How to Suppress Relay Coil Turn-Off Spikes: TVS, Freewheel Diode or RC Snubber for Inductive Loads
One-line takeaway: the turn-off spike of an inductive load is not a question of whether to protect, it is a question of how high to clamp. The correct chain has five steps: 1 identify coil type and drive topology, 2 estimate single-event turn-off energy with E = 1/2 L I squared, 3 choose the clamp (freewheel diode / diode plus TVS / bidirectional TVS / RC snubber), 4 fit the clamping voltage window inside the driver MOSFET voltage budget, 5 check TVS single-event energy and average power. The two most common field failures — the driver MOSFET still blows after moving to a bigger part, and the TVS fails even though it is fitted — usually come from steps 3 and 4: a clamp mismatched to the release-speed requirement, or a clamping voltage never checked against the real turn-off current.
flowchart TD
A["1 Identify coil type and drive topology<br/>relay / contactor / solenoid, high-side or low-side MOSFET"] --> B["2 Estimate turn-off energy<br/>E = 1/2 L I squared, I at the turn-off instant"]
B --> C{"3 Choose the clamp"}
C --> D["Freewheel diode<br/>lowest clamp, slow release"]
C --> E["Diode plus TVS in series<br/>controlled clamp, fast release"]
C --> F["Bidirectional TVS across coil or MOSFET<br/>simplest fit, watch the energy"]
C --> G["RC snubber<br/>contact arc suppression, power to calculate"]
D --> H["4 Fix the clamping voltage window<br/>supply plus clamp no more than MOSFET rating minus derating"]
E --> H
F --> H
G --> H
H --> I["5 Check TVS energy and average power<br/>single event E, and E times operating rate"]
I --> J["6 Measure the turn-off waveform<br/>spike amplitude, ringing, release time"]
J -- "fails" --> C
J -- "passes" --> K["7 Retest at temperature extremes and across production spread"]This article covers only the coil side, the low-voltage drive loop. For bus-side TVS selection on energy storage and BMS main loops see How to Select a TVS for a BMS Protection Board; for system-level graded surge protection see How to Design Surge Protection.
Two Questions Answered First
Q: Why must a relay coil have a clamp? What happens without one?
Because the coil is inductive and inductor current cannot change instantaneously. At the instant the driver opens, the coil generates a reverse spike to maintain current; unclamped, the amplitude easily reaches tens to hundreds of volts depending on coil inductance, current and loop parasitics. The spike hits the driver MOSFET or transistor first (drain-source or collector overvoltage breakdown), then couples into the supply rail and damages the MCU and surrounding low-voltage parts. It presents as relay driver circuits failing for no clear reason, or bigger devices still blowing after an upgrade — but this is a voltage stress problem, and a higher-current part does not fix it. To judge the failure mechanism, see What Is MOSFET Avalanche Breakdown.
Q: What is the difference between a freewheel diode and a TVS, and which one do I fit?
In one line: the freewheel diode buys stability, the TVS buys speed. With only a freewheel diode the clamp is about 1 V of forward drop, coil current decays slowly and relay release time lengthens noticeably — irrelevant for indicator lamps or holding loads, but fatal where fast disconnection matters (protection relays, contactor breaking, safety loops). Put a TVS in series after the diode (or use a bidirectional TVS directly) and the clamp rises to a chosen level: current decays fast, release is crisp, and the spike is still held inside the driver voltage budget. That is the mainstream choice when you need both speed and safety. RC snubbers are used more on the contact side for arc suppression and on AC.
Step 1: Coil Type, Drive Topology and Turn-off Energy
Answer three questions before choosing a clamp:
- What coil, and what current? Small-signal relay coils usually draw tens of milliamps with inductance from millihenries to tens of millihenries, so single-event turn-off energy is typically in millijoules. Contactor and solenoid coils carry far more current and inductance, so energy must be calculated from measurement or coil specification. Rough estimate: E = 1/2 L I squared, L from coil inductance and I from coil current at the turn-off instant. This number sets the TVS energy requirement and whether an additional absorption branch is needed.
- What interrupts the loop? Low-side MOSFET, high-side MOSFET, direct MCU I/O, or a dedicated driver IC — this sets the upper bound of the clamping window: supply voltage plus clamp voltage no more than driver rating minus derating margin, typically 10 to 20 percent. Driving a small relay directly from MCU I/O is the most common way to fail, because the I/O clamp structure cannot absorb an inductive spike.
- Is release speed specified? If it is, use the diode-plus-TVS or bidirectional TVS route. Only when it is not may a freewheel diode be used alone. Slow release on a protection relay means delayed fault interruption, which is a functional risk and not merely a parameter issue.
Step 2: Choosing Among Four Clamp Options
| Option | Clamping level | Characteristics and cost | Best for |
|---|---|---|---|
| Freewheel diode, reverse-parallel across coil | about 1 V, diode forward drop | Simplest circuit, lowest cost; slow current decay and noticeably longer release time | Holding loads with no release-speed requirement |
| Diode plus TVS in series across coil | TVS breakdown plus diode drop | Clamp height selectable against the driver voltage budget, fast release; the TVS absorbs most of the energy | Protection relays and contactors needing both fast release and driver protection |
| Bidirectional TVS across coil or driver D-S | TVS bidirectional clamping voltage | Simplest to fit, no polarity concern, usable where polarity alternates; energy and continuous dissipation must be checked separately | Local clamping at the coil or at MOSFET drain-source |
| RC snubber across contacts or coil | Soft, varies with current | Non-polarised, usable on AC; resistor carries continuous dissipation and values depend on loop parasitics | Contact arc suppression, AC coils, supplementary measure under vibration |
Three engineering judgements:
- Diode plus TVS is the default answer on the coil side. It turns how high to clamp into an explicit selection parameter instead of leaving it to chance. Derive the clamp height backwards from the voltage budget in step 1; select the TVS grade low, but above coil operating voltage including ripple, or it conducts continuously and heats.
- A bidirectional TVS across MOSFET drain-source is essentially setting a ceiling on drain overshoot. Its job differs from the bus-side TVS on the main loop and the two complement each other; their voltage windows cannot be borrowed from one another. For the bus side see A TVS Is Fitted, the MOSFETs Still Blow: The Voltage Account on 23S / 24S BMS Boards.
- An RC snubber and a TVS do not conflict. Where contact arcing is severe or EMC conducted emissions run high, the common combination is a TVS clamp on the coil side plus an RC snubber across the contacts, each handling its own job.
Step 3: Clamping Voltage Window and Driver MOSFET Withstand
| Check | Criterion | Common mistake |
|---|---|---|
| Steady-state window at turn-off | Supply plus clamp no more than driver rating x (1 minus 10 to 20 percent derating) | Counting coil voltage but not clamp height, or applying zero derating |
| Actual TVS clamp | Take Vc at the real turn-off current, not the datasheet nominal pulse current | Copying Vc straight from the datasheet table and ignoring that Vc rises with current |
| TVS energy | Single-event turn-off energy no more than TVS allowable energy, derated by pulse width and starting junction temperature | Computing E = 1/2 L I squared and stopping there, never reading the pulse-width derating curve |
| TVS average power | E times operating rate no more than TVS rated power | Ignoring cumulative heating where switching is frequent, such as periodic cycling or automated test |
| Contact-side stress | Contacts also see arc and ringing at opening; add RC suppression where needed | Protecting the driver but ignoring the contacts, which then erode and weld |
Of these, taking the real clamping voltage at the real current is the one most easily missed: the datasheet Vc is given at the nominal pulse current, for instance the Ipp point of the 10/1000 microsecond waveform. At a fraction of that current the clamp is somewhat lower, and above the nominal point it rises significantly. TVS datasheets such as the AMSEMI 5.0SMDJ series use the 10/1000 microsecond convention throughout (datasheets 8060014 and 8060055, no 8/20 microsecond data), so pulse width and current conditions must be converted to one convention before comparison, and specific parameters follow the manufacturer datasheet. The foldback TVS used on high-voltage buses and the general-purpose TVS used on low-voltage coils are two separate selection tracks and must not be mixed; see AMSEMI Foldback TVS: Full Series and Cross-Reference.
Step 4: Common Failure Symptoms and Troubleshooting
| Field symptom | Likely mechanism | Where to look first |
|---|---|---|
| Driver MOSFET or transistor breaks down at turn-off | No clamp or failed clamp: TVS reversed, missing or poorly soldered | Check TVS orientation and placement first, then measure the turn-off waveform |
| MOSFET rating looks adequate but failures are intermittent | Vc selected at a low-current grade; clamping is higher at the real turn-off current | Re-read Vc at the real current and redo the window with derating margin |
| Relay releases too slowly, breaking is delayed | Only a freewheel diode fitted, clamp too low and current decays slowly | Change to diode plus TVS or bidirectional TVS and measure release time |
| TVS runs hot, cracks or drifts in resistance | Single-event energy beyond rating, or average power exceeded at high operating rate | Check E = 1/2 L I squared and operating rate; move to a larger energy grade or multiple stages |
| Contacts erode, weld, arc badly | No arc suppression on the contact side | Add an RC snubber across the contacts and check contact rating and load category |
| MCU pins damaged, spikes on the supply rail | Spike coupled into the control side through layout and ground bounce | Place the clamp close to the coil, shorten the high-current loop and review the ground path |
Symptoms are only clues; the final judgement must come back to a measured waveform. Capture driver drain-source voltage and coil voltage at the turn-off instant and compare against the clamp setting and the voltage budget. Retest at temperature extremes and across production spread, because inductive spikes drift noticeably with temperature and lot-to-lot parasitics.
Related Reading
- Bus-side TVS selection for BMS protection boards: How to Select a TVS for a BMS Protection Board
- Troubleshooting boards that still blow with a TVS fitted: What to Do When a BMS Board Blows MOSFETs
- Full AMSEMI foldback TVS type list: AMSEMI Foldback TVS: Full Series and Cross-Reference
- Judging avalanche failure in the driver: What Is MOSFET Avalanche Breakdown
- Relay and precharge sequencing: How to Design a BMS Precharge Circuit
- System-level graded surge protection: How to Design Surge Protection
- Signal port protection for RS485 and CAN: Why Do RS485 and CAN Ports Keep Failing
- Driver loop and gate resistor tuning: How to Design a MOSFET Gate Drive Circuit
Frequently Asked Questions
Q1: Why does a relay coil need a diode or TVS? What happens without one?
The coil is an inductive load and inductor current cannot change instantaneously. When the driver opens, a reverse spike appears across the coil; unclamped it easily reaches tens to hundreds of volts, far beyond a low-voltage driver MOSFET rating. The symptoms are repeated driver breakdown, damaged MCU pins and short driver life. A clamp gives coil current a controlled discharge path and holds the spike inside the driver voltage budget. Whether one is needed and which type depends on coil specification, switching frequency and release speed.
Q2: What is the difference between a freewheel diode and a TVS? Which one fits a relay coil?
The core difference is clamping height. A freewheel diode reverse-parallel across the coil clamps at roughly 1 V of forward drop; current decays slowly and release time lengthens noticeably, which suits applications with no release-speed requirement. A diode in series with a TVS across the coil clamps at roughly the TVS breakdown plus the diode drop; current decays fast and release is crisp, and this is the mainstream choice when you need both fast release and driver protection. A bidirectional TVS can sit directly across the coil or across switch drain-source, is the simplest to fit and suits alternating polarity. RC snubbers are mostly for contact arc suppression and AC. Decide from release-speed requirement, clamping voltage budget and energy.
Q3: The driver MOSFET keeps failing at relay turn-off. How do I troubleshoot it?
Four steps. 1 Confirm the clamp is actually on the board and correctly oriented — a reversed TVS, a missing TVS or a reversed diode defeats the clamp completely. 2 Measure switch drain-source voltage at the turn-off instant and inspect spike amplitude and ringing against device rating and derating budget. 3 Check whether the TVS clamp was selected for the real turn-off current — datasheet Vc is given at the nominal pulse current and clamping is higher at higher current. 4 Verify TVS energy and average power — where operation is frequent, estimate average power as single-event energy times rate, because exceeding it thermally kills the TVS and protection is lost. Base every judgement on measured waveforms and the device datasheet.
Q4: How do I choose the resistor and capacitor for an RC snubber?
An RC snubber usually goes across the relay contacts or the coil. Common practice: size the capacitor against the oscillation frequency formed with loop stray inductance and parasitic capacitance, and the resistor near the characteristic impedance to damp the ringing. Fit the starting values, then tune step by step on the oscilloscope until ringing is suppressed and the resistor does not heat noticeably. Two items must be checked: resistor power rating, estimated from the AC component of the loop, which overheats or burns if undersized; and capacitor voltage rating and dielectric, which must survive repeated spike charging stress. RC values depend heavily on actual loop parasitics, so empirical formulas only give a starting point and the measured waveform decides.
Contact Us
For TVS selection and sample test support on inductive load clamping, for driver withstand window calculations, or for a complete protection device set across coil side, bus side and signal ports, contact us. Send the coil specification (voltage, current, inductance), drive topology and operating rate, and we will work the account through in the order energy, clamp type, voltage window, TVS specification.
Shenzhen Intek Technology Co., Ltd — electronic component distributor and system solution provider Phone / WeChat: 136-3264-8484 Address: 4F, Building 2, Jingwei Center, No. 309 Ping an Avenue, Pinghu Street, Longgang District, Shenzhen Website: www.intek.vip
Disclaimer: This article is technical education and industry exchange for reference only and is not selection, procurement or other commercial decision advice. The magnitudes given for coil inductance, current, energy and clamping voltage are typical industry ranges or formula estimates; real values differ markedly with coil type, drive topology and operating conditions, and final design must follow rated values and test conditions in the manufacturer datasheet, your own circuit calculations and measured waveforms on the actual machine. Where contact ratings and safe breaking are concerned, follow the relay manufacturer specification and the applicable safety standard for the equipment.