What Is a Bridge Rectifier?

A bridge rectifier converts AC voltage into pulsating DC using four diodes. It is one of the most common rectifier circuits used in linear power supplies, battery chargers and electronic equipment.

              D1              D2
 AC ~ ───────|>|─────+─────|<|────── ~ AC
                       │
                      +│
                      LOAD
                      -│
                       │
 AC ~ ───────|<|─────+─────|>|────── ~ AC
              D3              D4

During each half-cycle, two diodes conduct while the other two remain off. The load current therefore flows in the same direction during both halves of the AC waveform.

Bridge Rectifier Output

For a sinusoidal AC voltage, the peak voltage is:

Vpeak = Vrms × √2

Because current normally passes through two diodes in a conventional bridge, a simplified estimate of the peak DC voltage is:

VDC(peak) ≈ Vrms × √2 - 2Vf

where Vf is the forward voltage of one conducting diode.

Bridge Rectifier Calculator

Example — 24 VAC Bridge Rectifier

Suppose a transformer supplies 24 VAC RMS to a bridge rectifier. Assuming each conducting diode drops approximately 0.8 V:

Vpeak = 24 × √2

Vpeak ≈ 33.94 V

VDC(peak) ≈ 33.94 - 1.6

VDC(peak) ≈ 32.34 V

This is the approximate capacitor charging voltage before allowing for transformer regulation, wiring resistance, diode characteristics and load-dependent ripple.

Full-Wave Rectification

A bridge rectifier uses both halves of the AC waveform. Consequently, the ripple frequency is twice the AC input frequency.

fripple = 2 × fAC

For a 50 Hz supply:

fripple = 2 × 50

fripple = 100 Hz

Bridge Rectifier Diode Current

The average DC load current is not identical to the current waveform through each diode. In a bridge rectifier, each diode conducts only during part of the AC cycle.

For basic power-supply calculations, the load current provides a useful starting point for selecting a rectifier with sufficient current capacity.

The rectifier should also account for surge current caused by charging the filter capacitors at startup.

Bridge Rectifier Power Loss

Two diodes normally conduct at the same time in a conventional bridge. A simplified estimate of rectifier loss is:

Pdiodes ≈ 2 × Vf × I

where:

• Vf = forward voltage of one diode
• I = current through the rectifier

Example — 5 A Bridge Rectifier

If a bridge carries 5 A and each conducting diode has approximately 0.8 V forward drop:

P ≈ 2 × Vf × I

P ≈ 2 × 0.8 × 5

P ≈ 8 W

Approximately 8 W can therefore be dissipated in the two conducting diodes under this simplified assumption.

Actual diode forward voltage varies with current and temperature, so the manufacturer's forward-voltage data should be used for accurate thermal design.

Peak Inverse Voltage

The reverse voltage that a diode must withstand is an important parameter when selecting bridge rectifier diodes.

For a simple bridge connected to a transformer secondary, a useful starting estimate for the reverse voltage across an individual diode is related to the peak secondary voltage.

VRRM requirement should be
comfortably above the maximum
peak secondary voltage.

The actual required rating depends on the transformer configuration, source impedance, transients and circuit arrangement.

Peak Voltage Calculator

Bridge Rectifier With Capacitor Filter

A capacitor connected across the rectifier output charges toward the peak voltage. Between the peaks, the capacitor supplies current to the load and its voltage decreases.

        Bridge
AC ─── Rectifier ────+──── +DC
                     │
                    ─── C
                    ───
                     │
                     +──── Load
                     │
                    GND

The approximate ripple voltage for a capacitor-input full-wave rectifier is:

Vripple ≈ I / (fripple × C)

Therefore:

C ≈ I / (fripple × Vripple)

Bridge Filter Capacitor Calculator

Example — 3 A Full-Wave Supply

Suppose a power supply provides 3 A DC and the allowable ripple is 2 V. The transformer operates at 50 Hz.

fripple = 2 × 50

fripple = 100 Hz

C = I / (f × Vripple)

C = 3 / (100 × 2)

C = 0.015 F

C = 15,000 µF

A practical design would use a suitable standard capacitor value or combination of capacitors while also checking voltage rating, ripple current and temperature rating.

Capacitor Voltage Rating

The filter capacitor must be rated above the highest voltage that can appear across it.

For an unloaded transformer secondary:

Vpeak ≈ VAC × √2

However, transformer regulation can cause the no-load secondary voltage to be higher than its nominal rating. Mains voltage can also vary. These effects must be considered when selecting the capacitor voltage rating.

Bridge Rectifier for Dual-Rail Supplies

A transformer with a center-tapped secondary can be used to produce positive and negative supply rails.

        +V
         │
         │
        Load
         │
         0V
         │
        Load
         │
         │
        -V

The exact rectifier arrangement depends on whether a conventional four-diode bridge or a center-tapped full-wave configuration is being used.

For amplifier power supplies, the transformer winding configuration, rectifier topology and reservoir capacitor arrangement should be considered together.

Bridge Rectifier vs Center-Tapped Rectifier

Bridge Rectifier Surge Current

When a large filter capacitor is initially discharged, the rectifier can experience a high charging current when the power supply is switched on.

This startup surge can be significantly higher than the normal DC load current.

The rectifier and transformer should therefore be selected with appropriate surge capability.

A soft-start circuit, inrush-current limiter or other protection may be required in higher-power supplies.

Diode Selection

Important rectifier diode specifications include:

• Average forward current rating
• Peak repetitive reverse voltage
• Surge current rating
• Forward voltage
• Reverse recovery characteristics
• Maximum junction temperature

For low-frequency mains rectification, conventional power diodes are often suitable. High-frequency switching applications require rectifiers selected according to switching frequency and recovery requirements.

Bridge Rectifier Heat Dissipation

Rectifier heat depends primarily on forward voltage and current. Higher current produces greater diode losses.

If the calculated dissipation is significant, the bridge rectifier may require a heatsink or adequate thermal path.

The thermal design should consider:

• Diode junction-to-case thermal resistance
• Case-to-heatsink thermal resistance
• Heatsink thermal resistance
• Ambient temperature
• Continuous load current
• Actual forward voltage

Bridge Rectifier Efficiency

A bridge rectifier itself does not provide voltage regulation. Its output voltage changes with the AC input, diode losses and load.

The two conducting diode drops also reduce the available DC voltage.

For low-voltage supplies, the diode drops can represent a relatively large percentage of the available voltage.

Common Mistakes

• Using RMS voltage as if it were peak voltage.
• Forgetting that two diodes conduct in a bridge at a time.
• Ignoring transformer voltage regulation.
• Ignoring capacitor charging surge current.
• Selecting a diode with insufficient reverse-voltage rating.
• Selecting a rectifier with insufficient surge-current capability.
• Ignoring diode heat dissipation.
• Using insufficient filter capacitance.
• Using a capacitor with insufficient voltage rating.
• Assuming the calculated unloaded DC voltage is the actual loaded output voltage.

Key Points

• A bridge rectifier uses four diodes.
• Two diodes normally conduct during each half-cycle.
• Both halves of the AC waveform are used.
• For a capacitor-input supply, DC voltage approaches the AC peak minus two diode drops.
• Full-wave ripple frequency is twice the AC frequency.
• Large filter capacitors can produce substantial startup surge current.
• Rectifier losses increase with load current.
• Diode voltage, current and surge ratings must all be considered.
• Transformer regulation and mains variation affect the actual DC voltage.