Size an RC snubber for a switching transistor, a contact buffer for a relay, the bootstrap capacitor for a high-side gate driver, or a surge capacitor for contact protection — all in one tool. Every result is computed live from the same energy-balance and vendor-guide formulas used in power-electronics practice, with the practical margins (E12 rounding, X7R derating) made explicit.
Four Protection Modes, One Tool
RC snubbers · relay contact buffers · bootstrap caps · surge caps. All live, all mobile-ready.
Snubber Capacitance C = Lp·I²/ΔV²
10nF
Peak clamp voltage: 500 V
Loop inductance used: 1 µH
Snubber Resistor R = √(Lp/C)
10Ω
ζ ≈ 0.5 · use 2×R for ζ ≈ 1
Resistor Power C·ΔV²·fsw
2W
Use a 2–3× derated non-inductive part
Energy per Turn-off
50µJ
½·Lp·I² absorbed by the snubber
Recommended Fit
10nF
Next E12 up · ×2 margin → 22 nF
📌 Core formulas:C = Lp·I²/ΔV² (energy balance ½LpI² = ½CΔV²) · R = √(Lp/C) · P = C·ΔV²·fsw. The ×2 margin covers X7R DC-bias derating if you choose a ceramic instead of film.
The Physics
Why Switching Transients Demand a Snubber
Every power-switching loop has parasitic inductance. The bus capacitor's leads, the PCB traces, the emitter and collector busses and even the transistor package itself form a current loop with a stray inductance Lp that no layout can fully eliminate. While the switch conducts, a current I flows through that inductance and the energy ½·Lp·I² sits stored in its magnetic field. When a MOSFET or IGBT turns off in 20–100 ns, that current cannot stop instantaneously — the only path left is into stray capacitance, and the loop begins to ring. The voltage across the switch climbs to Vbus + Lp·di/dt before the ring even starts: interrupt 10 A in 1 µH within 50 ns and Faraday's law adds 1 µH × 10 A ÷ 50 ns = 200 V on top of a 400 V bus — before any LC overshoot.
Stray capacitance across the switch then turns that energy into a classic LC tank. The ring frequency is f = 1/(2π√(Lp·C_stray)) — typically 5–50 MHz in a compact layout — and its first peak can exceed the transistor's avalanche or breakdown rating. Even when the switch survives, every cycle radiates broadband EMI in exactly the band that conducted-emission scans sweep, and the steep dv/dt couples through the Miller capacitance Cgd to momentarily turn the device back on. A snubber — a capacitor in series with a resistor placed directly across the switch — gives that loop energy somewhere harmless to go: the capacitor absorbs the inductor's energy and holds the first peak near Vbus + ΔV, while the resistor dissipates it and damps the ring dead.
💡 Tip: measure the ring before you design the fix. Probe across the switch with a short ground lead, note the ringing frequency and amplitude, then let this calculator turn your allowed overshoot ΔV into a capacitance. Place the snubber within 10–15 mm of the switch terminals — a long snubber loop simply adds more Lp.
⚠️ Warning: ceramic snubber caps derate hard under DC bias. An X7R part marked 100 nF can fall to 50–70 nF at its working voltage, and high temperature makes it worse. Choose a voltage rating at or above the peak clamp voltage, and prefer film where the capacitance value must actually hold.
❌ Don't: never fit a bare capacitor across a switching transistor without the series resistor — you build an undamped LC tank that rings worse than the layout did. And don't copy a value from another board: with C ∝ I²/ΔV², doubling the interrupted current quadruples the capacitance you need.
The Math
The RC Snubber Formula, Term by Term
The Switch RC Snubber mode above solves three small equations. Understanding what each symbol means tells you exactly which knob to turn when the prototype misbehaves — and which knob does nothing at all.
Snubber capacitance: C = Lp·I²/ΔV²
The formula is the energy balance ½·Lp·I² = ½·C·ΔV² rearranged: the snubber capacitor must absorb the entire energy stored in the stray inductance while its own voltage rises by no more than the overshoot you allow. Lp is the loop inductance in henries — measured, simulated, or estimated at roughly 10 nH per centimetre of current loop. I is the current flowing at the instant of turn-off — the peak being interrupted, not the RMS load current. ΔV is the overshoot you tolerate above the bus: 100 V on a 400 V bus means the clamp engages at 500 V. C is the snubber capacitance that makes the two energies equal.
C = Lp · I² / ΔV²
Note the quadratic sensitivities: doubling the interrupted current quadruples the capacitance, and halving the allowed overshoot doubles it. Frequency does not appear — switching frequency only enters through resistor heating below. Example: Lp = 1 µH, I = 10 A, ΔV = 100 V → C = (1×10⁻⁶ × 100) / 10 000 = 10 nF.
Snubber resistance: R = √(Lp/C) — damp, don't resonate
The series resistor sets the damping of the Lp–C tank. A loop's characteristic impedance is Z₀ = √(Lp/C), and the calculator returns R = Z₀ as the starting point: for the defaults that is √(1 µH / 10 nF) = 10 Ω — which is also, not coincidentally, ΔV/I. At this value the damping ratio of the series loop is ζ ≈ 0.5, so the first overshoot is absorbed and the residual ring dies within about one cycle. If you need the overshoot itself suppressed to near zero, double the resistor to 2·√(Lp/C) for critical damping (ζ ≈ 1): the clamp engages slightly later and dissipates exactly the same energy.
R = √(Lp / C) · critical damping: R = 2·√(Lp/C)
What you must never do is pick R ≈ 0 or R ≫ Z₀. Too small and the tank still rings, with the resistor carrying huge pulse currents; too large and the capacitor charges too slowly, so the initial spike leaks past the clamp before it engages. Damping is the whole point — the resistor converts the ring energy to heat, it does not tune the tank.
Resistor power: P = C·ΔV²·fsw
Each switching cycle the snubber capacitor charges to ΔV and discharges back, and both ½·C·ΔV² halves end up as heat in the resistor — hence P = C·ΔV²·fsw. The defaults give 10 nF × (100 V)² × 20 kHz = 2 W: a quarter-watt resistor would burn open in seconds, so this calculation exists to be taken seriously. Fit a 3–5 W part for the worked example below.
P = C · ΔV² · fsw
Use non-inductive wirewound or metal-plate resistors: a few microhenries of winding inductance reintroduce the very Lp you are fighting, push the resistor's own resonance into the ring band, and turn your snubber into a small antenna. Also check the part's pulse/overload rating — the average power can look fine while the turn-off current step exceeds it on every cycle.
Step by Step
Worked Example: 400 V Bus, 10 A Switch, 1 µH Loop
These are the calculator's default inputs, and the arithmetic below is exactly what the tool computes live — change any input and every line updates.
Step 1 — Stray energy. The loop stores E = ½·Lp·I² = 0.5 × 1 µH × (10 A)² = 50 µJ at the moment the switch turns off. That is the entire budget the snubber must swallow every cycle.
Step 2 — Allowed overshoot. The bus sits at 400 V and we accept ΔV = 100 V, so the clamp must hold the switch at 500 V peak — pick a device rated ≥ 600 V and a capacitor rated ≥ 630 V.
Step 3 — Capacitance. C = Lp·I²/ΔV² = (1×10⁻⁶ H × 10² A²) / (100 V)² = 10 nF. Ten nanofarads absorbs 50 µJ while rising exactly 100 V.
Step 4 — Resistance. R = √(Lp/C) = √(1 µH / 10 nF) = 10 Ω, which damps the ring at ζ ≈ 0.5. Double it to 20 Ω if the scope still shows a second overshoot and you want ζ ≈ 1.
Step 5 — Resistor power. P = C·ΔV²·fsw = 10×10⁻⁹ F × 100² V² × 20×10³ Hz = 2 W continuous. A 3–5 W non-inductive wirewound is the correct fit, mounted with airflow in mind.
The bill of materials writes itself: a 10 nF (already an E12 value) film capacitor rated 630 V or more — or a 22 nF part to cover the ×2 margin if a DC-bias-derating X7R ceramic is fitted — in series with a 10 Ω, 3–5 W non-inductive resistor, placed within centimetres of the switch. The recommended-fit card in the calculator shows the same E12 rounding and the ×2 margin note automatically.
Contact Life
Relay Contact Protection: Taming the Arc
When relay contacts open an inductive load — a solenoid, motor winding, transformer or even a long cable — the load inductance keeps pushing current through the shrinking gap. The voltage across the gap rises until the air ionizes and an arc strikes; on a 24 V DC relay breaking 1 A, that arc can sustain for milliseconds and reach temperatures that erode contact plating in a single event. Each operation blackens the surface, and the mechanism eventually welds shut — especially on make, where contact bounce lets the arc re-strike half a dozen times before the contacts settle. An RC network across the contacts gives the load current somewhere to flow after the contact opens: the capacitor carries the transient while the contacts separate, and the resistor limits the capacitor's discharge current when the contacts reclose.
The Omron sizing rule
Rather than solving loop equations for every relay, the industry leans on a rule published in Omron's relay technical FAQ (FAQ02804): size the buffer capacitor at 0.5–1 µF per 1 A of contact current, and the series resistor at 0.5–1 Ω per 1 V of contact voltage. For a 1 A / 24 V contact that is 0.5–1 µF and 12–24 Ω, with the midpoint suggestion of 0.75 µF + 18 Ω as a sensible first build. The Relay Contact Buffer mode above applies the same rule to any current and voltage pair, and the quick-reference table below lists the common contact currents. Always verify against the source: Omron FAQ02804.
AC versus DC, and polarity notes
On AC contacts the arc self-extinguishes at each zero crossing, so the RC network's job is limiting the first strike and the bounce re-ignition; place the RC directly across the contact, and for mains voltages use an X2-rated (275 VAC) metallized film capacitor — never a DC electrolytic, which has no place across an AC line in any polarity. On DC contacts there is no zero crossing, so protection matters more: a freewheel diode across the load is the classic fix but it delays release, and the RC-across-contact or RC-plus-zener options trade a little release delay for genuine arc suppression. In all DC cases use a non-polarized film or MLCC across the contacts — an electrolytic installed with reversed polarity on a DC line will vent — and remember that the series resistor also discharges the capacitor before the next closure, limiting the make-inrush that a bare capacitor would dump into fresh contacts.
Every number in this table is computed arithmetically from the cited Omron rule — 0.5–1 µF per ampere of contact current, 0.5–1 Ω per volt of contact voltage — not from measured data. Pick the row nearest your contact current and the column matching the voltage being switched; intermediate currents scale linearly between rows.
Contact Current
C Range (µF)
R Range @ 24 V (Ω)
R Range @ 230 V (Ω)
0.5 A
0.25 – 0.5
12 – 24
115 – 230
1 A
0.5 – 1
12 – 24
115 – 230
2 A
1 – 2
12 – 24
115 – 230
5 A
2.5 – 5
12 – 24
115 – 230
Note: R depends only on the contact voltage in this rule, so the R columns repeat down the table — that is the rule working as published, not a typo. The calculator's Relay Contact Buffer tab reproduces any intermediate row live.
High-Side Drive
Bootstrap Capacitors for High-Side Gate Drivers
High-side switches need a gate drive referenced to their own source or emitter — a node that rides up and down with the switching point. The IR2110 family of floating drivers, and its many descendants, solve this with a bootstrap supply: a diode from a low-voltage rail charges a floating capacitor Cboot whenever the low-side switch is on and the switching node pulls low; when the high-side switch turns on, that capacitor powers the floating driver and delivers the gate charge Qg the MOSFET needs. There is no isolated supply behind it — every joule the high-side gate consumes must have been stored in Cboot beforehand.
The refresh cycle and droop
Each high-side pulse draws Qg out of Cboot, plus the driver's own quiescent charge, so the floating rail sags by ΔVbs = Qg/Cboot between refreshes. If ΔVbs grows past a few hundred millivolts, the delivered gate voltage falls below the Miller plateau and the MOSFET switches in its linear region — slow, hot and lossy. The hard floor is therefore Cboot_min = Qg/ΔVbs: with Qg = 30 nC and an allowed droop of 0.5 V, the minimum is 60 nF. Long on-times (high duty cycle, low switching frequency) stretch the discharge window, which is why this calculation yields a minimum, not a target.
Why the 2× and 10× ladder exists
Infineon's AN-978 — the reference application note for these floating drivers — recommends fitting at least twice the calculated minimum. The further multiplier is physics rather than conservatism: MLCCs lose capacitance under DC bias, and a 25 V X7R sitting at 12–15 V can shed 20–40% of its nominal value, while elevated temperature multiplies leakage and the driver's standby draw. The Bootstrap Capacitor mode therefore shows three numbers: the strict minimum, the AN-978 ≥2× value, and a practical 10× recommendation. For 30 nC gates that ladder reads 60 nF → 120 nF → 600 nF — i.e. fit a 680 nF or 1 µF X7R rated 25 V or more. Source: Infineon/IR AN-978.
The Quiet Leaker
Surge (Contact) Capacitors and Their Leakage Current
A surge capacitor — Omron's relay literature calls the whole network a contact surge suppressor — is simply a capacitor placed across a set of contacts, or from each contact to frame, to absorb the leading edge of a switching surge and limit the dv/dt the contacts and downstream electronics see. You will find them across relay and contactor contacts, across switch contacts in test and telecom gear, and as the C leg of RC snubbers on thyristors and triacs. Unlike a switch snubber, a surge capacitor usually works at mains frequency and sits idle across the contacts for hours at a time.
That idle state is exactly where its leakage current matters. A capacitor across the contacts is a reactance at mains frequency, and it passes I = 2πf·C·V continuously even when the load is switched off. The Surge (Contact) Capacitor mode evaluates this for you: 100 nF across 230 V at 60 Hz passes 8.7 mA — enough to keep LED pilot lamps glowing, hold in a small relay, or trip a 10 mA residual-current device. Size the capacitor for the contact current using the Omron range shown in the mode, then read the leakage before committing: the very value that protects the contact can also make the "off" state leak.
⚠️ Remember the resistor: a capacitor alone across the contacts limits inrush poorly on inductive loads — the unresisted LC path can even intensify the make-bounce arc. Omron explicitly recommends the resistor in series with the capacitor; keep it, sized at 0.5–1 Ω per volt of contact voltage.
Component Selection
Choosing the Capacitor Family for Each Mode
Film for switch snubbers. Metallized polypropylene or PET film is the default: capacitance holds within ±5–10% under DC bias and temperature, ESR and ESL are low, dV/dt ratings are high, and self-healing metallization clears pinhole faults instead of shorting. Rate the film at least 1.5–2× the peak clamp voltage — a 630 V DC part for the 500 V clamp in the worked example. C0G/NP0 ceramics are electrically excellent but impractically large and expensive at tens of nanofarads; X7R is acceptable only with the ×2–×3 derating the calculator flags.
Film again for relay buffers — safety-rated on AC. The same film virtues apply to contact protection, and the values land in the microfarad range, so box film parts in 630 V DC or 275 VAC are typical. Across AC mains the safety classification rules the choice: X2 (275 VAC) metallized film is mandatory, and polarization is not negotiable — never an electrolytic across an AC line, never an electrolytic reverse-biased on DC.
X7R MLCC for bootstrap capacitors. Here ceramics win: the values are small (hundreds of nanofarads), the voltage is low (12–18 V floating rail on a 25–50 V part), and the packages are tiny. Choose X7R or X5R in 1206 or larger to limit DC-bias derating, avoid Y5V/Z5U dielectrics entirely — they can lose 80% of their capacitance — and add a small film or C0G in parallel if the switching node's dV/dt is extreme and high-frequency stability matters.
Safety-rated film for surge capacitors. Anything that sits across mains belongs in the X2/Y2 classes: 275 VAC X2 metallized polypropylene from 100 nF to 2.2 µF is the standard contact-protection building block. Whatever family you pick, run the leakage check in the surge mode first — it is the number most often forgotten.
Every mode on this page shares one engine: the snubber capacitor calculator on our homepage — the same live, in-browser tooling that powers the full design suite on our calculators hub, from RC timing and filtering to DC-link sizing. Bookmark both: transient-protection values change whenever the layout, the load or the switching frequency changes, and re-entering three numbers here is far faster than re-deriving the formulas by hand.
Snubber & Bootstrap Capacitor FAQ
How do I calculate a clamping (snubber) capacitor for an IGBT?▼
Use the energy balance ½·Lp·I² = ½·C·ΔV². Take the parasitic loop inductance Lp (measure the ring, simulate the layout, or estimate ≈10 nH per cm of current loop), the turn-off current I, and the overshoot ΔV you accept above the bus, then C = Lp·I²/ΔV². Example: Lp = 1 µH, I = 10 A, ΔV = 100 V gives C = 1 µH × 100 / 10 000 = 10 nF. The series resistor is R = √(Lp/C) = 10 Ω and it dissipates P = C·ΔV²·fsw — at 20 kHz that is 2 W. Round C up to the next E12 value and double it if you fit a DC-bias-derating ceramic instead of film.
What value RC snubber should I use across a relay contact?▼
Follow the Omron rule from FAQ02804: 0.5–1 µF of capacitance per 1 A of contact current, and 0.5–1 Ω of series resistance per 1 V of contact voltage, with C and R in series across the contacts. A 1 A, 24 V contact therefore gets 0.5–1 µF in series with 12–24 Ω (a sensible first build is the midpoint 0.75 µF + 18 Ω). A 230 VAC contact needs 115–230 Ω and the capacitor must be an X2-rated film type — never a DC electrolytic. Mode 2 of the calculator above scales the rule to any current and voltage pair.
How do I calculate the bootstrap capacitor for an IR2110?▼
Start from the charge balance: Cboot_min = Qg / ΔVbs. A MOSFET with Qg = 30 nC and an allowed floating-rail droop of 0.5 V needs 60 nF as the strict minimum. Infineon's AN-978 recommends fitting at least twice that (120 nF), and because an X7R MLCC loses 20–40% of its capacitance under DC bias at elevated temperature, the practical fit is 10× the minimum — 600 nF, so a 680 nF or 1 µF X7R rated 25 V or more. Re-check the droop if your duty cycle exceeds roughly 80% or the switching frequency drops below a few kHz, since the refresh window then stretches.
What is a surge capacitor?▼
A capacitor placed across relay or switch contacts (or from each contact to frame) to absorb the leading edge of a switching surge and limit the dv/dt the contacts see. It sits across mains permanently, so it passes a reactive leakage current I = 2πf·C·V even when the load is off: a 100 nF part across 230 V at 60 Hz passes 8.7 mA — enough to keep LED pilot lamps glowing, hold in a small relay or trip a 10 mA RCD. Omron's guidance still calls for the series resistor (0.5–1 Ω per volt), because a bare capacitor limits inrush poorly on inductive loads.
Why does my snubber resistor get hot?▼
Because it is doing its job every switching cycle: P = C·ΔV²·fsw. With C = 10 nF, ΔV = 100 V and fsw = 20 kHz the resistor dissipates 2 W continuously — far beyond a generic ¼ W part. The heating scales linearly with switching frequency and with the square of the allowed overshoot, so raising ΔV by 50% costs 2.25× the power. Fit a 3–5 W non-inductive wirewound or metal-plate resistor, verify its pulse/overload rating (not just average power), and expect the part to run warm — that heat is the ring energy leaving your circuit for good.
Can I omit the snubber resistor and use just a capacitor?▼
Not across a switching transistor. Without R, the snubber capacitor and the parasitic inductance form an undamped LC tank: the energy simply swaps between L and C and the ring reappears at f = 1/(2π√(Lp·C)). Damping requires resistance on the order of the loop's characteristic impedance √(Lp/C), and true critical damping (ζ = 1) takes 2√(Lp/C). The partial exception is the relay-contact surge capacitor — cap-only networks exist there — but even in that case Omron recommends the series resistor, mainly to tame the capacitor's own discharge inrush when the contacts reclose.
Traceability
Sources & Notes
1. Omron — Relay contact protection rule (FAQ02804). The 0.5–1 µF per ampere and 0.5–1 Ω per volt guidance behind Mode 2, the quick-reference table, and Mode 4's recommended C range: ia.omron.com/support/faq/answer/36/faq02804/
2. Infineon/IR — AN-978, “HV Floating MOS-Gate Driver ICs”. The bootstrap-capacitor guidance behind Mode 3's ≥2× and 10× ladder: infineon.com/dgdl/an-978.pdf
Traceability. The relay quick-reference table is generated arithmetically from the Omron rule quoted above; the bootstrap ladder follows AN-978's derating guidance; and every RC-snubber number derives from the energy-balance equations shown in this page, matching the worked example line for line.
The 10 nH/cm rule is an estimate. The loop-inductance shortcut used in Mode 1 — Lp ≈ 10 nH per centimetre of current loop — is a widely used PCB and wiring estimate, not a measurement. It is labeled as such in the tool; for production designs, measure the actual ring or extract the loop inductance from your layout.
Scope of this page. This tool covers switch-transient and contact protection. For RC timing (τ = R × C and the 5τ settling rule), use the RC time constant calculator; for signal filtering (cutoff frequency and roll-off), use the RC filter capacitor calculator.
Why this helps you. Everything computes live in your browser — no sign-up, nothing leaves your device — and every formula is printed next to the number it produces, so you can audit the result before it reaches your bill of materials.
Maintained by the CapacitorCalculator.com editorial team · Last updated: November 2, 2026
The snubber protects the switch; the switch feeds the bus. Pair this tool with the DC link capacitor calculator to size the bus capacitance the snubbered switches draw from, so both halves of the power stage are covered.