A customer messaged me last week: "I turned the light off, but the ceiling is still glowing. A faint grey circle I can see even with my eyes closed. I can't sleep."
I've been getting this question a lot lately — more than flicker, more than dropped connections. It's called ghosting (or afterglow), and it has a counterintuitive cause: it's almost never the light fixture. It's the circuit and the driver.
Why an LED can glow on almost nothing
LEDs and old incandescent bulbs are fundamentally different here:
- An incandescent bulb needs a large current to heat its filament until it glows — a few milliamps does nothing.
- An LED is a semiconductor. It only takes 0.3–1 mA to excite the phosphor and produce visible light.
So any tiny residual current that "leaks" onto the LED will show up as a faint glow. In a dark room, the human eye is extremely sensitive to this. It's not dangerous, but it hurts sleep and, over time, that continuous micro-current ages both the LED and the driver.
The four causes, ranked by probability
1. The switch cuts the neutral line (common in older buildings)
A standard mechanical switch should break the live (L) wire. If the electrician wired the neutral into the switch instead, then when the switch is open the live wire is still connected to the driver — the fixture stays "half-live," and distributed capacitance in the walls forms a weak return path. The LED glows.
Self-check: switch it off, then test the live terminal at the lamp socket with a voltage tester. If it's still live, the wires are reversed. This one needs a licensed electrician — don't DIY it.
2. Single-live smart switches and illuminated switches
This is the #1 reason ghosting has exploded in recent years.
Many smart switches skip the neutral wire and instead power themselves by leaking a small current through the load — i.e. through your lamp. Typically 0.3–1 mA, which is exactly enough to light up a sensitive low-wattage LED. Mechanical switches with a neon indicator lamp leak about the same.
Self-check: swap in a plain mechanical switch for one night, or temporarily disconnect the smart switch's load terminal. If the glow disappears, that's your culprit.
3. Induced voltage from long parallel wiring
If the switch wire runs alongside the live wire in the same conduit or raceway for a long distance (noticeable beyond ~3 m), the two behave like a capacitor and induce a voltage — measured at 10–40 V in practice. Not enough to drive a load, but plenty to make an LED glow faintly.
Self-check: typically shows up in large apartments, villas, and retrofits with long cable runs. A quick tell: lights on short runs don't glow, lights on the long run do.
4. The driver's own bleed-off design
This is the one people miss most — and the one we care about most as a driver manufacturer.
Cheap drivers use a capacitive-dropper front end (power factor around 0.5) with no bleeder resistor. Any leakage current slowly charges the input capacitor until the voltage reaches the LED's forward threshold — then the light glows, or blinks every few seconds.
Quality drivers use active PFC (PF > 0.9) with a built-in bleeder resistor that safely dissipates that micro-current as heat. On the same circuit, swap in a good driver and the glow is simply gone.
Fix it by cause
| Cause | Fix | Cost |
|---|---|---|
| Live/neutral reversed | Have an electrician move the switch to the live wire | labor |
| Single-live smart switch leakage | Use a neutral-required smart switch, or one rated for LED loads | switch upgrade |
| Induced voltage | Ground a metal conduit over the switch wire, or add an X2 safety cap to filter noise | a few dollars |
| Insufficient driver bleed-off | Replace with an active-PFC driver with bleeder design | driver upgrade |
If you don't want to rewire right away, there's a quick fix: wire a 100 kΩ–470 kΩ / 1 W metal-film resistor in parallel across the lamp socket to give the residual current a low-impedance path to drain. A few dollars, but keep it in a ventilated spot away from anything flammable.
The cleaner solution is to fix it at the driver — choose a constant-current driver with a bleeder circuit and a high startup threshold, and you cut the leakage off at the source.
How to avoid this when buying
Three rules if you're wiring or renovating:
- Run a neutral to your switch boxes. A little extra wire now saves you from smart-switch pain later.
- Check the driver's power factor. PF ≥ 0.9 with active PFC is far more resistant to ghosting. Buying on wattage alone is the biggest trap.
- Prefer isolated drivers on long runs. Isolated topologies have roughly an order of magnitude less leakage than non-isolated — important for villas and large homes.
The takeaway
When a light glows after switch-off, most people replace the fixture — and it still glows, because the real problem lives in the circuit and the driver, two things you can't see.
Whether a light turns off cleanly isn't just about the LED specs. The leakage control, bleed-off design, and power factor of the driver behind it decide whether you sleep well at night.
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