Protecting Switches from Inductive Spikes

 

Protecting Switches from Inductive Spikes

A Beginner's Guide

Safety note: Several circuits in this guide involve mains voltage (200-240 V AC), which can be lethal. Use appropriately rated components, keep high-voltage sections enclosed and insulated, never probe live mains circuits casually, and use proper isolation and safe measurement practices (for example, a differential probe or an isolating transformer with a properly rated scope setup). If you are unsure, don’t work on live circuits.

1. The one idea you need

A coil (inductor) resists sudden changes in current.

While current flows, the coil stores energy in its magnetic field. When you switch OFF, that energy needs somewhere to go. Without a suitable current path or voltage clamp, the inductor can generate a large voltage to keep the current flowing. The voltage may rise until a protection device conducts, a parasitic path carries the current, or something breaks down, potentially damaging the switch or insulation.

Think of a long pipe of fast-flowing water. Slam the valve shut and you get a pressure spike (“water hammer”). Protection circuits give the water somewhere to go.

Key question: Every protection circuit answers one question: when the switch turns OFF, where does the stored energy go?

2. The tools

Tool	Its job	Think of it as
Flyback diode	Lets DC coil current circulate in a loop	A bypass road
Zener / TVS diode	Limits voltage to a ceiling	A pressure relief valve
RC snubber	Damps ringing and reduces rapid voltage changes	A shock absorber
RCD clamp	Captures a spike in a capacitor, bleeds it off through a resistor	A catch basin with a drain
MOV	Clamps voltage surges on suitably rated AC or DC circuits	A relief valve that can degrade with repeated or severe surges

 

Zener vs TVS: a Zener is a general voltage-limiting part. A TVS is specifically designed for transient-voltage suppression and has specified breakdown, clamping-voltage and pulse-power characteristics. Its actual energy capability depends on the device and the pulse conditions. For power circuits, check the clamping voltage at your peak current, not just the nominal breakdown voltage.

Frequency: you don’t choose a topology by frequency alone. Pick it by what the energy must do. Then use frequency to size the parts, because the faster you switch, the more often energy is dissipated (clamp losses rise with frequency).

3. How to choose: three questions

Question 1: Is there an obvious inductor (coil, motor, relay, solenoid, transformer)?

•     Yes: continue.

•     No: you may not need an inductive clamp, but wiring inductance, capacitive loads and parasitic LC circuits can still cause spikes or ringing. Add protection if a measured waveform or the component ratings show a problem.

Question 2: Is the load DC or AC?

•     DC coil: diode, Zener/TVS or a combination.

•     AC coil: never connect a plain diode directly across an ordinary AC coil. Depending on the application, use a suitably rated RC snubber, MOV or bidirectional TVS. Check the standoff voltage, clamping voltage and energy rating against the coil’s normal operating voltage.

Question 3: Where is the energy supposed to go?

Goal	Typical choice
Simple suppression, slow relay release	Flyback diode across the DC coil
Faster coil-current decay and relay release	Diode in series with a Zener or TVS, keeping the voltage within the switch’s rating
Electrically brake a motor	A proper motor-driver or braking circuit. Stopping time also depends on inertia, load and friction.
Transfer to a secondary winding (flyback converter)	The secondary rectifier is the intended path. An RCD clamp or TVS limits the extra spike from leakage inductance.
Release as a high-voltage pulse (ignition coil)	No plain diode. Use a suitably designed ignition driver with controlled voltage clamping, or a self-clamping ignition IGBT.
Ringing or fast voltage edges	RC snubber

4. Two things people mix up

dv/dt vs ringing

•     dv/dt is how fast voltage changes. A steep edge is a high dv/dt.

•     Ringing is voltage that wobbles after a sharp edge, like a plucked string. It comes from stray inductance and capacitance forming a tiny resonant circuit.

A steep edge can exist without ringing, and ringing contains steep edges. An RC snubber adds damping and limits the rate of voltage change, which helps with both, though it won’t necessarily remove them completely.

Why TRIACs may need snubbers

A TRIAC has two weak spots:

1.       False triggering (static dv/dt). A fast voltage jump on the line can turn it ON with no gate signal. This can happen even with a resistive load.

2.       Retriggering during turn-off (commutating dv/dt). When the current falls below the holding current, the TRIAC attempts to turn OFF. With an inductive load, the voltage across it may rise rapidly during recovery. If the commutating dv/dt exceeds the device’s capability, the TRIAC may retrigger.

Resistive loads cause fewer commutation problems than inductive ones, but whether you need a snubber depends on the TRIAC, the circuit and the electrical noise around it. A snubber is not a universal cure. Sometimes you also need a TRIAC with a higher dv/dt rating, or changes to the gate network.

5. RC debounce vs RC snubber

•     Debounce (Arduino button): smooths a noisy signal so one press reads as one press. Tiny parts, usually with a Schmitt-trigger input.

•     Snubber: damps power transients across a switch or load. The parts must handle real voltage, current and energy.

6. Worked examples

Circuit	What happens at turn-off	Protection
DC relay + MOSFET	Coil current needs a loop	Flyback diode across the coil
Fast-release relay	Same, but quicker decay wanted	Diode + Zener/TVS in series
Flyback converter	Magnetizing energy transfers to the secondary during the switch’s OFF-time, while leakage inductance can produce an additional voltage spike across the primary switch	Secondary rectifier for the intended energy, RCD clamp or TVS on the primary for the leakage spike
TRIAC dimmer + motor	Steep voltage step across the TRIAC	RC snubber, checked against the TRIAC’s ratings
Ignition coil	Rapid current interruption produces the high voltage needed for the spark	A suitably designed ignition driver with controlled voltage clamping, never a plain flyback diode

7. Common mistakes

1.       A plain diode across an ignition coil. The current decays slowly and the spark dies.

2.       A diode across an AC load. It conducts every half-cycle.

3.       A diode across a flyback transformer primary. It blocks energy transfer to the secondary.

4.       Assuming “X2” means “suitable”. Across mains, use a safety-rated capacitor (typically Class X2), but also check its AC voltage, pulse and surge ratings for your circuit. Use a pulse-rated resistor too.

5.       Ignoring snubber leakage. The capacitor passes a small current even when the TRIAC is OFF, so LED lamps and small loads can glow faintly or flash.

6.       Adding protection “just because there’s an inductor.” Add it when the real waveform or ratings call for it.

7.       Assuming every joule becomes heat in the clamp. In a converter, most of the energy goes to the output.

8. Two stages of design

1.       Pick the topology. Start by reading the schematic and identifying the intended current and energy paths. You can often choose an initial protection approach without measurements.

2.       Size the parts and verify. The stored energy in an ideal inductor is E = ½LI² (L in henries, I in amps, E in joules). In a real circuit that energy is shared between the clamp, winding resistance, the switch, parasitics and any intended output path. Then account for voltage limits, switching frequency and average power dissipation, and check the real waveform, because a schematic alone won’t show parasitic inductance or the true size of the transient.

 

TRIAC snubber starting point: about 100 Ω + 0.1 µF is a common value for some 200-240 V AC designs, but it is not universal. Check the TRIAC’s static and commutating dv/dt ratings, the load’s inductance and power factor, capacitor discharge current at turn-on, resistor pulse rating, and leakage when OFF. Verify with an oscilloscope, following the safety note above.

9. The summary

•     Diode: give DC coil current a place to go.

•     Zener/TVS: set a maximum voltage.

•     RC: damp ringing and reduce rapid voltage changes.

•     RCD: catch a leakage spike and burn it off.

•     Combine when needed, for example an RCD clamp for the big spike plus a small RC snubber for ringing.