Test: Diode Rectifiers - Electrical Engineering (EE) MCQ

Unilateral Circuit: 

• In unilateral circuits, the property of the circuit changes with the change of direction of supply voltage or current.
• In other words, a unilateral circuit allows the current to flow only in one direction.
• A diode rectifier is the best example of a unilateral circuit because it does not perform the rectification in both directions of supply.

• The circuit consisting of Diodes like the rectifier circuit is an example of a Unilateral Circuit.
• In this circuit, the current can flow in one direction only because the diode allows the flow of current in one direction only.

Difference Between Half Wave and Full Wave Rectifier:

Maximum Average Forward Current (IFAVM):
Diodes are often used in rectifier circuits supplying a DC (average) current to the load.
In such cases, the average load current and the diode forward current usually have a simple relationship.
Therefore, it will be of interest to know the maximum average current, a diode can conduct in the forward direction.
This specification gives the maximum average value of power frequency half cycle sine wave current allowed to flow through the diode in the forward direction.
The average current rating of a diode decreases with a reduction in conduction angle due to an increase in the current “form factor”.

Full-wave rectifier output waveform:

Average current value = 2Im/π
Half-wave rectifier output waveform:

 
Average current value = Im/π

DC output waveform:

Average current value = Im
So average current rating will be in the order of DC > Full-wave rectifier > Half-wave rectifier.

Ripple factor:

The amount of AC present in the output of the signal is called as ripple.

The ripple factor indicates the number of ripples present in the DC output.

The output of the power supply is given by


It is given as:

 

Thus if the ripple factor is less, the power supply has less AC components and power supply output is purer (i.e more DC without much fluctuations)

Thus ripple factor is an indication of the purity of output of the power supply.

Concept:

Total harmonics distortion is given by,

Now, total power due to all harmonic component

Calculation:
Given that, THD = 0.1
IL = 4 A, Rc = 8 Ω
⇒ PT = (4)²(1 + (0.1)2) × 8/2

P_T = 64.64 W

Three-Phase Full Wave Diode Rectifier:

• The full-wave three-phase uncontrolled bridge rectifier circuit uses six diodes, two per phase.
• A 3-phase full-wave rectifier is obtained by using two half-wave rectifier circuits. The advantage here is that the circuit produces a lower ripple output than the previous half-wave 3-phase rectifier as it has a frequency of six times the input AC waveform.
  
  
• As before, assuming a phase rotation of Red-Yellow-Blue (VA – VB – VC) and the red phase (V_A) starts at 0^o. Each phase connects between a pair of diodes as shown. One diode of the conducting pair powers the positive (+) side of the load, while the other diode powers the negative (-) side of the load.
• Diodes D₁ D₃ D₂ and D₄ form a bridge rectifier network between phases A and B, similarly diodes D₃ D₅ D₄ and D₆ between phases B and C and D₅ D₁ D₆, and D₂ between phases C and A.
• If we start the pattern of conduction at 30^o, this gives us a conduction pattern for the load current of D_1-4 D_1-6 D_3-6 D_3-2 D_5-2 D_5-4, and return again to D_1-4 and D_1-6 for the next phase sequence as shown.
• So the average DC value of the output voltage waveform from a 3-phase full-wave rectifier is given as:
  

Formula Used:

efficiency (%η) = P_o/P_i

Here, P_o is the output power

And, P_i is the input power

Application:

Given,

Input AC Power (P_i) = 150 W

Output DC Power (P_o) = 90 W

Hence, Efficiency (η) = 90/150 = 0.6 = 60 %

Transformer Utilization Factor (TUF):

It is defined as the ratio of dc power output to the transformer Volt-Ampere (VA) rating required by the secondary winding.

TUF = (DC power output) / (Effective VA rating of transformer)

It is a quantitative indication of the utilization of the VA Rating of Transformer. The more the value of TUF, the more will be the utilization. In other words, the VA rating of the required transformer will be less if TUF is more and vice versa.

Application:

Considering R load,

The output average voltage of a single-phase full-bridge diode rectifier is given by
V₀ = 2V_m/π

Here, The output average current for R load is given by 

Supply voltage V_{s RMS} is given by Vm/√2 

Supply current I_{s RMS} is given by

Now,

= 0.81
= 81%

Concept:

In a single-phase uncontrolled diode rectifier

The average output voltage, Vo = 2V_m/π

Supply RMS current i_sr = I_o

Supply average current = 0

In a three-phase uncontrolled diode rectifier

The average output voltage, V_o = 3V_ml/π

Supply RMS current i_sr = 

Supply average current = 0

Calculation:

Given that, load current (I₀) = 10 A

Supply average current = 0 A

Supply RMS current = 10 A

Rectification is AC to DC. In DIODE biased rectification, control is not possible.