Full Wave Center Tap Rectifier

The full wave center tap rectifier is a two-diode rectifier circuit that uses a center-tapped transformer to convert both half cycles of the AC input into a unidirectional DC output. It offers significantly better efficiency and lower ripple compared to a half wave rectifier, making it a foundational circuit in DC power supply design and a frequently tested topic in GATE Electronics.

Core Concept Explanation

The circuit diagram below shows the basic configuration with two diodes and a center-tapped transformer, illustrating how current flows through alternate diodes on each half cycle.

In a half wave rectifier, only one half cycle of the AC waveform is used. The center tap rectifier resolves this inefficiency by using the center tap of the secondary winding as a common reference, effectively splitting the secondary into two equal halves with opposite polarity.

During the positive half cycle of the input, the upper half of the secondary becomes positive with respect to the center tap. Diode D1 is forward biased and conducts current through the load RL. During the negative half cycle, the lower half becomes positive with respect to the center tap, and D2 conducts. In both cases, current flows through RL in the same direction, producing a unidirectional pulsating output.

The output waveform contains pulses at twice the input frequency. For a 50 Hz supply, the output ripple frequency is 100 Hz. This higher ripple frequency makes filtering significantly easier and more effective compared to a half wave rectifier.

Mathematical Expression

Let Vm be the peak voltage of each half of the secondary winding. The average DC output voltage for a full wave rectifier is given by Vdc = 2Vm / pi, which is approximately 0.637 Vm. This is exactly double the average output of a half wave rectifier (Vm / pi).

The ripple factor for a full wave rectifier is 0.482, compared to 1.21 for a half wave rectifier. A lower ripple factor means the output is much smoother and closer to pure DC. The rectifier efficiency is 81.2 percent, which is double that of a half wave rectifier.

The Peak Inverse Voltage (PIV) is an important design parameter. For the center tap rectifier, the non-conducting diode has the full secondary voltage plus the output voltage appearing across it. The PIV across each diode equals 2Vm. This is the main disadvantage compared to the bridge rectifier where PIV is only Vm.

Practical Understanding

The center tap transformer is a specialized component that increases circuit cost and size. The need for a high PIV rating on the diodes (2Vm) means diodes must be rated for higher reverse breakdown voltage. In high voltage applications, this can be a significant constraint.

Despite this, the two-diode configuration is simpler than the four-diode bridge rectifier. For low power applications where a center-tapped transformer is already available, this circuit remains practical. The output DC voltage is slightly higher than a bridge rectifier because only one diode voltage drop (0.7 V) occurs in the conduction path, compared to two diode drops in a bridge circuit.

Given:
Vm = 12 V (peak voltage per half of secondary)
RL = 1 kΩ
Diode forward voltage Vf = 0.7 V

Why this formula applies:
Full wave rectifier average output formula Vdc = 2Vm/π, subtract one diode drop.

Formula:
Vdc = (2Vm / π) - Vf

Substitution:
Vdc = (2 × 12 / 3.1416) - 0.7

Calculation:
Vdc = (24 / 3.1416) - 0.7 = 7.639 - 0.7

Final Answer:
Vdc ≈ 6.94 V
PIV across each diode = 2Vm = 24 V

Exam Tip: PIV for center tap rectifier is 2Vm, not Vm. A very common GATE trap is confusing PIV of center tap (2Vm) with bridge rectifier (Vm). Also remember ripple frequency = 2f for full wave rectifiers.

Quick Revision

• Uses two diodes and a center-tapped transformer.
• Average DC output: Vdc = 2Vm / π ≈ 0.637 Vm.
• Ripple factor = 0.482; efficiency = 81.2%.
• PIV per diode = 2Vm — higher than bridge rectifier (Vm).
• Output ripple frequency = 2 × input frequency (100 Hz for 50 Hz supply).
• Only one diode drop in conduction path — slightly higher Vdc than bridge.
• GATE trap: Do not confuse PIV formula between center tap and bridge configurations.