RC snubber design calculator

When a relay contact or MOSFET interrupts current through an inductive load, the collapsing magnetic field drives a high voltage spike that can arc the contact, avalanche the FET, or couple noise into nearby circuitry. An RC snubber—series resistor and capacitor placed across the switch—absorbs that inductive energy and damps the ring so the voltage rise stays within a design peak. This RC snubber design calculator takes the three numbers every bench design already has on the schematic: supply voltage, interrupted load current, and load inductance, optionally a higher allowed peak clamp voltage and a switching frequency for average resistor power. It applies the inductive-energy method C equals L times I squared over V peak squared and R equals V peak over I, which also equals the characteristic impedance square root of L over C, and returns cleaned farad and ohm values plus stored energy and a recommended discrete power rating.

How the inductive-energy RC snubber is sized

At turn-off the inductor current cannot change instantly, so the energy stored in the magnetic field, one half L I squared, must go somewhere. With an RC snubber across the opening switch, that energy is steered into the snubber capacitor. Setting one half C V peak squared equal to one half L I squared and solving for capacitance gives the core design identity C equals L times I squared over V peak squared.

The series resistor is then chosen to match the characteristic impedance of the LC tank formed by the load inductance and the snubber capacitor: R equals V peak over I, which is identical to the square root of L over C when the energy identity holds. That match critically damps the resonance so the voltage does not ring for many cycles, while also limiting the capacitor discharge current when the switch closes again to roughly the original load current.

If you allow a higher peak clamp voltage than the supply—for example a MOSFET with avalanche headroom or a contact rated well above the rail—enter that larger max voltage so C shrinks proportionally to one over V peak squared and R rises with V peak. Optional switching frequency multiplies the single-event energy by f to estimate average power dumped in the resistor each cycle, P approximately equals E times f, which is the same as one half C V peak squared times f.

The algorithm is a handful of floating-point multiplications, divisions, and one square root, plus a lookup of the next common discrete power rating at or above twice the calculated dissipation when frequency is given, or a conservative floor from energy when frequency is omitted.

Where RC snubber calculators show up in real work

Relay and contactor manufacturers publish contact-protection application notes that walk engineers through choosing R and C so the arc energy at break stays within the contact rating; a trusted energy-balance calculator turns coil inductance and load current into first-pass part values before the prototype board is ordered.

Power electronics teams placing RC snubbers across MOSFET or IGBT drains on inductive half-bridges use the same identities with a higher allowed peak voltage set by the device avalanche rating or a parallel TVS, then record farads, ohms, and wattage in the bill of materials. Repair technicians reconstructing a burned snubber network from a failed industrial controller often only know the original supply rail and approximate coil inductance; a clean C and R estimate avoids another undersized metal-film resistor.

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