Induced voltage spikes

While coil current is steady, flux is steady — little induced EMF.

Interrupt current (open switch, transistor off, contact bounce) → flux collapses fast → large dΦ/dt → large induced voltage per e = −L (ΔI/Δt).

The minus sign is Lenz: induced EMF polarity opposes the decrease of current — it adds in series to push current through whatever path exists.

If the only path is air between opening contacts, voltage rises until arc ionizes the gap — spike energy goes into heat and pitting.

Spike magnitude depends on:

• Inductance L
• How fast current changes (Δt very small → huge EMF)
• Total resistance of the loop during collapse

Example logic: 1.5 H coil, current drops 2 A in 5 ms:

|EMF| ≈ L × ΔI/Δt = 1.5 × (2 / 0.005) = 600 V

Open switch with infinite gap resistance → inductor attempts any voltage needed to maintain current — arcs measured in kV on small control coils are common without suppression.

Resistor in parallel with coil (or coil+driver): when supply opens, induced current circulates through coil + resistor — series path.

Higher resistance → higher peak voltage, slower decay.

Lower resistance → lower peak, faster energy burn.

Trade heat in resistor vs contact damage. Often used on large DC contactors where diode drop is unacceptable for fast release timing.

Silicon diode across coil, cathode to positive side of coil (reverse-biased while coil energized).

Switch on: diode blocks; coil charges normally.

Switch opens: induced polarity forward-biases diode → current circulates through coil + diode → voltage clamped ~0.7 V (silicon) across the loop.

Energy dissipates as heat in coil wire and diode. DC only — AC reversals would short half-cycles through the diode.

Perfect for PLC-driven relays, solenoids, small DC motors field windings where release delay of one τ path is acceptable.

Metal oxide varistor (MOV) connects across coil (or supply lines). Normally megohms of resistance.

When voltage exceeds rating (e.g., 140 V on 120 V control), MOV resistance drops sharply in nanoseconds — clamps spike.

Used on AC relays, motor starters, line-side protection — conducts either polarity.

Choose rating above normal operating V but below insulation breakdown of sensitive parts.

Common on surge strips and across starter coils in MCC buckets.

Resistor + capacitor in series across contacts absorbs switching energy and limits dV/dt on solid-state outputs.

Tuned for coil L and supply — too small C → still arcs; too large → leakage or slow release.

Manufacturer kits often specify snubber module per contactor family.

Combine strategies on harsh loads: diode internal to DC coil driver + MOV at board input.

• Never drive bare coil from transistor/PLC output without suppression spec.
• Replace pitted contacts with root-cause fix — snubber missing beats filing contacts every month.
• DC diode polarity wrong = short on energize — verify cathode to +V side of coil.
• AC coil → no single diode; use MOV or RC rated for AC.
• Scope or transient recorder on first article if spikes exceed device absolute max.

Situation. New building: hundreds of 24 VDC damper actuators on one PLC output card. Within six months, 30% of outputs fail shorted. Coil drivers have no diodes; actuators are inductive 0.5 H class.

Root cause: Each off command generates spike across open collector; integrated driver dies before mechanical limit is reached.

Fix: External diode at each actuator (or relay with suppressed coil), verify 5τ release still meets control sequence, replace failed modules once — not monthly.