ProcirelSizing Rectifier Filter Capacitors Without Ripple Regret
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.
1. Compute peak DC (VAC × 1.414 − 2 diode drops)
2. Choose ripple budget above regulator dropout
3. Apply C = I·t ÷ ΔV and round up a standard value
4. 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)](/tools/capacitor-code) and [RC Time Constant](/tools/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](/tutorial/power-batteries-complete-guide) pillar page for the full map, or continue with [supply families](/tutorial/smps-vs-linear-supplies) and [regulator choice](/tutorial/linear-vs-switching-regulators). For the arithmetic, open the [Capacitor Code (3-Digit)](/tools/capacitor-code) or [RC Time Constant](/tools/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)](/tools/capacitor-code) and [RC Time Constant](/tools/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](/tutorial/power-batteries-complete-guide)
– Read next: [bms design tutorial: battery management systems explained](/tutorial/bms-design-tutorial)
– Also in this track: [solar mppt explained: maximum power point tracking](/tutorial/solar-mppt-explained)
– Continue with: [linear vs switching regulators: choosing correctly](/tutorial/linear-vs-switching-regulators)
– Calculate as you go: [runtime estimator](/tools/battery-life) · [regulator designer](/tools/lm317-regulator) · [capacitor code tool](/tools/capacitor-code)
– 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.