If your LED lights flicker at low dimming levels, flash when turned off (ghosting), or cause a smart switch to reboot, you need a bypass (bleeder) resistor. For a standard 120V AC smart switch struggling with a low-wattage LED load, wire a 100kΩ 1/2W resistor or a commercial 10W dummy load (like the Lutron LUT-MLC) in parallel across the line and load terminals. This provides the minimum holding current required by the switch's internal electronics, stopping the strobe effect and stabilizing the circuit.
The Physics of Ghosting and Flicker in LED Circuits
Unlike incandescent bulbs, which act as simple resistive loads, LED fixtures contain internal drivers with bridge rectifiers and bulk capacitors. When you install a smart switch or a dimmer on a circuit with only one or two LED bulbs, the total wattage often drops below the switch's minimum load requirement.
Smart switches (like the Lutron Caséta PD-5S-DV or Leviton Decora Smart) require a tiny amount of standby current to power their internal WiFi or Zigbee radios. Without a neutral wire, this standby current leaks through the LED bulb itself. Because the LED driver's input capacitor stores this leaked charge, it eventually reaches the threshold voltage to fire the LEDs, discharges, and recharges. This results in a rhythmic flashing known as ghosting. Wiring a bleeder resistor in parallel gives this leakage current a dedicated path back to the source, bypassing the LED driver entirely and eliminating the flash.
Circuit Impact Math: Inrush Current and Power Factor
Before sizing a resistor or swapping a dimmer, you must understand the hidden math of LED drivers: Power Factor (PF) and inrush current. According to All About Circuits, the bulk capacitors inside LED drivers draw massive instantaneous current when first energized.
Imagine a circuit with ten 10W LED bulbs. The steady-state real power is 100W. At 120V, you might assume the current is 0.83A (100W / 120V). However, cheap LED drivers often have a poor Power Factor of 0.6. This means the apparent power is 166VA, and the actual RMS current drawn from the breaker is 1.38A.
More critically, the inrush current to charge those ten capacitors simultaneously can spike to 100 times the steady-state current for 2 to 5 milliseconds. That is a 138A spike. This transient surge is why a dimmer rated for 600W of incandescent load will instantly destroy its internal TRIAC if you connect 600W of LED load to it.
This inrush phenomenon dictates why dimmers must be heavily derated for LED use, and why adding a properly sized dummy load resistor must be calculated against the dimmer's minimum holding current, not just its maximum wattage.
Lumens, Watts, and Efficacy: Sizing Your LED Load
When calculating your total circuit load to determine if you need a dummy load resistor, you must look at efficacy (lumens per watt), not just raw wattage. The US Department of Energy's Solid-State Lighting data shows that modern high-efficacy LEDs produce the same light output for a fraction of the electrical draw, which directly impacts your minimum load calculations.
| Incandescent Equivalent | Modern LED Wattage | Lumens Output | Efficacy (lm/W) | Typical Driver PF |
|---|---|---|---|---|
| 40W | 4.5W | 450 lm | 100 lm/W | 0.65 |
| 60W | 8.5W | 800 lm | 94 lm/W | 0.70 |
| 75W | 11W | 1100 lm | 100 lm/W | 0.75 |
| 100W | 15W | 1600 lm | 106 lm/W | 0.90 (Active PFC) |
Efficacy Context: A 15W bulb with active Power Factor Correction (PFC) draws current much more smoothly than an 8.5W bulb with a passive capacitive dropper. If your circuit consists entirely of low-wattage, low-PF bulbs (like the 4.5W models), your dimmer will struggle to maintain the TRIAC latch, making a bleeder resistor mandatory.
Dimmer and Driver Compatibility: Trailing Edge vs. Leading Edge
Which dimmer or driver should you use for your specific fixture count? The answer depends on the switching topology. Standard leading-edge dimmers use TRIACs and require a high minimum holding current (often 20mA to 50mA). Trailing-edge (ELV) dimmers use MOSFETs or IGBTs and can operate with much lower minimum loads.
According to the Lutron LED Compatibility Guide, trailing-edge dimmers are vastly superior for low-wattage LED circuits. If you have a circuit with three 8.5W LED bulbs (25.5W total), a leading-edge dimmer will likely flicker at 30% brightness because the current drops below the TRIAC's holding threshold. A trailing-edge dimmer, however, will dim smoothly down to 1%.
Heat, Enclosures, and Wiring Constraints
Resistors dissipate energy as heat. When wiring a resistor for LED lights, thermal management is your primary safety constraint. A standard 1/2W carbon film resistor (100kΩ) used for smart switch ghosting dissipates roughly 0.14W on a 120V circuit (P = V²/R). This generates negligible heat and can safely be tucked behind the switch yoke using a Wago 221-212 lever nut.
However, if you are using a 10W or 50W wirewound dummy load resistor to stabilize a magnetic transformer or a high-capacity LED driver, the thermal constraints are severe. A 10W resistor operating at full load will reach surface temperatures of 150°C to 200°C.
- Enclosure Rules: Never bury a high-wattage dummy load inside wall insulation, and never mount it inside a sealed plastic canopy. It must be mounted in a ventilated metal junction box or attached to a ceramic standoff.
- Clearance: Maintain at least 1 inch of air gap between the resistor body and any THHN/NM-B wire insulation to prevent thermal degradation of the PVC jacket.
- Wiring Method: Always wire the resistor in parallel (Line to Load), never in series. Wiring in series will drop the voltage to the LED driver, causing it to brownout and strobe.
The Decision Path: Pick Your Resistor, Dimmer, or Driver
Use this decision tree to terminate your troubleshooting and select the exact part required for your circuit.
| Symptom / Scenario | Root Cause | Concrete Fix (Part / Value) |
|---|---|---|
| Smart switch ghosting (LEDs flash every few seconds when OFF). | Switch standby current leaking through the LED driver capacitor. | Wire a 100kΩ 1/2W carbon film resistor OR the Lutron LUT-MLC in parallel across Line and Load. |
| Flickering at low dim levels (below 30% brightness). | Leading-edge TRIAC dropping out due to low LED holding current. | Replace dimmer with a Trailing-Edge ELV model: Lutron DVELV-300P. |
| Halogen transformer buzzing after swapping to LED MR16 bulbs. | Magnetic transformer requires minimum 20W load; LEDs only draw 5W. | Swap the transformer for an electronic LED driver: HitLights 12V 60W LED Driver. |
| Breaker tripping on startup with large LED arrays. | Capacitive inrush current exceeding the magnetic trip curve of the breaker. | Install an NTC Thermistor (e.g., Ametherm MS35 10018) in series on the hot leg to limit inrush. |
By matching your specific symptom to the correct component, you eliminate the trial-and-error phase. If your issue is purely ghosting on a smart switch, the 100kΩ resistor or LUT-MLC is your definitive fix. If the issue is dimming performance, no resistor will save a mismatched leading-edge dimmer; upgrade to the DVELV-300P and let the MOSFETs handle the low-wattage load natively.
