After the bridge rectifier, the reservoir capacitor holds the rail between AC peaks. Too small and the supply hums under load. Oversized and it hammers the transformer with narrow charging spikes. The sizing rule is one line of arithmetic.
At a glance: 7 minute guide · part 4 of 10 in the power supplies and batteries complete guide track · includes a worked example and a quick-reference table.
The ripple equation
Between charging peaks, the capacitor discharges into the load. ΔV = I × t ÷ C, with t ≈ 8 ms (full-wave, 50 Hz) or 10 ms at 60 Hz. Rearranged: C = I × t ÷ ΔV. For 1 A of load and 1 V of ripple at 50 Hz: C ≈ 8000 µF.
| Parameter | Rule | Example (1 A, 50 Hz) |
|---|---|---|
| Ripple ΔV target | 1-2 V typical | 1.5 V |
| Capacitance | C = I·t ÷ ΔV | ≈ 5300 µF → 6800 µF |
| Voltage rating | ≥ 1.5 × Vpeak | 50 V for 24 VAC winding |
| Ripple current rating | ≥ load current | Check datasheet derating |
| Inrush | NTC or resistor | Protects diodes and fuse |
Peak and surge currents
The cap charges only near the AC crest, in short, tall pulses. Transformer regulation, winding resistance and diode surge ratings all face these spikes. Massive capacitors "for safety" make the spikes worse and the diodes hotter.
After the reservoir
Ripple at the reservoir is fine as long as the regulator's dropout and PSRR handle it the regulator is the ripple eraser. Size for ~1-2 V of ripple, then let regulation do its job. Inrush limiting (NTC) protects the first charging cycle.
How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
- Compute peak DC (VAC × 1.414 − 2 diode drops)
- Choose ripple budget above regulator dropout
- Apply C = I·t ÷ ΔV and round up a standard value
- Add inrush limiting and verify ripple under full load
Worked example
A 12 VAC/2 A transformer after a bridge peaks near 15.6 V DC. For 1.5 A load at 1.5 V ripple: C ≈ 8000 µF, 25 V rated. The 5 V regulator afterwards erases the remaining ripple entirely. Run the numbers yourself with the Capacitor Code (3-Digit) and the result should agree to within rounding.
Practical note from the bench. The ripple formula is the first thing we teach after the bridge rectifier it converts supply design from folklore to one line of maths.
Common mistakes to avoid
- Rating capacitors at the RMS rather than the peak voltage
- Omitting inrush limiting on big caps
- Measuring ripple unloaded and declaring victory
Key takeaways
- The ripple equation the foundation of this guide; revisit it if any measurement here surprises you.
- Peak and surge currents the foundation of this guide; revisit it if any measurement here surprises you.
- After the reservoir the foundation of this guide; revisit it if any measurement here surprises you.
Prerequisites and preparation
Before starting: compute peak dc (vac × 1.414 − 2 diode drops) and choose ripple budget above regulator dropout. Keep the Capacitor Code (3-Digit) and RC Time Constant open every number in the worked example is reproducible. Total time including the bench steps: about 6-7 minutes.
Who benefits most
Hobbyists meeting this topic for the first time, students who want the version with real numbers instead of abstract symbols. Returning engineers refreshing a corner of the craft. The mistake list alone justifies the visit every entry in it was learned the expensive way.
Quick reference card
| Aspect | Where to find it in this guide |
|---|---|
| Core theory | The ripple equation |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Common mistakes to avoid |
How this fits the power supplies and batteries complete guide track
This guide is one stop in the structured learning path. Start from the power supplies and batteries complete guide complete guide for the full map, or continue with supply families and regulator choice. For the arithmetic, open the Capacitor Code (3-Digit) or RC Time Constant.
Frequently asked questions
Full-wave or half-wave? Full-wave bridges double the recharge rate, halving capacitor needs half-wave survives only in the cheapest products.
Do two caps in parallel help? Yes ripple current shares, ESR drops; two 4700 µF often beat one 10 000 µF thermally.
Is there a calculator for this? Yes the Capacitor Code (3-Digit) and RC Time Constant tools run the formulas from this guide instantly, client-side, with no signup.
Keep going with this track
- The complete power & batteries guide: Power & Batteries complete guide
- Read next: bms design tutorial: battery management systems explained
- Also in this track: solar mppt explained: maximum power point tracking
- Continue with: linear vs switching regulators: choosing correctly
- Calculate as you go: runtime estimator · regulator designer · capacitor code tool
- Bookmark this page against the day a measurement surprises you. Most readers return to the table and the mistake list first, and that is the correct order.
Working method notes
Keep a lab notebook entry for every build in this track. The measured values, the deviations from the guide and the reason for each. Six months from now, those notes are worth more than any tutorial. They describe your bench and your components rather than a general case.
When a result here disagrees with your expectation, write down both numbers before changing anything. The gap between predicted and measured is where the real engineering lives. It is usually a tolerance, a parasitic or an assumption that was never checked.
Formulas and checks from this guide
Verification checklist for this track. Verify regulation under load, not just open-circuit, measure inrush where it matters. Treat every lithium cell as energetic chemistry that has earned its protection chain.
Bookmark this page against your next build in the track. The checklist above is the same one used across 18 guides in this series.
What the bench taught us
No. Oversizing hammers the diodes with inrush and buys ripple you no longer need once regulation follows.
Any lithium pack of 2S or more, without exception.
Extended Application Notes
This section expands the practical application of sizing rectifier filter capacitors without ripple regret beyond the worked example, into the situations builders actually meet. Component substitution: when the exact specified part is unavailable, the substitution logic follows the governing parameter of this design, not the nominal value, and the verification step after any substitution is to re-measure the one quantity this guide identified as critical. Batch variation: components vary, and the design margins recommended in the sections above absorb that variation; if a second build behaves differently, the difference itself is diagnostic and points to the tolerance that dominated. Environmental limits: temperature, supply variation and ageing each push a real circuit away from its bench behaviour, and the recommended practice is to test the extremes deliberately rather than discover them in the field. These notes exist because the bench taught them, repeatedly, and each one was once a real troubleshooting session that ended in understanding.
Failure Analysis in Depth
The mistakes section above lists the traps; this section explains why each trap exists and how to recognize it early. Rating capacitors at the RMS rather than the peak voltage Omitting inrush limiting on big caps Measuring ripple unloaded and declaring victory. Each of these failures has a signature that appears in measurement before it appears in smoke: a reading that drifts, a waveform that differs from the prediction, a temperature that climbs faster than the calculation. The discipline this guide teaches is to measure at the first sign, not at the last, and the sections above give the specific instrument and setting for each check. Failure analysis is not pessimism; it is the fastest curriculum in electronics, because a fault understood once is a fault prevented forever.
Pre-Build Checklist
Before powering any build of this design, run the list: every component value verified against the specification above, the critical measurement points identified and accessible, the instrument modes and ranges chosen in advance, the expected values written down beside the bench, and the power source current-limited for first application. The checklist takes two minutes and replaces the most expensive class of beginner error, which is not ignorance but confidence outrunning verification. Builders who adopt the checklist across the guides in this track report first-apply success rates that feel like cheating, but it is not cheating, it is engineering.
What Comes Next
Having worked through this guide, the natural next steps are the adjacent guides in the track index above, each of which assumes exactly the vocabulary this page built. The calculators linked in the tools section verify every number in seconds, and the complete guide at the head of this track maps the entire curriculum. Read once, build once, measure always: that is the method this site teaches and the method every section above followed before publication.
Theory in Practice, Extended
The theory section of sizing rectifier filter capacitors without ripple regret deserves one more pass with the bench in mind, because knowing a relationship and applying it under constraint are different skills. In application, the relationship is never isolated: it interacts with tolerances, with temperature, with the behaviour of adjacent stages, and with the measurement itself. The extended practice is to take the governing formula from the sections above and stress it, deliberately. Push the input to the edge of its specified range and watch the output follow the prediction, then push past it and watch the prediction break, because the edge of the specification is exactly where the formula stops being the whole story. That boundary, found on the bench rather than in the datasheet, is the real knowledge this guide offers beyond the mathematics.
Component Sourcing and Substitution Notes
Real builds meet real supply chains, and this section addresses the practical reality. The specified components in this guide were chosen for the reasons stated in the design sections, but equivalent parts from reputable manufacturers almost always serve, provided the governing parameters match, not merely the nominal ones. The substitution checklist: match the parameter this guide identified as critical, verify the package and pinout against the physical part before layout, check the datasheet revision for silent changes, and re-run the verification measurement after installation. Avoid unbranded surplus and marketplace components for anything this guide treats as safety-relevant; the failure mode of a counterfeit is not degradation, it is unpredictability, and unpredictability defeats every other design decision in the chain.
Instrumentation for This Design
Every measurement recommended in this guide maps to a specific instrument configuration, and this section consolidates them. Voltage checks: DC range selected before probing, leads verified against a known source, meter burden considered when the node is high impedance. Current checks: circuit broken at the defined point, meter inserted with the correct range and fuse status confirmed first. Waveform checks: probe compensated against the reference before any amplitude claim, ground lead kept short, bandwidth sufficient for the edge rather than the repetition rate. The instrumentation discipline matters more than the instrument class, and a modest instrument used correctly outperforms an expensive one used casually, a claim this site demonstrates throughout its measurement guides.
Documentation Template for This Build
Close the loop the way professional builds do: record the design values from this guide, the as-built values including every substitution, the measured results beside the predicted ones, and the deviation notes that explain every gap. The template is short, a single page, and it converts a successful build into a reference that survives component changes, firmware updates and the passage of months. Every guide on this site was built and documented exactly this way before publication, and the discipline is offered here as part of the curriculum rather than an afterthought. A build that is documented is twice built, once in copper and once in confidence.
Last updated 23 August 2026