Uncontrolled Rectifiers

Uncontrolled rectifiers use diodes to convert AC input to a fixed DC output voltage. They are commonly used as the front end in variable frequency drives and basic DC power supplies.

Core Concept

In a single phase full wave bridge four diodes arrange to steer positive and negative half cycles to a single DC polarity. Conduction shifts naturally based on source voltage polarity.

Because diodes cannot be controlled via a gate the output voltage is entirely dependent on the AC input amplitude. A 230V AC RMS input yields approximately 207V DC average.

A large DC link capacitor is often added to smooth the output ripple resulting in a peak voltage near 325V DC. However this draws pulsed currents generating high harmonics.

Key Formulas

For a single phase full bridge rectifier Vdc average equals 2 multiplied by Vm divided by pi. Vrms equals Vm divided by the square root of 2.

Given: Vac rms = 230 V
Formula: Vm = 230 * sqrt(2), Vdc = 2 * Vm / pi
Steps: Vm = 325.27 V, Vdc = (2 * 325.27) / 3.1416
Final Answer: 207.0 V

Exam Tip: The ripple frequency of a single phase full wave rectifier is 2f which is 100 Hz for a 50 Hz supply. Half wave rectifiers have a ripple frequency of f.

Performance Parameters

• Form factor: 1.11 for pure resistive load
• Ripple factor: 48.2 percent
• Rectification efficiency: 81.2 percent
• Peak Inverse Voltage PIV: Vm for bridge topology
• Transformer Utilization Factor: 0.812

Quick Revision

• Fixed DC output voltage
• Natural line commutation
• Bridge needs four diodes
• Center tap needs two diodes but higher PIV
• Ripple frequency is double the line frequency
• Exam trap: Confusing bridge PIV with center tap PIV which is 2Vm