rectifier.

Three Phase Half-wave Rectifier Circuits

Three-phase half wave uncontrolled rectifier requires three diodes, each connected to a phase. The three-phase rectifier circuit suffers from a high amount of harmonic distortion on both DC and AC connections. There are three distinct pulses per cycle on the DC side output voltage.

A three phase HWR is mainly utilized for converting three phase AC power into three phase DC power. In this, in the place of diodes, switched are used which are called uncontrolled switches. Here, uncontrolled switches correspond that there exists no approach of regulating ON and OFF times of the switches. This device is constructed using a three-phase power supply which is connected to a 3-phase transformer where the transformer’s secondary winding has always star connection.

Here, only star connection is followed due to the reason that a neutral point is necessary to have the connection of load again to the secondary winding of the transformer, thus offering a return direction for the power flow.

The general construction of 3-phase HWR providing a purely resistive load is shown in the below picture. In the construction design, the transformer’s each phase is termed as an individual AC source.

The efficiency gained through a three phase transformer is nearly 96.8%. Though the efficiency of three phases HWR is more than a single phase HWR, it is less than the performance of three phases full-wave rectifier.

Half-wave Rectifier Characteristics

The characteristics of a half-wave rectifier for the following parameters

PIV (Peak Inverse Voltage)

During the reverse biased condition, the diode has to withstand because of its maximum voltage. During the negative half-cycle, no current flows through the load. So, an entire voltage appears across the diode because there is a no-voltage drop through load resistance.

PIV of a half-wave rectifier = V_SMAX

This is the PIV of half wave rectifier.

Average and Peak Currents in the Diode

Assuming, the voltage across the secondary of the transformer be sinusoidal and its peak value is V_SMAX. The instantaneous voltage which is given to the half wave rectifier is

Vs = V_SMAXSin wt

The current flowing through the load resistance is

I_MAX = V_SMAX / (R_F+R_L)

Regulation

Regulation is the difference between no-load voltage to full-load voltage with respect to the full-load voltage, and the percentage voltage regulation is given as

%Regulation = {(Vno-load – Vfull-load) / Vfull-load} *100

Efficiency

The ratio of input AC to output DC is known as efficiency (?).

?= Pdc / Pac

A DC power that is delivered to the load is

Pdc = I²_dc R_L = (I_MAX/ᴨ)² R_L

The input AC power to the transformer,

Pac=Power dissipation in load resistance + power dissipation in the junction diode

= I²_rmsR_F + I²_rmsR_L = {I²_MAX/4} [R_F + R_L]

?= Pdc/Pac = 0.406/{1+R_F/R_L}

The efficiency of a half wave rectifier is 40.6% when R_F is neglected.

Ripple Factor (γ)

Ripple content is defined as the amount of AC content present in the output DC. If the ripple factor is less, the rectifier performance will be more. The ripple factor value is 1.21 for a half wave rectifier.

The DC power generated by the HWR is not an exact DC signal, but a pulsating DC signal, and in the pulsating DC form, there exist ripples. These ripples can be decreased by using filter devices like inductors and capacitors.

To calculate the number of ripples in the DC signal, a factor is used and is called a ripple factor which is represented as γ. When the ripple factor is high, it shows an extended pulsating DC wave whereas a minimal ripple factor shows a minimal pulsating DC wave,

When the value of γ is very minimal it represents that the output DC current is almost the same as a pure DC signal. So, it can be stated that the lower the ripple factor, the smoother the DC signal is.

In a mathematical form, this ripple factor is denoted as the proportion of RMS value of the AC section to the DC section of the output voltage.

Ripple factor = RMS value of the AC section/ RMS value of the DC section

I² = I²_dc + I²₁ + I²₂+ I²₄ = I²_dc+ I²_ac

γ = I_ac / I_dc = (I² – I²_dc) / I_dc = {( I_rms / I²_dc) / Idc = {(I_rms /I²_dc)-1} = k_f²-1)

Where kf – form factor

kf= Irms / Iavg = (Imax/2)/ (Imax/ᴨ) =ᴨ/2 = 1.57

So, γ = (1.572 – 1) = 1.21

Transformer Utilization Factor (TUF)

It is defined as the ratio of AC power delivered to the load and transformer secondary AC rating. The TUF of half wave rectifier is about 0.287.

HWR with Capacitor Filter

As per the general theory which was discussed above for the output of a half-wave rectifier is a pulsating DC signal. This is obtained output when an HWR is operated without implementing a filter. Filters are the device that is employed to transform pulsating DC signal into steady DC signals which means (conversion of the pulsating signal into smooth signal). This can be achieved by suppressing direct current ripples that happen in the signal.

Even though these devices can be theoretically used having no filters, but they are supposed to be implemented for any practical applications. As the DC apparatus will need a steady signal, the pulsating signal has to be converted into a smooth one in order to be used for real applications. This is the reason HWR is used with a filter in practical scenarios. In the place of a filter, either an inductor or capacitor can be used, but HWR with a capacitor is the most generally used device.

The below picture explains the circuit diagram of the construction of half wave rectifier with capacitor filter and how it smoothens the pulsating DC signal.

Advantages and Disadvantages

When compared with full wave rectifier, a half wave rectifier is not that much employed in the applications. Even though there are few benefits to this device. The advantages of half wave rectifier are:

• Cheap – Because a minimal number of components are used
• Simple – Due to the reason that the circuit’s design is completely straightforward
• Easy to use – As the construction is easy, the device utilization will also be so streamlined
• A low number of components

The disadvantages of half wave rectifier are:

• At the load section, the output power is included with both the DC and AC components where the basic frequency level is similar to the frequency level of the input voltage. Also, there will be an increased ripple factor which means that the noise will be high, and extended filtering is needed to provide constant DC output.
• As because there will be power delivery only at the time of one half-cycle of the input AC voltage, their rectification performance is minimal, and also the output power will be less.
• Half wave rectifier has minimal transformer utilization factor
• At the transformer core, there happens DC saturation where this results in magnetizing current, hysteresis losses, and also the development of harmonics.
• The amount of DC power which got delivered from a half wave rectifier is not adequate to generate even a general amount of power supply. Whereas this can be utilized for a few applications such as battery charging.

Applications

The main application of half-wave rectifier is to gain AC power from DC power. Rectifiers are mainly employed internal circuits of the power supplies in almost every electronic device. In power supplies, the rectifier is generally located in a series way thus consisting of the transformer, a smoothing filter, and a voltage regulator. Few of the other applications of HWR are:

• Implementing a rectifier in the power supply allows for the conversion of AC to DC. Bridge rectifiers are extensively utilized for huge applications, where they hold the ability to convert high-level AC voltage to minimal DC voltage.
• The implementation of HWR assists to gain the required level of DC voltage through step-down or step-up transformers.
• This device is also used in welding iron types of circuits and is also utilized in mosquito repellent so as to push the lead for the vapors.
• Used on AM radio device for the detection purposes
• Used as firing and pulse generation circuits
• Implemented in voltage amplifier and modulation devices.

This is all about the Half Wave rectifier circuit and working with its characteristics. We believe that the information given in this article is helpful for you for a better understanding of this project. Furthermore, for any queries regarding this article or any help in implementing electrical and electronics projects, you can feel free to approach us by commenting in the comment section below. Here is a question for you, what is the main function of half wave rectifier?