An LED light bulb glows when off because micro-ampere leakage currents pass through the driver circuitry, or induced voltage couples from adjacent live wires. The immediate fix for a glowing bulb on a smart switch is installing a parallel bleeder resistor (typically 100kΩ to 220kΩ, 1W) across the fixture, or upgrading to a smart switch that requires a dedicated neutral wire.
The Physics of the Ghost Glow: Leakage, Induction, and Driver PF
To understand why an LED light bulb glows when off, you have to look at the driver topology and the switch wiring. LEDs are hyper-efficient; a standard 9W bulb draws roughly 75mA at 120V AC to produce 800 lumens. However, it only takes 0.5mA to 2mA of leakage current to excite the phosphor and create a faint 'ghost' glow.
This leakage usually comes from three specific sources on the jobsite:
- Smart Switches Without a Neutral: Devices like older Lutron Caséta or Leviton Decora Smart switches power their internal Wi-Fi or Zigbee radios by leaking a tiny current (~1mA) through the load (your bulb) when the switch is in the 'off' state.
- Induced Voltage in Multi-Gang Boxes: A live 120V wire running parallel to a switched leg in the same conduit or NM-B cable can induce a phantom voltage (often 20V-40V) via capacitive coupling.
- 3-Way Switch Travelers: In 3-way and 4-way switch setups, the long traveler wires act as a capacitor. A 50-foot run of 14/3 NM-B cable has enough parasitic capacitance to couple 10V to 20V into the switched leg, even when the mechanical switch is open.
Circuit Impact Math: Inrush and Power Factor
The driver's Power Factor (PF) dictates how it handles this leakage. Cheap bulbs use a capacitive dropper circuit with a PF around 0.5. These lack a bleeder resistor, meaning any leakage current charges the input capacitor until it hits the LED's forward voltage threshold, causing a glow or a periodic strobe. High-quality bulbs use Active Power Factor Correction (PFC) with a PF >0.9. These drivers include a bleeder resistor that safely dissipates micro-currents as heat, preventing the glow.
When you turn the switch on, the math flips to inrush current. A 9W LED with a 0.5 PF can spike to 30A for a few microseconds as the bulk capacitors charge. This is why standard mechanical switches last for years, but cheap solid-state relays or under-specified smart dimmers weld their internal TRIACs shut after a few months. Always check the dimmer's surge rating (e.g., 160A for 800µs) against the driver's inrush spec.
Lumens, Watts, and Efficacy: Sizing the Right LED Driver
When selecting bulbs to avoid ghosting and ensure proper dimmer loading, you cannot just look at wattage. You must evaluate luminous efficacy (lumens per watt), which directly correlates to the driver topology used. High-efficacy bulbs often use more complex, active PFC drivers that inherently solve the ghosting problem.
| Wattage | Lumens | Efficacy (lm/W) | Driver Topology | Ghosting Risk |
|---|---|---|---|---|
| 4W | 450 | 112 | Capacitive Dropper (PF ~0.5) | High (Requires bypass) |
| 9W | 800 | 88 | Linear IC (PF ~0.7) | Medium (Depends on switch) |
| 11W | 1100 | 100 | Active PFC (PF >0.9) | Low (Built-in bleed) |
| 15W | 1600 | 106 | Active PFC w/ Bleeder (PF >0.9) | None |
Dimmer Compatibility: Minimum Loads and Flicker Fixes
If your LED light bulb glows when off on a dimmer, or flickers at low settings, you are dealing with a minimum load or phase-cut mismatch.
Trailing Edge vs. Leading Edge
Incandescent bulbs are purely resistive, so they work with Leading Edge (TRIAC) dimmers. LED drivers are highly reactive (capacitive/inductive). You must use a Trailing Edge (ELV / Electronic Low Voltage) dimmer. Trailing edge dimmers use MOSFETs to chop the back half of the AC sine wave, which prevents the harsh voltage spikes that cause LED driver capacitors to whine and fail.
The Minimum Load Trap
Every dimmer has a minimum load requirement. A dimmer rated for '150W LED' might actually require a minimum of 10W to keep its internal circuitry stable.
- Which dimmer for this fixture count? If you have six 9W bulbs (54W total) on a single switch, a standard Lutron Diva DVCL-153P (min load 1 bulb) will work flawlessly.
- The Single Bulb Problem: If you have a single 4W vanity bulb on that same dimmer, you fall below the 10W minimum. The dimmer will misfire, causing the bulb to flicker at 50% brightness, or leak current and glow when off.
- The Fix: Install a LUT-MLC (Minimum Load Capacitor) in parallel at the fixture, or switch to a dimmer specifically rated for a 0W minimum load, like the Leviton DDMX.
Flicker at low dimming levels happens when the driver's internal rectifier cannot maintain DC bus voltage during the dimmer's 'off' phase of the chopped sine wave. At the driver level, the AC phase-cut is converted to a DC pulse-width modulation (PWM) or constant current reduction (CCR) signal. If the dimmer's chopped waveform doesn't cross the driver's zero-crossing detection threshold cleanly, the PWM controller stutters, resulting in visible 120Hz flicker. According to the U.S. Department of Energy's Solid-State Lighting program, ensuring the dimmer and driver are tested together on a compatibility matrix (like the Lutron LED Compatibility Tool) is the only way to guarantee flicker-free operation below 20% brightness.
Thermal Constraints and Enclosure Derating
The final piece of the LED circuit puzzle is heat. Unlike incandescent bulbs that radiate heat forward as infrared, LED drivers trap heat at the base. When an LED bulb is installed in an enclosed fixture (like a recessed can, a flush-mount globe, or a sealed porch light), the ambient temperature inside the enclosure can easily exceed 45°C (113°F).
Electrolytic capacitors inside the LED driver lose roughly 20% of their lifespan for every 10°C increase in operating temperature. To prevent catastrophic failure, high-quality LED bulbs feature thermal foldback circuitry. When the Tc (case temperature) point hits 85°C, the driver intentionally reduces current to the LEDs, dropping a 9W bulb down to 5W to save the electronics.
By matching the driver topology to your switch type, respecting dimmer minimum loads, and accounting for thermal derating, you eliminate the ghost glow and ensure your lighting circuit lasts for its rated 50,000 hours.
