Written from a luminaire manufacturer's bench. Fair warning: this is not a product pitch — it is what you learn testing lamps and switches on the same circuit for a living.
When 'Off' Isn't Off
You tap Off in the app. The app confirms it is off. You hear the click on the wall. The lamp stays on.
The cost of a wrong switch is not the money — it is the rework, the tripped breaker, and the lamp you cannot switch off at 1 a.m. One blog put it better: a nineteenth-century mechanical switch on a twenty-first-century solid-state load.
[Figure: relay contact welded by LED inrush]
First, Separate Three Kinds of 'Won't Turn Off'
Do not rush to call the switch counterfeit. 'Won't turn off' is at least three different faults.
- Smart-bulb always-on plus gateway logic. Tapping Off only sends the bulb into standby; the switch never cut power, so it changes back later.
- Single-live-wire ghost glow. The lamp glows faintly after switch-off — leakage current the switch steals through the lamp, not full brightness.
- Welded relay contacts. The coil moves, the click is normal, but the lamp is at full brightness and will not go off — an irreversible physical weld.
Only the third matters here, and it is the fault most often mistaken for a software bug: you reinstall the app and re-pair the network all night while the contacts welded hours ago.
[Figure: three causes vs. cold-start peak current]
Why a Smaller Lamp Is More Likely to Kill the Switch
Every LED lamp contains a rectifier bridge and a large filter capacitor. At power-on the capacitor is empty, so to the mains it looks almost like a short circuit, and cold-start inrush pours in — 20 to 100 times the steady-state value, lasting only a few hundred microseconds to a few milliseconds.
A single 10 W LED lamp, in that first millisecond of closing, is equivalent to a 1000 W load sitting on the contacts.
Measured: ten 20 W LED lamps draw about 0.9 A steady-state; the cold-start peak is 31 A, decaying to 2.1 A after roughly 1.5 ms. The relay contact is rated for 10 A. Thirty-one amps onto ten — of course something gives.
Capacitive loads are sized for 5 to 10 times the steady-state current, or higher. Counter-intuitively, the smaller and more numerous the lamps, the more each driver delivers its own inrush while one contact does the switching — a heap of small lamps on one circuit is the most dangerous case.
How the Weld Happens: Three Actions, a Few Hundred Microseconds
Too fast for the eye to follow. Slow it down.
- The capacitor looks close to a short, inrush rushes in, and current density piles up on the contact.
- The relay armature snaps closed, but metal is elastic, so the contacts bounce and open a tiny gap within milliseconds.
- Voltage breaks down that gap and draws an arc, locally thousands of degrees, melting the silver-alloy plating. The bounce stops, and the two contacts are glued together by molten metal the return spring cannot pull apart.
Traditional wall switches rarely do this: bigger contacts, stiffer springs, so a micro-weld is flung apart. A smart switch must fit an antenna, chip and power supply in one box, so it uses a miniature pulse relay — small contacts, weak spring. Inrush it survives, you cannot.
[Figure: three actions that weld a contact]
Match the Solution to the Load
Choosing a switch comes down to how it switches off.
| Type | How it switches off | Inrush tolerance | Good for | Watch out for |
|---|---|---|---|---|
| Mechanical relay | Small relay makes and breaks | Weak; capacitive loads need heavy derating | Ceiling lights, strips, circuits with margin | Small lamps, many circuits, frequent switching — welds first |
| Zero-cross detection | Closes only at the zero crossing | Strong; kills the arc by principle | High power, many circuits, commercial panels | Datasheet only — ask support |
| Triac dimming | Phase chopping, no contacts | Medium | Rooms needing smooth dimming | Needs a dimmable driver; cheap units flicker low; small downlights glow |
| Solid-state relay | Fully solid-state, no contacts | Strong; no wear | Frequent switching, automation-heavy circuits | Small leakage; an ordinary LED may glow when off |
A headline rating like 400 W / 10 A is quoted for incandescent lamps — a resistive load. Your LED is capacitive.
[Figure: four switch types and their pitfalls]
Already Welded? Diagnose It, Then Deal With It
Step 1. Send Off from the app and listen for the click: the coil moves, control works, yet the lamp does not budge — that is welding.
Step 2. Pull the main breaker, wait a few minutes, close it. Lamp still on means software and automation are ruled out.
Step 3. Measure the normally-open contact with a multimeter in continuity mode. Conducting while idle confirms the weld.
Emergency: a gentle tap on the housing sometimes shakes a micro-weld loose, but the contact is already pitted and will weld again next power-up. Temporary only.
The real fix is replacing the switch: establish the circuit's load type (resistive or capacitive LED) and lamp count, size by the derated figure, and prefer models labeled zero-cross switching, high inrush, or lamp load.
Three Checks Before You Buy
- Do the load math. Back the circuit's total LED wattage out with a 5 to 10 times inrush margin to see what contact capacity the switch needs. Many small lamps on one circuit is the easiest trap.
- Ask support. Zero-cross switching? Contact material? AgSnO₂ silver tin oxide resists high-temperature arcs better than ordinary silver cadmium. Any rated capacitive or LED load capability? If they cannot answer, do not order.
- Check material and installation. With no neutral you are stuck with single-live-wire — watch for ghost glow, and do not cram a heap of small lamps onto one circuit.
Someone who really hit this swapped a 10 A ordinary intermediate relay for a higher-inrush type with 25 A contact capacity and a wider gap, then added an NTC inrush limiter at the driver input. Cold-start current fell from 31 A to around 15 A, and 1000 consecutive power-ups produced no re-weld.
The lamp stays on — did I buy a fake switch?
No — it is the wrong load class. Relay makers treat this as the expected result of sizing without derating.
Will a more expensive, fancier smart switch fix it?
Only if it has zero-cross detection and high-inrush contact material, not because of price. An expensive one without zero-cross welds the same.
Can I just use smart bulbs and keep the wall switch permanently on?
It works, but the lamps stay online, so you need a local button or wireless switch when the network drops — and watch for leakage and heat.
Wrapping Up
Lamp makers and switch makers sit on the same circuit. Look at the lamp and the switch parameters together.
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