What Is Ripple Voltage?

Ripple voltage is the small AC variation that remains on a DC power supply output after rectification and filtering.

A rectifier converts AC into pulsating DC. A filter capacitor then stores electrical energy and supplies current to the load between successive peaks of the rectified waveform.

AC
 │
 ▼
Bridge Rectifier
 │
 ▼
 ┌─────────┐
 │    C    │
 │  Filter │
 └────┬────┘
      │
      ▼
    DC + ripple
      │
      ▼
     Load

The capacitor voltage slowly falls between charging peaks. This periodic voltage variation is the ripple voltage.

Ripple Voltage Formula

For a capacitor-input filter, a commonly used approximation is:

Vripple ≈ I / (f × C)

where:

• Vripple = peak-to-peak ripple voltage
• I = DC load current in amperes
• f = ripple frequency in Hz
• C = filter capacitance in farads

The equation can also be rearranged to calculate capacitance:

C ≈ I / (f × Vripple)

This is an approximation for capacitor-input rectifier supplies. Real power supplies are affected by transformer impedance, diode characteristics, capacitor ESR, load variation and conduction angle.

Ripple Voltage Calculator

Example — 5 A, 10,000 µF, 100 Hz

Suppose a full-wave rectifier supplies a 5 A load through a 10,000 µF filter capacitor. At 50 Hz mains frequency, the ripple frequency is 100 Hz.

I = 5 A

f = 100 Hz

C = 10,000 µF

C = 0.01 F

Therefore:

Vripple = I / (f × C)

Vripple = 5 / (100 × 0.01)

Vripple = 5 V

The approximate ripple voltage is therefore 5 V peak-to-peak.

Full-Wave Rectifier Ripple Frequency

With a full-wave bridge rectifier, both halves of the AC waveform are used. Therefore the ripple frequency is approximately twice the AC frequency.

fripple = 2 × fAC

For 50 Hz mains:

fripple = 2 × 50

fripple = 100 Hz

For 60 Hz mains:

fripple = 2 × 60

fripple = 120 Hz

Required Filter Capacitor

If the maximum acceptable ripple voltage is known, the required capacitance can be estimated from:

C = I / (f × Vripple)

Example — Designing a Filter

Suppose a power supply must deliver 3 A and the maximum acceptable ripple is 1 V peak-to-peak. The supply uses a full-wave rectifier from a 50 Hz source.

fripple = 2 × 50

fripple = 100 Hz

Required capacitance:

C = 3 / (100 × 1)

C = 0.03 F

C = 30,000 µF

The theoretical capacitance is therefore approximately 30,000 µF. A practical design would select an appropriate standard capacitance while checking capacitor voltage rating, ripple-current rating, ESR, temperature and physical size.

Ripple Voltage From Capacitor Size

Ripple Voltage and Load Current

From:

Vripple = I / (f × C)

ripple voltage is directly proportional to load current.

Vripple ∝ I

If the load current doubles while capacitance and frequency remain constant, the approximate ripple voltage also doubles.

For example, if a supply has approximately 2 V ripple at 2 A, the simplified calculation predicts approximately 4 V ripple at 4 A under the same conditions.

Ripple Voltage and Capacitance

Ripple voltage is inversely proportional to capacitance:

Vripple ∝ 1 / C

Increasing the filter capacitance reduces the approximate ripple.

For example, doubling the capacitance from 10,000 µF to 20,000 µF approximately halves the calculated ripple voltage, assuming the same load current and ripple frequency.

Ripple Voltage and Frequency

Ripple voltage is also inversely proportional to ripple frequency:

Vripple ∝ 1 / f

This is one reason full-wave rectification produces easier-to-filter ripple than half-wave rectification.

At 50 Hz mains, a full-wave bridge produces approximately 100 Hz ripple, while a half-wave rectifier produces approximately 50 Hz ripple.

Half-Wave vs Full-Wave Ripple

For the same load current and capacitance, the higher ripple frequency of a full-wave rectifier reduces the theoretical ripple voltage.

Multiple Capacitors in Parallel

Filter capacitors can be connected in parallel to increase total capacitance.

Ctotal = C1 + C2 + C3 + ...

For example:

10,000 µF
+
10,000 µF
+
10,000 µF

= 30,000 µF

Using multiple capacitors can also provide increased total ripple current capability when suitable capacitors are selected.

Capacitor Ripple Current

The capacitor in a rectifier supply experiences ripple current. The capacitor's ripple-current rating is therefore important, especially in high-power supplies.

A capacitor that is subjected to excessive ripple current can heat internally and suffer reduced service life.

The actual capacitor ripple current waveform in a capacitor-input rectifier is not simply equal to the DC load current. Accurate design requires consideration of the rectifier conduction angle, transformer impedance, capacitance and load.

Ripple Voltage in Audio Power Supplies

Ripple voltage is particularly important in audio amplifier power supplies because residual ripple can appear as audible hum if it reaches sensitive amplifier stages.

High-current amplifier supplies often use large reservoir capacitors to reduce low-frequency ripple.

However, simply increasing capacitance indefinitely is not always the best solution. Rectifier surge current, transformer heating, capacitor ripple current, wiring resistance and charging pulses must also be considered.

Ripple Voltage After a Regulator

A voltage regulator can further reduce ripple appearing at its output. The amount of rejection depends on the regulator type, operating conditions and frequency.

A linear regulator generally requires sufficient voltage above its output to remain in regulation. Therefore, the minimum point of the ripple waveform must remain high enough for the regulator to operate correctly.

Unregulated DC
     ╲____╱╲____╱
          │
          ▼
      Regulator
          │
          ▼
    Lower-ripple DC

Peak-to-Peak Ripple

The formula used on this page estimates the change in capacitor voltage between charging peaks:

Vripple ≈ I / (f × C)

The result is therefore normally interpreted as an approximate peak-to-peak ripple voltage.

It should not be confused with RMS ripple voltage.

Ripple Percentage

Ripple can also be expressed as a percentage of the DC output voltage:

Ripple (%) =
Vripple
──────── × 100
VDC

Practical Filter Design

The theoretical capacitor value is only the starting point. A practical power supply should also consider:

• Transformer regulation
• AC mains variation
• Rectifier forward voltage
• Rectifier surge current
• Capacitor ESR
• Capacitor ripple-current rating
• Capacitor voltage rating
• Load variation
• Temperature
• Wiring and PCB resistance
• Required output regulation

Common Mistakes

• Using 50 Hz instead of 100 Hz for a full-wave bridge on a 50 Hz supply.
• Confusing peak-to-peak ripple with RMS ripple.
• Ignoring the load current.
• Using a capacitor with insufficient voltage rating.
• Ignoring capacitor ripple-current capability.
• Assuming the theoretical formula predicts exact real-world ripple.
• Ignoring transformer impedance and regulation.
• Increasing capacitance without considering rectifier and transformer surge current.
• Assuming a regulator can operate correctly when its input ripple falls below the required headroom.

Key Points

• Ripple voltage is the residual AC variation on a DC supply.
• For a capacitor-input filter, Vripple ≈ I/(fC).
• Full-wave rectification produces ripple at twice the AC frequency.
• Higher load current produces more ripple.
• Larger capacitance reduces ripple.
• Higher ripple frequency reduces ripple.
• Capacitor voltage and ripple-current ratings are both important.
• Real power supplies can deviate from the simplified ripple equation.
• High-current supplies require particular attention to capacitor and rectifier surge currents.