Capacitor Ripple Calculator

The Capacitor Ripple Calculator estimates ripple voltage and current in power supplies from load, frequency, capacitance, and ESR.

About the Capacitor Ripple Calculator

This Calculator estimates voltage ripple caused by capacitors in DC supplies and converters. It models both capacitive discharge between charge events and voltage drop across a capacitor’s ESR. You can analyze classic rectifier-and-filter supplies or switching-regulator outputs. The tool also helps you back-calculate the capacitance required to hit a ripple target.

Enter your operating frequency, load or ripple current, capacitance, and ESR. The Calculator then computes peak-to-peak ripple and separates the capacitive and ESR components. It reports the result with notes on assumptions. You can compare scenarios quickly by adjusting one variable at a time.

The interface favors clear inputs and consistent units. That makes the derivation behind each formula easier to follow. You will see which term dominates and where to focus design effort. Use the results as a starting point and refine with measurements.

The Mechanics Behind Capacitor Ripple

Ripple arises when a capacitor charges in short bursts and discharges while the source is lower than the capacitor voltage. In linear supplies, diodes conduct only near voltage peaks. In switching converters, the inductor’s ripple current flows in and out of the capacitor. Both cases create a time-varying voltage on the output.

• Between charge pulses, the load draws current, and the capacitor voltage falls roughly linearly for small ripple.
• When the source rises above the capacitor voltage, a surge refills the lost charge and the voltage peaks again.
• The discharge interval sets the capacitive ripple component; longer intervals mean larger drops.
• ESR adds an immediate step-like component: voltage equals ripple current times ESR, superimposed on the capacitive sag.
• Ripple frequency depends on topology: twice the mains frequency for full-wave rectifiers, and the switching frequency for many DC/DC converters.

Real capacitors also have ESL and frequency-dependent ESR. At higher frequencies, ESR often falls, and ESL can create ringing. The Calculator focuses on the dominant low-to-mid frequency behavior that sets peak-to-peak ripple in most designs.

Capacitor Ripple Formulas & Derivations

Two simple relationships explain most ripple behavior. First, charge relation: Q = C × V. Second, current is the time rate of change of charge. Combining these gives dV = I × dt / C. For small ripple, this linear approximation is accurate and easy to apply.

• Rectifier supply, capacitive ripple: For load current I and ripple frequency f_ripple, V_ripple_pp ≈ I / (f_ripple × C). With a full-wave rectifier on mains f_line, f_ripple = 2 × f_line. For half-wave, f_ripple = f_line.
• Derivation for rectifiers: Over the hold time Δt ≈ 1 / f_ripple, the capacitor loses charge ΔQ = I × Δt, so ΔV = ΔQ / C = I / (f_ripple × C).
• ESR contribution: V_ripple_ESR_pp ≈ ΔI_pp × ESR, where ΔI_pp is the peak-to-peak ripple current through the capacitor during each cycle.
• Switching buck output, triangular current: The capacitor sees a near-triangular current with peak-to-peak ΔI_pp. The resulting capacitive ripple is V_ripple_pp_C ≈ ΔI_pp / (8 × f_sw × C). Add ESR ripple in quadrature or linearly for an estimate: V_total_pp ≈ V_ripple_pp_C + ΔI_pp × ESR.
• RMS ripple (when needed): For a triangular voltage ripple of peak-to-peak V_pp, V_rms ≈ V_pp / (2√3). Most datasheets and EMI checks focus on peak-to-peak.

These expressions assume the ripple is small relative to the DC level and that ESR is roughly constant over the relevant band. They also assume the source impedance is low during charging. When conduction angles are very narrow or ESL is significant, the simple derivation underestimates high-frequency spikes. Use the results for sizing and then verify on the bench.

What You Need to Use the Capacitor Ripple Calculator

Gather a few key specifications from your circuit. You will enter frequency, currents, and capacitor data. The Calculator uses consistent units, so confirm each entry before running a scenario. A quick sketch of your topology helps avoid mistakes.

• Topology and ripple frequency: mains half-wave or full-wave (f_ripple = f_line or 2 × f_line), or switching frequency f_sw.
• Load or ripple current: steady load current I (rectifier) or capacitor ripple current peak-to-peak ΔI_pp (switching).
• Capacitance C: single value or effective total when several capacitors are in parallel.
• ESR: from the capacitor datasheet at the operating frequency and temperature.
• Target ripple: desired maximum peak-to-peak voltage ripple for sizing C.
• Supply or line frequency: 50 Hz or 60 Hz for mains systems when applicable.

Use realistic ranges. Extremely small ripple assumptions break if ESR or ESL dominate. Very high capacitance can push diode conduction into short, high-current spikes. Near-zero load current can make discharge negligible and skew the result. If your device operates over temperature, remember ESR and capacitance change with heat.

Step-by-Step: Use the Capacitor Ripple Calculator

Here’s a concise overview before we dive into the key points:

1. Select your topology: rectifier filter or switching converter output.
2. Enter the ripple frequency: 2 × mains for full-wave, mains for half-wave, or the switching frequency.
3. Enter the relevant current: load current I or ripple current ΔI_pp.
4. Enter the capacitance C and the capacitor ESR at your frequency.
5. Optionally enter a target ripple to compute the required capacitance.
6. Run the calculation to see capacitive ripple, ESR ripple, and total peak-to-peak ripple.

These points provide quick orientation—use them alongside the full explanations in this page.

Example Scenarios

Rectified mains supply: A 60 Hz mains feeds a full-wave bridge and a 2,200 µF filter capacitor. The load draws I = 0.5 A. Ripple frequency is 120 Hz. Capacitive ripple V_pp ≈ I / (f_ripple × C) = 0.5 / (120 × 0.0022) ≈ 1.89 V. If ESR is 0.05 Ω and the charging pulse peak-to-peak current effectively adds about 0.2 A across ESR during the event, ESR ripple adds ~10 mV, which is negligible compared to 1.89 V. Total ripple is dominated by the capacitive term, near 1.9 V. What this means: To cut ripple in half, double the capacitance or double the ripple frequency.

Buck converter output: A 5 V buck runs at f_sw = 500 kHz with inductor current ripple ΔI_pp = 0.4 A. Output capacitor is 47 µF with ESR = 0.02 Ω. Capacitive ripple V_pp_C ≈ ΔI_pp / (8 × f_sw × C) = 0.4 / (8 × 500,000 × 47e−6) ≈ 2.1 mV. ESR ripple V_pp_ESR ≈ ΔI_pp × ESR = 0.4 × 0.02 = 8 mV. Estimated total peak-to-peak ripple is about 10 mV. What this means: Lowering ESR matters more than increasing C in this case.

Accuracy & Limitations

The Calculator uses widely accepted approximations for ripple estimation. They fit most power designs well. However, every physical capacitor and supply has quirks. You should understand where error creeps in and how to tighten agreement with measurements.

• ESR and C vary with frequency, voltage bias, and temperature; datasheet curves show these effects.
• Diode conduction angle and transformer/source impedance can alter the effective hold time in rectifiers.
• ESL and PCB inductance create high-frequency spikes not captured by simple formulas.
• Capacitor tolerance (often ±20%) shifts actual ripple away from the nominal result.
• Measurement bandwidth and probe setup can either hide or exaggerate ripple.

Use the tool for design direction and sizing. Then validate on hardware with proper probing and bandwidth limits. If a mismatch appears, revisit ESR and layout parasitics first. Adjust the model to reflect your parts and environment.

Units Reference

Correct units keep calculations consistent and prevent large errors. Power equations mix current, voltage, capacitance, and frequency. Always confirm base units before comparing one derivation to another or entering values in the Calculator.

Read the table as a legend. Match each symbol to its units before you compute. For microfarads, use 1 µF = 1e−6 F. For kilohertz, use 1 kHz = 1,000 Hz. Keep units consistent to avoid errors in the final result.

Tips If Results Look Off

If your calculated ripple does not match the oscilloscope trace, isolate the source of error. Verify the basic inputs first. Then focus on parasitics and measurement technique. Small setup changes can produce big differences in observed ripple.

• Recheck ESR at operating temperature and frequency; use datasheet graphs, not only the table.
• Confirm the actual ripple frequency with a scope; harmonics can confuse readings.
• Measure at the load with a short ground spring, not a long probe ground clip.
• Try adding a small ceramic capacitor in parallel; if ripple shrinks, ESR or ESL was dominant.
• Reduce bandwidth on the scope to compare against low-frequency derivation.

When the mismatch persists, simulate with a simple RLC model including source impedance. Compare the simulated waveform to the measurement. Adjust ESR and ESL until it aligns. Update the Calculator inputs with the refined values.

FAQ about Capacitor Ripple Calculator

Does the Calculator output peak-to-peak or RMS ripple?

The primary result is peak-to-peak ripple. If you need RMS, apply an appropriate factor based on the waveform shape. For triangular ripple, V_rms ≈ V_pp / (2√3).

How do I choose f_ripple for a rectifier filter?

Use f_ripple = f_line for half-wave and f_ripple = 2 × f_line for full-wave rectifiers. For 50 Hz mains, that means 50 Hz or 100 Hz. For 60 Hz mains, 60 Hz or 120 Hz.

Can the Calculator size the capacitor for a target ripple?

Yes. Enter the target peak-to-peak ripple, frequency, and current. The Calculator solves for C using the derivation ΔV = I / (f × C), and includes an option to budget ESR.

How does ESR affect high-frequency spikes?

ESR creates a step proportional to ripple current, while ESL can cause overshoot and ringing. The Calculator estimates ESR ripple, but very fast spikes often need layout fixes and parallel ceramics.

Glossary for Capacitor Ripple

Ripple Voltage

The periodic variation in a DC output voltage due to charging and discharging of capacitors. Often expressed as peak-to-peak magnitude.

Peak-to-Peak (pp)

The difference between the maximum and minimum value of a waveform within one cycle. It is the most common ripple metric for supplies.

RMS

Root-mean-square value representing equivalent DC heating. For ripple, RMS is used in noise budgets and compliance checks.

ESR

Equivalent Series Resistance of a capacitor. It causes instantaneous voltage drops proportional to ripple current.

ESL

Equivalent Series Inductance of a capacitor. It limits high-frequency performance and can create ringing.

Rectifier

A circuit, usually using diodes, that converts AC to DC. It sets the ripple frequency seen by the filter capacitor.

Hold Time

The interval between charge events when a capacitor supplies current to the load. It largely determines capacitive ripple.

Switching Frequency

The operating frequency of a DC/DC converter. It sets the fundamental ripple frequency at the output capacitor.

Sources & Further Reading

Here’s a concise overview before we dive into the key points:

• Wikipedia: Ripple (electrical)
• All About Circuits: Capacitor-Input Filter
• Murata: Understanding Capacitor ESR and ESL
• Analog Devices: How to Measure Switching Regulator Output Ripple
• Electronics Tutorials: Smoothing Capacitors in Power Supplies

These points provide quick orientation—use them alongside the full explanations in this page.