Every time you switch off current through an inductor — a relay coil, transformer primary, brushed motor, or even a long PCB trace — the collapsing magnetic field tries to keep current flowing. With nowhere to go, it generates a voltage spike governed by V = L·di/dt. Open a switch fast enough on a 100 mH coil carrying 1 A, and you can easily see hundreds of volts ringing across your switching device. That's how MOSFETs die.

A snubber absorbs that energy and damps the resulting ringing. Three flavors you'll meet most often:

• RC snubber — a resistor and capacitor in series, placed across the switch (or across the inductive load). The cap absorbs the spike's energy; the resistor dissipates it and damps the LC ringing between load inductance and parasitic capacitance.
• RCD snubber — adds a diode so the cap charges quickly through the diode but discharges slowly through the resistor. Common in flyback converters where you must clamp the leakage-inductance spike across the primary MOSFET.
• TVS or MOV — a transient voltage suppressor clamps hard above a threshold. Simple, but lossy and can mask design problems.

Concrete example: a 24 V relay with 200 mH coil, 50 mA holding current, switched by a small N-channel MOSFET. When the FET turns off in ~100 ns, di/dt ≈ 5×10⁵ A/s and the spike potential is L·di/dt ≈ 100 kV (clamped in practice by parasitics, but easily 200+ V). A flyback diode across the coil is the cheapest fix; add a 100 Ω + 10 nF RC across the FET drain–source to kill the residual ringing that EMI tests will otherwise flag.

Rule of thumb for RC snubbers: measure the parasitic ringing frequency f_r with the snubber off. The parasitic capacitance is roughly C_p ≈ 1/((2π f_r)²·L). Pick C_snub ≈ 3–10× C_p, then choose R = √(L/C_snub) for critical damping. Verify the resistor's power rating: P ≈ ½·C_snub·V²·f_switch. At 100 kHz with 10 nF and 50 V, that's already 1.25 W — don't size it casually.

Place the snubber physically close to the switching node. A snubber three centimeters away with a long ground return is barely a snubber at all — its own loop inductance defeats the purpose.