Snubber Circuit Diagram: RC Values, Placement, and Inductive Load Protection

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A snubber circuit suppresses voltage transients caused by rapid switching of inductive loads. This guide covers RC snubber design across relay contacts, TRIACs, and power transistors — with component value selection and placement rules.

When a switch, relay contact, TRIAC, or transistor interrupts current through an inductive load — a motor, solenoid, transformer primary, or contactor coil — the inductance resists the sudden cessation of current. According to the fundamental equation V = L × (dI/dt), a rapid change in current through an inductance generates a voltage spike that can be many times the supply voltage. In a 230V AC circuit, this transient can easily reach 1000–2000V and last for microseconds.

This transient has three damaging effects: it arcs across relay contacts, eroding and welding them; it causes false triggering or latch-up in TRIACs and SCRs; and it can exceed the voltage ratings of semiconductor devices in the same circuit, causing immediate failure or accelerated degradation.

The RC snubber (also called an RC suppressor or RC damper) addresses all three by placing a series resistor-capacitor network either directly across the switching device (device-side snubber) or across the inductive load terminals (load-side snubber). When the switch opens, the inductor's back-EMF drives current through the snubber's capacitor rather than across the open contacts. The capacitor absorbs the energy of the spike, and the resistor limits the rate of charge and provides damping to prevent LC ringing.

For relay contacts switching AC inductive loads, the snubber is typically placed across the contact set. Standard component values that work well across a broad range of general-purpose relay contact applications are R = 100 ohms and C = 0.1µF (100nF). These values limit the peak transient voltage while keeping the continuous current through the snubber (which flows even when the contacts are closed, since the snubber is always across the contacts) at a safe level for the resistor's power rating. Power dissipated in the resistor: P = (V² / R) × (X_C / (R + X_C)) — for most practical AC applications with R = 100Ω and C = 0.1µF at 50Hz, continuous dissipation is typically under 0.5W. Use a 1W or 2W resistor for margin.

For TRIACs and SCRs, the snubber is placed across the device (anode to cathode or MT1 to MT2). The snubber limits the rate of voltage rise (dV/dt) during commutation, which is critical because TRIACs can self-trigger (false fire) if dV/dt exceeds the device's rated dV/dt specification. Typical values: R = 33–100 ohms, C = 10–100nF. Exact values depend on the TRIAC's dV/dt rating and the inductance of the load — consult the TRIAC datasheet's application notes.

For DC circuits with relay contacts or MOSFETs switching inductive loads (motors, solenoids), the preferred suppression method is a freewheeling diode (also called a flyback diode) placed in anti-parallel across the inductive load. The diode clamps the back-EMF to approximately 0.7V above supply and provides a current path for the inductor's stored energy. An RC snubber can supplement a flyback diode in high-frequency switching applications but is typically secondary to the diode for DC relay contacts.

How to wire snubber circuit diagram

Identify the switching device and load type Determine whether the switching device is a relay contact, TRIAC, SCR, MOSFET, IGBT, or BJT. Determine whether the load is AC or DC, and whether it is inductive (motor, solenoid, coil, transformer) or resistive. Inductive AC loads with relay contacts or TRIACs are the primary application for RC snubbers. Resistive loads rarely require snubbers.
Determine snubber placement For relay contacts switching AC inductive loads: place the RC snubber directly across the contact set (in parallel with the contacts). For TRIACs or SCRs: place the RC snubber from MT2 (or anode) to MT1 (or cathode) — directly across the device terminals. Position the snubber physically as close to the device as possible to minimise stray inductance in the snubber leads.
Select capacitor value For general-purpose relay contact snubbers on 50Hz or 60Hz mains: C = 0.1µF (100nF) is a standard starting value. For TRIAC snubbers, calculate C from the load inductance and the TRIAC's rated dV/dt: C = I_line / (dV/dt_rated), where I_line is the load current and dV/dt is from the TRIAC datasheet. Always use X2-rated film capacitors for across-the-line applications. Voltage rating must exceed peak mains voltage with margin (use 400V or 630V rated capacitors for 230V AC mains).
Select resistor value For relay contact snubbers: R = 100 ohms is a standard starting value. The resistor limits the surge current into the capacitor at contact closure and damps LC ringing. Calculate power dissipation and select appropriate wattage. For TRIAC snubbers: R = sqrt(L/C) is the critically damped value derived from the load inductance (L) and chosen capacitor (C). Values typically range from 33 to 100 ohms. Use non-inductive resistors (metal film or carbon film — not wirewound).
Verify resistor power rating The resistor in a mains-connected snubber carries continuous current even when the contacts are closed (because the RC network is always connected in parallel with the load when the contacts are closed and in parallel with the open circuit when they are open). For R = 100 ohms on 230V AC at 50Hz with C = 0.1µF, the continuous dissipation is approximately 0.3–0.5W. Use a 1W or 2W resistor. Use metal film, not wirewound — wirewound resistors have inductance that degrades snubber performance.
Assemble and install the snubber Solder or crimp the resistor in series with the capacitor. For relay-contact snubbers, connect the series RC network across the contact terminals. For PCB-mounted relay contacts, add the snubber components directly on the PCB across the contact pads. Ensure the capacitor is rated for the application voltage and is X2-type for mains connections. Secure the assembly to prevent vibration-induced lead fatigue.
Test with an oscilloscope if available With an oscilloscope and a suitably rated differential probe, capture the voltage waveform across the switching device when switching the inductive load. Without a snubber, a large spike will be visible. With the snubber installed, the spike should be substantially reduced and the waveform should show damped ringing decaying quickly after switching. Adjust component values if the transient still exceeds the device's rated voltage or if ringing takes many cycles to decay.

Specifications

Typical snubber R for relay contacts (mains AC)	100 ohms, non-inductive (metal film or carbon film)
Typical snubber C for relay contacts (mains AC)	0.1µF (100nF), X2-rated film, 400V or 630V
Typical snubber R for TRIAC (mains AC)	33–100 ohms — depends on load inductance and TRIAC dV/dt rating
Typical snubber C for TRIAC (mains AC)	10–100nF, X2-rated film, 400V or 630V
DC inductive load — preferred suppression	Flyback diode anti-parallel across load (e.g. 1N4007 for standard relay applications)
Capacitor type for mains applications	X2 safety-rated polypropylene film — never ceramic or electrolytic
Resistor type	Metal film or carbon film — never wirewound (inductive)
Resistor wattage (relay contact snubber on 230V AC)	1W minimum; 2W recommended for continuous operation

Safety warnings

Any snubber or suppression circuit connected directly across mains voltage conductors (230V AC, 120V AC) must use components specifically rated for across-the-line service. Capacitors must be X1 or X2 safety-rated types. Standard ceramic or electrolytic capacitors must never be used across mains voltage — they can fail short-circuit and cause fire.
Mains voltage circuits must only be worked on by qualified electricians with the circuit fully isolated and confirmed dead. Snubber components placed across mains contacts are permanently connected to live mains when the supply is energised — they cannot be considered safe to touch.
Verify that the total continuous power dissipation of the snubber resistor is within the component's rated wattage. A resistor in a mains-connected snubber carries continuous AC current. Undersized resistors overheat and can ignite surrounding materials.
Ensure snubber components are rated for the peak (not RMS) supply voltage. For a 230V RMS AC supply, the peak voltage is 325V. Use capacitors and varistors rated at 400V or higher.
This guide is for illustrative and reference purposes only. Electrical and power electronics design must be performed by qualified engineers and all mains-connected equipment must meet applicable safety standards and be installed by licensed electricians.

Tools needed

Soldering iron and solder (for PCB assembly)
Digital multimeter (for component verification and circuit testing)
Oscilloscope with differential probe (for transient measurement — strongly recommended for TRIAC circuit design)
Component leads cutters and PCB mounting hardware
Insulating sleeving or heat-shrink (for bare leads)
Component value tester or LCR meter (to verify actual capacitor and resistor values)

Common mistakes

Using a standard ceramic or electrolytic capacitor across mains voltage: these are not rated for continuous across-the-line connection and can fail short-circuit, causing fire. Always use X2-rated film capacitors.
Using a wirewound resistor in the snubber: wirewound resistors have significant parasitic inductance, which reduces the snubber's effectiveness at high frequencies and during fast transients. Use metal film or carbon film only.
Placing the snubber far from the switching device: stray inductance in long leads between the snubber and the device reduces transient suppression effectiveness. The snubber must be as close to the device as physically possible.
Ignoring continuous power dissipation in the resistor: the snubber resistor in a mains circuit carries current continuously — it is not only active during switching. Calculate steady-state dissipation and select appropriate wattage.
Using a snubber alone for a DC relay contact switching a solenoid: for DC inductive loads, a flyback diode is a far more effective first-line suppression measure. The RC snubber in DC circuits provides secondary benefit and should supplement rather than replace the flyback diode.

Troubleshooting

Relay contacts arc and pit despite snubber being installed: Cause: Snubber component values too high (insufficient suppression), snubber placed too far from the contact, resistor or capacitor values out of specification, or X2 capacitor has failed open Fix: Test the capacitor with an LCR meter to confirm it is at the specified capacitance. Check resistor value. Verify the snubber is wired directly across the relay contact terminals and not at a remote point in the circuit. Consider reducing R to 47–68 ohms and increasing C to 220nF to improve suppression.
TRIAC false-fires (triggers without gate signal): Cause: dV/dt across the TRIAC exceeds the device's rated dV/dt specification at commutation; snubber capacitance is too small or resistor is too large Fix: Consult the TRIAC's datasheet for its critical dV/dt rating. Increase snubber capacitance to reduce the rate of voltage rise. Reduce snubber resistance slightly to improve the RC time constant. Ensure the snubber is placed directly across the TRIAC's MT1–MT2 terminals with short leads.
Snubber resistor overheats during operation: Cause: Resistor power rating is insufficient for the continuous AC current flowing through the RC network Fix: Calculate continuous power dissipation for the actual circuit voltage, frequency, and component values. Replace the resistor with one of a higher wattage rating (2W or 5W). If the resistor is the correct value but still overheating, the capacitor value may be too large, causing excessive continuous current — reduce C or increase R to lower steady-state current.

Frequently asked questions

Where exactly should an RC snubber be placed — across the load or across the switch?

Both positions suppress transients but with different trade-offs. Across the switch (relay contacts, TRIAC): protects the switching device directly and limits the arc. Across the load: reduces the transient seen by the entire circuit including the supply wiring. For relay contacts, placement across the contacts is most effective. For TRIACs, the snubber must be across the device (MT1 to MT2) to control dV/dt.

What are the standard RC snubber values for a relay contact switching a 230V AC inductive load?

A widely used starting point is R = 100 ohms and C = 0.1µF (100nF), both rated for the AC mains voltage (use X-rated capacitors for across-the-line applications). These are not universal — the optimal values depend on the load inductance and circuit impedance. For critical applications, calculate from the load inductance using the formulas in application notes or test with a transient capture oscilloscope.

What type of capacitor must be used in a mains-connected snubber?

For snubbers connected directly across mains voltage (230V AC or 120V AC) — whether across relay contacts or across a TRIAC — use X-rated (X1 or X2) film capacitors. X2 capacitors are rated for across-the-line use and are designed to fail safely (open-circuit) rather than short-circuit in case of overvoltage. Standard electrolytic or ceramic capacitors must never be used across mains voltage.

Can I use a single RC snubber for multiple relay contacts in the same enclosure?

Each relay contact that switches an inductive load should have its own snubber directly across it. A shared snubber at one point in the circuit will not adequately suppress transients at each individual contact. The snubber must be placed as close as possible to the switching device to minimise the length of wiring between the snubber and the inductive spike source — parasitic inductance in long leads reduces suppression effectiveness.

Does a DC snubber circuit use the same RC values as an AC snubber?

For DC relay contacts switching inductive loads, a freewheeling diode in anti-parallel across the load is the preferred and most efficient suppression method — not an RC snubber. An RC snubber can be used to supplement the diode in circuits where diode recovery time is too slow, but for standard DC relay applications, a correctly rated diode (e.g. 1N4007) handles back-EMF far more effectively than an RC network.

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