Half Wave rectifier | Properties | Frequency| Ripples

What is a Half Wave Rectifier?

what is half wave rectifier

A half-wave rectifier can be explained as a type of rectifier that only passes one half-cycle of an AC voltage waveform to pass, blocking the remaining half-cycle. Half-wave rectifiers are normally used to convert AC power to DC power, and only require one diode to construct.

A simple Half Wave Rectifier is nothing more than a single PN junction diode which is connected in series to the load resistor. As you know a diode is to electric current like a one-way valve is to water, it passes electric current to flow in the only uni-direction. This property of the diode is very useful in developing simple rectifiers which are normally used to convert AC power to DC power.

Here the alternating current is applied as input. Input voltage is provided to a step-down transformer and the resulting reduced voltage of the transformer is supply to the diode ‘D’ and load resistor RL. The output voltage is measured across the load resistor RL.

As part of our “Basic Electronics Tutorial” series, we have seen that rectification is the most important application of a PN junction diode. The process of rectification is transferring alternating current (AC) to direct current (DC).

Half wave rectifier definition

A half-wave rectifier is a simple type of rectifier which transfers the positive half cycle (positive current) of the input signal into pulsating DC (Direct Current) output signal.

or

A half-wave rectifier is a type of rectifier that passes only half cycle (either positive half cycle or negative half cycle) of the input AC signal while the other half cycle is blocked.

For example, if the positive half cycle of the power is allowed then the negative half cycle will be blocked. Similarly, if the negative half cycle is passed then the positive half cycle will be blocked. However, a half-wave rectifier can not pass both positive and negative half-cycles at the same time.

Therefore, the half-cycle (either positive or negative) of the input power supply is wasted.

Half Wave Rectifier Theory

A rectifier is the simplest and basic form of the rectifier available. We will observe at a complete half-wave rectifier circuit later – but let’s first understand exactly what this type of rectifier is doing.

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 main parts:

Components requirements

When designing a half-wave rectifier circuit, it is necessary to ensure that the diode is capable of providing the required performance. While there are very many parameters that define individual diodes, and these may need to be taken into account for a given design, some of the major parameters are detailed below:

AC source

The AC source supplies Alternating Current to the circuit. The alternating current is often represented by a sinusoidal waveform.

Transformer

A transformer is a device that reduces or increases the AC voltage. The step-down transformer reduces the AC voltage from high to low whereas the step-up transformer increases the AC voltage from low to high. In the half-wave rectifier, we generally use a step-down transformer because the voltage needed for the diode is very small. Applying a large AC voltage without using the transformer will permanently destroy the diode. So we use a step-down transformer in a half-wave rectifier. However, in some cases, we use a step-up transformer.

In the step-down transformer, the primary winding has more turns than the secondary winding. So the step-down transformer reduces the voltage from the primary winding to secondary winding.

Diode

A diode is a two-terminal device that allows electric current in one direction and blocks electric current in another direction.

Resistor

A resistor is an electronic component that restricts the current flow to a certain level.

•             Forward current:   It is necessary that the diode is able to handle the levels of average current and peak current flowing through it in a half-wave rectifier circuit. The current will peak as a result of the capacitor smoothing circuit. As the current only flows as the capacitor charge up, the current is in short bursts which are much higher than the average current.

•             Peak inverse voltage:   The diode must be able to reliably withstand the peak reverse or inverse voltages that appear across it. The peak voltages are not just the output voltage, but higher. The peak inverse voltage rating of the diode should be at least 2 x √2 times the RMS voltage of the input. This is because the output is normally smoothed by a capacitor, and this will take a value that is the peak of the input waveform. This will be √2 times the RMS voltage. With this voltage on the output, the input waveform on the “blocked” half of the cycle will fall and reach a peak value at the bottom of the crest of √2 times the RMS value. The maximum reverse value seen across the rectifier diode is the sum of these two voltages.

There should also be a significant margin, especially when used in a mains or line power supply. This is because voltage spikes can appear on the line.

•             Diode turn on voltage:   All diodes have a forward voltage drop needed to turn the diode on. This can be necessary for some applications. Typically a silicon diode requires is 0.7V and a germanium diode needed is 0.3V. A silicon Schottky diode is nearly about 0.2 to 0.3V. Reducing the forward voltage drop decreases power loss and in a few applications like signal detection, it makes the diode rectifier more sensitive.

•             Forward voltage drop:   Apart from the forward turn-on voltage, diodes also have a different level of resistance. As the current increase, so does the level of voltage reduces. Power diodes normally have a larger area for current conduction and therefore their voltage drop will be less at high current levels.

•             Diode capacitance:   When the diode is used in a half-wave rectifier as a signal detector, the capacitance may be a problem because the frequencies are present. Typically Schottky diodes have a very tiny junction capacitance.

Circuit precautions

When developing a half-wave rectifier circuit, it is a must to make sure there is a DC power return in the circuit. Often when using the diode rectifier for signal or peak detection it is easy to omit a DC power return. This must to be included either as a resistor or as part of a transformer or choke.

The rectifier circuit can be used to good effect. As a power rectifier, it only detects half of the waveform making smoothing an is a later problem. As a result, a full-wave system is usually used for power rectification. The half-wave rectifier is sometimes used for signal and peak detection.

Half  Wave Rectifier Operation

operation of half wave rectifer

Simply place a half-wave rectifier clips the negative half cycle of an AC input and passes only the positive cycles to creating a DC flow.

To understand the working of a half-wave rectifier exactly, you must know the theory section really well. If you are new to the concepts of a PN junction and its characteristics, I suggest you read the half-wave rectifier theory first.

The working of a half-wave rectifier is pretty easy. From the theory part, you should know that a PN junction diode conducts current only in one or single direction. In other words, a PN junction diode conducts current only when it is forward biased. The same phenomenon is made use of in a half-wave rectifier to transfer AC power to DC power. The input we supply here is alternating current. This input voltage is stepped down by a transformer. The reduced voltage is provided to the diode ‘D’ and load resistance RL. During the positive half cycles of the input wave signal, the diode ‘D’ will be forward biased and during the negative half cycles of the input wave signal, the diode ‘D’ will be reverse biased. We achieve the output across the load resistor RL. Since the diode flows current only during the one-half cycle of the input wave, we get an output as shown in the diagram. The output is positive and significant during the positive half cycles of the input supply. At the same time output is 0 or insignificant during negative half cycles of the input wave. This is known as half-wave rectification.

Explaining Half Wave Rectification in academic words!

When a one rectifier diode unit is placed in series with the load across an ac power supply, it converts the alternating voltage into a uni-directional pulsating voltage, using a one-half cycle of the applied voltage, the other half cycle being clipped due to it conducts only in a uni-direction. Unless there is an inductance or battery present in the circuit, the current will be 0, therefore, for half the time. This is known as half-wave rectification. As already told, a diode is an electronic component consisting of two elements known as cathode plate and anode plate. Since a diode current can pass in one direction only i.e. from the cathode to anode, the diode provides the unilateral conduction necessary for rectification. This is fact for diodes of all types-vacuum, gas-filled, crystal or semiconductor, metallic (copper oxide and selenium types) diodes. Semiconductor diodes, because of their inherent advantages are normally used as a rectifying device. However, for very huge voltages, vacuum diodes may be employed.

Working of a Half wave rectifier

The half-wave rectifier circuit using a semiconductor diode (D) with a load RL but no smoothing filter is provided in the figure. The diode is connected in series with the secondary side of the transformer and the load RL. The primary of the transformer is being connected to the ac main power supply.

WORKING of halfwave rectifer

The ac voltage across the secondary winding continuously changes its polarities after every half cycle of the input wave. During the positive half-cycles of the input ac voltage i.e. when the upper-end side of the secondary winding is positive w.r.t. its lower end side, the diode is forward biased and therefore conducts current at the time. If the forward resistance of the diode is assumed to be 0 (in practice, however, a small resistance exists) the input voltage during the positive half-cycles is directly supplied to the load resistance RL, making its upper-end side positive w.r.t. its lower end side. The waveforms of the output current and output voltage are of the exact same shape as that of the input ac voltage and current.

During the negative half cycles of the input ac voltage i.e. when the lower end side of the secondary winding is positive w.r.t. its upper-end side, the diode is now on reverse-biased mode and so does not conduct current. Thus during the negative half cycles of the input ac voltage, the current and the voltage across the load remains 0. The reverse current is very little in magnitude, is neglected. Thus for the negative half-cycles, no power is provided to the load.

Thus the output voltage (VL) shows across the load resistance RL  is a series of positive half cycles of alternating voltage, with intervening very little constant negative voltage levels, It is obvious from the figure that the output is not a steady dc, but only a pulsating dc wave power. To make the output wave smooth and useful in a DC power source, we must have to use a filter across the load. Since only half-cycles of the input wave are used, it is known as a half-wave rectifier. 

Half Wave Rectifier Theory

Rectification is an application of the simple PN junction diode. A half-wave rectifier is a circuite that makes use of important properties of a PN junction diode. So to understand the underlying theory behind a half-wave rectifier, you need to clearly understand the PN junction and the characteristics and properties of the PN junction diode.

Power Supply Specifications of a rectifier

The most important parameters which are needed to be specified for a power supply are the needed output dc voltage, the average current and peak currents in the diode, the peak inverse voltage (PIV) of the diode, the regulation, and the ripple factor.

A half-wave rectifier is sometimes used in practice. It is never preferred as the power supply of an audio circuit due to of the very huge ripple factor. huge ripple factor will develop noises in the input audio signal, which in turn will affect audio quality.

The advantage of a half-wave rectifier is only that it’s very cheap, simple, and easy to develop. It is cheap because of the few components involved. Simple because of the straightforwardness in circuit design. Apart from this, a half-wave rectifier has a huge number of disadvantages than advantages!

Disadvantages of Half wave rectifier

1. The output current in the load contains, in addition to the dc component, ac components of basic frequency equal to that of the input supplied voltage frequency. Ripple factor is huge and elaborate filtering is, therefore, required to give steady dc output.

2. The output power and, therefore, rectification efficiency is very low. This is because the fact that power is supplied only during the one-half cycle of the input alternating voltage source.

3. The transformer utilization factor is low.

4. DC saturation of the transformer core resulting in magnetizing current and hysteresis losses and developing of harmonics.

The  DC output provided from a half-wave rectifier is not satisfactory to make a  general power supply. However, it may be used for a few applications such as battery charging.

Half Wave Rectifier with Capacitor Filter 

The output of the half-wave rectifier is not a fixed or constant DC voltage. You can see from the output diagram that it’s a pulsating dc voltage including ac ripples. In real-life applications, we required a power supply with smooth waveforms. In other words, we want a DC power supply with a constant or fixed output voltage. A constant output voltage which comes from the DC power supply is very important as it directly impacts the reliability of the electronic device we connect to the power supply.

We can make the output of a half-wave rectifier smooth by utilizing a filter (a capacitor filter or an inductor filter) across the diode.  In some cases, a resistor-capacitor coupled filter (RC) is mostly used. The circuit diagram below shows a half-wave rectifier with a capacitor filter.

Half-wave Rectifier Characteristics

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

PIV (Peak Inverse Voltage)

During the reverse biased mode, the diode must withstand because of its maximum voltage. During the negative half-cycle, zero current passe through the load. So, an all voltage appears across the diode due to there is a no-voltage drop by the load resistance.

PIV of a half-wave rectifier = VSMAX

Average current and Peak Currents in the Diode

Assuming, the voltage across the secondary side of the transformer will be sinusoidal and its peak value is VSMAX. The instantaneous voltage which is provided to the half-wave rectifier is

Vs = VSMAXSin wt

The current passes through the load resistance is

IMAX = VSMAX / (RF+RL)

Regulation of Half wave rectifier

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

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

Efficiency of half wave rectifier

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 = I2dc RL = (IMAX/ᴨ)2 RL

The input AC power to the transformer,

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

= I2rmsRF + I2rmsRL = {I2MAX/4} [RF + RL]

?= Pdc/Pac = 0.406/{1+RF/RL}

The practical efficiency of a half-wave rectifier is 40.6% when RF is neglected.

Ripple Factor of half wave rectifier (γ)

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

I2 = I2dc + I21 + I22+ I24 = I2dc+ I2ac

γ = Iac / Idc = (I2 – I2dc) / Idc = {( Irms / I2dc) / Idc = {(Irms /I2dc)-1} = kf2-1)

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 provided to the load and transformer secondary AC power rating. The TUF of half wave rectifier is nearly 0.287.

Power Supply Specifications of a rectifier

The most important characteristics which are needed to be specified for a power supply are the required output dc voltage, the average current and peak currents in the diode, the peak inverse voltage (PIV) of the diode, the regulation, and the ripple factor.

•            

Three Phase Half-wave Rectifier

three phase half wave rectifier

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

All of the theory above has a deal with a single-phase half-wave rectifier. Although the phenomena of a 3 phase half-wave rectifier are the same the properties are different. The waveform, ripple factor, efficiency, and RMS output values are not the same as single-phase half-wave.

The three-phase half-wave rectifier is normally used for the transferring of three-phase AC power to DC power. Here the switches are diodes, and we know that they are uncontrolled switches. That is to say, there is no way of controlling the on and off times of these diodes.

The 3 phase half wave diode rectifier is generally designed with a three-phase power supply connected to a three-phase transformer where the secondary winding of the transformer is always connected using star connection. This is because the neutral point is needed to connect the load back to the transformer secondary windings, providing a back path for the flow of power.

A typical configuration of a three-phase half-wave rectifier supplying to a purely resistive load is shown below. Here, each phase of the transformer is used as an individual alternating source.

So we can observe from the above figure that the diode D1 conducts when the R phase has a value of the voltage that is much higher than the value of the voltage of the other two phases have, and this condition starts when the R phase is at a 30o and repeats after every complete cycle. That is to say, the next time diode DI starts to conduct is at 390o. Diode D2 takes over conduction from D1 which stops conducting at angle 150o due to this instant the value of voltage in the B phase becomes much higher than the voltages in the other two phases. So each diode conducts for an angle of 150o – 30o = 120o.

Here, the waveform of the resulting DC power signal is not purely DC as it is not flat, but rather it contains a more ripple. And the frequency of the ripple is 3 × 50 = 150 Hz.

Even though the efficiency of the 3 phase half-wave rectifier is seemingly high, it is still less than the efficiency provided by a 3 phase full wave diode rectifier. Although three-phase half-wave rectifiers are cheaper, this cost-saving is insignificant compared to the money lost in their higher power losses. As such, three-phase half-wave rectifiers are not commonly used in the industry.

Applications of Half wave rectifier

Any rectifier used to make DC power supplies. The practical application of any rectifier (be it half wave or full wave) is to be needed as a component in building DC power supplies.  A half-wave rectifier is not special as compare to a full-wave rectifier in any terms. In order to develop an efficient & smooth DC power supply, a full-wave rectifier is always preferred.  However, for applications in which a constant or fixed DC voltage is not very important, you can use power supplies with a half-wave rectifier.

Limitations

If the load resistor is very small for a given capacitor rating, a high current will pass through the load which discharges the capacitor very quickly (Because of the RC time constant) and results in much-increased ripples. As long as the RC time constant is much greater than the period, the capacitor remains almost fully charged, and we will get a perfect DC output power. To have a greater RC time constant, we need a huge value capacitor. This is not practical due to there are limits on both the cost as well as the size of the capacitor.

Also, there is zero output during the negative half cycle of wave hence half of the power is wasted which results in lower output amplitude.

Because of their major disadvantages, the half-wave rectifiers are few times used.

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