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How to Size a Smoothing Capacitor
Heshan Fernando
Co-founder & COO
A transformer and a bridge rectifier give you a pulsating DC that touches zero twice per mains cycle. A capacitor across the output fills in those gaps by discharging into the load while the rectifier isn’t conducting.
How much it sags between peaks is the ripple, and sizing the capacitor is about deciding how much sag you can tolerate.
The ripple calculation
Between rectifier peaks, the capacitor supplies the load alone. The voltage it loses is:
V_ripple ≈ I ÷ (f_ripple × C)
Where I is load current, C is capacitance, and f_ripple is the ripple frequency.
At 500 mA into 2200 µF on a full-wave rectifier from 50 Hz mains: 0.5 ÷ (100 × 0.0022) ≈ 2.3 V peak-to-peak.
Note the ripple frequency is 100 Hz, not 50. A full-wave rectifier produces a peak on both halves of the mains cycle, so the capacitor only has to bridge half as long as it would with a half-wave rectifier.
That’s a genuinely significant advantage: a half-wave rectifier needs roughly twice the capacitance for the same ripple, which is a large part of why full-wave bridges are standard.
The number that actually matters
Ripple in isolation isn’t the design constraint. What matters is the minimum voltage — the bottom of the ripple trough — and whether it’s still high enough for whatever comes next.
With a linear regulator, that means the trough must stay above the regulator’s dropout voltage above its output. A 7805 needs perhaps 2 V of headroom; if the ripple trough dips below 7 V, the regulator falls out of regulation and the ripple appears on your supposedly clean 5 V rail.
V_min = V_peak − V_ripple
Design the ripple so that V_min clears the dropout requirement across the whole range of mains voltage variation, not just at nominal. A supply that regulates at 240 V and fails at 220 V is a supply that fails intermittently, which is the worst kind.
| Rectifier | Ripple frequency | Capacitance for same ripple |
|---|---|---|
| Half wave | 50 Hz (from 50 Hz mains) | ~2× larger |
| Full wave bridge | 100 Hz | Baseline |
Bigger isn’t free
Increasing capacitance reduces ripple. It also increases the inrush current at switch-on, because an uncharged capacitor briefly looks like a short circuit.
That inrush stresses the rectifier diodes, can weld relay contacts, and on a large supply will trip a breaker at the moment of switch-on. Which is why substantial supplies use an inrush limiter — an NTC thermistor or a relay-bypassed resistor — rather than simply fitting the biggest capacitor available.
There’s also a practical ceiling: at some point a bigger capacitor costs more, takes more space, and buys less ripple reduction than moving to a switching regulator would.
Common mistakes to avoid
- Using 50 Hz as the ripple frequency for a full-wave rectifier.
- Sizing for average load when the peak load is much higher.
- Checking ripple and never checking the trough against regulator dropout.
- Fitting a very large capacitor with no inrush limiting.
- Ignoring the capacitor’s ripple current rating, which is a real limit — an electrolytic runs hot and fails early if it exceeds it.
How to do it with Power Supply Ripple Calculator
The Power Supply Ripple Calculator solves in either direction.
- Enter the load current at its maximum, not typical.
- Set the capacitance, or set a ripple target and solve for it.
- Check the ripple trough against your regulator’s dropout requirement.
- Verify at the low end of your mains voltage tolerance, not just nominal.
Other electronics calculators are in the tools directory.
Frequently asked questions
Why is the ripple frequency double the mains frequency?
Because a full-wave rectifier produces a peak on both halves of the cycle, halving the discharge time. That’s why full-wave needs far less capacitance than half-wave for the same ripple.
What ripple is acceptable?
It depends on what follows. A linear regulator needs the trough above its dropout voltage; without a regulator, the ripple appears directly in your output.
Does a bigger capacitor always help?
It reduces ripple and increases inrush current at switch-on, which stresses the rectifier and can trip breakers. Very large smoothing capacitors often need an inrush limiter.
Final thought
Calculate the trough, not the ripple. The ripple figure is interesting; whether the minimum voltage clears dropout is what determines if the supply works.