If you are wiring a smart dimmer to low-wattage LEDs and fighting ghosting or flicker, the default LED protective resistor pick is a Vishay PR02FS 100kΩ 2W metal film resistor wired in parallel across the line and load. For high-power 12V/24V DC LED arrays requiring current limiting, use a Bourns 68Ω 5W wirewound resistor in series. For AC mains inrush protection on large commercial drivers, install an Ametherm SL32 2R015 NTC thermistor. The sections below break down the exact math, dimmer compatibility criteria, and thermal constraints to ensure your circuit operates without dropping out or burning up.
The Dual Role of LED Protective Resistors
In lighting circuits, the term 'protective resistor' bridges two entirely different domains depending on your voltage source. In low-voltage DC applications, it acts as a current limiter, dropping excess voltage to protect the LED junction from thermal runaway. In 120V/230V AC mains applications, it acts as a bleed resistor (dummy load) or an inrush limiter.
Modern LED drivers are highly efficient switching power supplies. While this is great for your electric bill, it creates havoc with legacy TRIAC dimmers and smart switches. Smart switches require a small amount of leakage current to power their internal Wi-Fi or Zigbee radios when the light is 'off'. Without a protective bleed resistor, this micro-current charges the LED driver's bulk input capacitor until it reaches the firing threshold, causing the bulb to flash or glow faintly (ghosting). The bleed resistor provides a parallel path to safely dissipate this leakage current.
Lumens, Watts, and Efficacy: Sizing the Load
To correctly size a protective resistor or select a compatible dimmer, you must understand the real power draw of your fixtures. Incandescent bulbs were purely resistive; their wattage directly correlated to their light output and dimmer load. LEDs are non-linear, and their efficacy (lumens per watt) dictates the actual load the dimmer sees.
| Legacy Incandescent | Modern LED Equivalent | Target Lumens | LED Efficacy (lm/W) | Real Power Draw (W) | Apparent Power (VA) @ 0.7 PF |
|---|---|---|---|---|---|
| 40W | 5W LED | 450 | 90 lm/W | 5W | 7.1 VA |
| 60W | 9W LED | 800 | 88 lm/W | 9W | 12.8 VA |
| 75W | 11W LED | 1100 | 100 lm/W | 11W | 15.7 VA |
| 100W | 15W LED | 1600 | 106 lm/W | 15W | 21.4 VA |
Dimmer Compatibility: Trailing Edge, Minimum Load, and Flicker
Flicker and ghosting in LED circuits almost always trace back to a mismatch between the dimmer's minimum load requirement and the LED driver's input impedance. According to the U.S. Department of Energy's SSL troubleshooting guidelines, incompatibility accounts for over 70% of LED dimming complaints.
Leading Edge (TRIAC) vs. Trailing Edge (ELV)
Leading Edge (Forward Phase): The standard for incandescent loads. It chops the front of the AC sine wave. These require a minimum holding current (typically 20mA to 50mA) to keep the TRIAC latched. If you connect three 5W LEDs (15W total) to a dimmer with a 25W minimum load, the current falls below the holding threshold during the zero-crossing, resulting in 60Hz strobing.
Trailing Edge (Reverse Phase / ELV): Uses MOSFETs or IGBTs to chop the back of the sine wave. These have much lower minimum load requirements (often 5W or less) and provide a smoother turn-on for the capacitive input stage of LED drivers. Always default to a trailing-edge dimmer for new LED installations.
Why Flicker Happens (and the Fix)
When a smart switch is in the 'off' state, it leaks 1mA to 3mA through the load to keep its internal logic alive. This current passes through the LED driver's bridge rectifier and slowly charges the primary bulk capacitor. Once the capacitor voltage hits the driver's startup threshold (usually around 40V-60V DC), the driver fires, the capacitor drains, the bulb flashes, and the cycle repeats. The fix: Wire an LED protective bleed resistor in parallel with the fixture (Line to Load) to divert this leakage current away from the driver's capacitor.
Circuit Impact Math: Inrush Current, Power Factor, and Heat
Selecting the right resistor requires balancing electrical dissipation with physical thermal constraints inside a junction box.
Bleed Resistor Sizing Math (120V AC)
To stop ghosting, we need to bleed off roughly 2mA of leakage current at 120V RMS.
- Resistance: R = V / I = 120V / 0.002A = 60,000Ω (60kΩ). The nearest standard E24 value is 68kΩ or 100kΩ.
- Power Dissipation: P = V² / R = (120)² / 100,000 = 14,400 / 100,000 = 0.144 Watts.
Inrush Current and Driver Power Factor
For large commercial LED drivers (e.g., 150W+ high bays), the inrush current when the contactor closes can trip upstream breakers. The peak inrush current is limited only by the circuit impedance and the driver's internal ESR (Equivalent Series Resistance).
Ipeak = Vpeak / RESR
If a 240V circuit (339V peak) feeds a driver with an ESR of 0.5Ω, the inrush is 678 Amps. To protect the circuit, an NTC thermistor (a temperature-dependent protective resistor) is placed in series. A cold NTC might have 10Ω of resistance, limiting inrush to 33.9A. As it heats up from the steady-state current, its resistance drops to <0.1Ω, minimizing steady-state power loss.
Heat and Enclosure Constraints
Wirewound resistors and NTC thermistors generate heat. Per RPI's Lighting Research Center data on thermal management, ambient temperature inside a sealed junction box can be 15°C to 20°C higher than room temperature. If you are mounting a 5W wirewound bleed resistor in a crowded, insulated junction box, you must apply a 50% thermal derating factor. If the resistor will dissipate 2W continuous, specify a 5W or 7W chassis-mount part to prevent scorching the NM-B cable insulation.
Decision Tree: Selecting Your Resistor and Driver
Use this decision matrix to terminate your design process with a specific, actionable component choice. Do not guess; match your exact circuit symptom to the required part.
| Circuit Scenario / Symptom | Fixture Count & Load | Required Action / Criteria | Concrete Component Pick |
|---|---|---|---|
| Smart switch ghosting (bulbs glow when off) | 1 to 4 LEDs (< 40W total) | Bleed 1-2mA; must be wired Line-to-Load; high voltage rating required. | Vishay PR02FS 100kΩ 2W (Metal Film, 500V rating) |
| 60Hz Flicker at low dim (TRIAC dropout) | 3+ LEDs on Leading Edge dimmer | Upgrade dimmer to Trailing Edge (ELV) with < 10W min load. No resistor needed. | Lutron Diva DVELV-300P (Trailing Edge Dimmer) |
| High-Power DC Array (12V strip, 3x 3W LEDs in series) | 12V Source, Vf=9.6V, Target I=700mA | R = (12 - 9.6) / 0.7 = 3.4Ω. P = I²R = 1.66W. Derate by 50% for heat. | Bourns 3.3Ω 5W Wirewound (Chassis mount to heatsink) |
| Mains Inrush Tripping Breaker (Large commercial driver) | 150W+ Driver, 120V/240V AC | Limit cold inrush to < 40A; steady state < 1A. Requires NTC thermistor. | Ametherm SL32 2R015 (15A steady, 2Ω cold) |
The Default Recommendation
If you are a DIYer or contractor dealing with the most common issue—smart switches causing LED ghosting or flickering on standard 120V AC residential circuits—and you do not want to swap out the dimmer for a $60 ELV model, your default pick is the Vishay PR02FS 100kΩ 2W metal film resistor. It costs less than $0.50, safely handles AC mains peak voltages without arcing, dissipates a negligible 0.14W of heat, and reliably bleeds the leakage current that causes ghosting. Wire it directly across the Line and Load terminals at the dimmer switch, torque the terminals to 14 in-lbs, and your circuit will operate flawlessly.
For comprehensive design parameters on switching power supplies and LED driver topologies, refer to the Texas Instruments LED Driver Design Application Notes to ensure your protective components align with the driver's specific input capacitance and control loop.
