To stop 120V AC LED dimmer flicker, strobing, or ghosting, you must install a dummy load resistor in parallel across the Line (Hot) and Load (Switched Hot) wires at the dimmer switch box. Older TRIAC dimmers require a minimum holding current—typically 15W to 25W—to stay latched in the ON state. Because modern LED bulbs draw as little as 7W, the dimmer misfires. Installing a 50W, aluminum-housed wirewound resistor (approx. 576Ω) bridges this gap, drawing the missing current without altering the LED's light output.
This procedure involves 120V AC mains wiring. Before touching any wires, de-energize the circuit at the breaker panel and apply a lockout/tagout device if possible. Verify the circuit is dead using a properly functioning CAT III or CAT IV non-contact voltage tester and a multimeter. Never rely solely on a wall switch to isolate power. Local electrical codes (NEC Article 404) may require this work to be performed or inspected by a licensed electrician. Always defer to your local Authority Having Jurisdiction (AHJ).
Why TRIAC Dimmers Need LED Load Resistors
Traditional incandescent dimmers use a TRIAC (Triode for Alternating Current) semiconductor to chop the AC sine wave, reducing the RMS voltage sent to the bulb. A TRIAC requires a minimum 'holding current' (usually 20mA to 50mA) to remain conductive through the zero-crossing point of the AC wave. A 60W incandescent bulb easily provides 500mA. A 9W LED equivalent provides only ~75mA, which often drops below the TRIAC's threshold as the dimmer is lowered. When the current drops out, the TRIAC snaps off, the dimmer's internal capacitor recharges, and the TRIAC violently snaps back on. This rapid cycling manifests as visible flicker, strobing, or a faint 'ghost' glow when the switch is fully off.
By wiring a dummy load resistor in parallel with the fixture, you provide an artificial current path. When the dimmer is ON, the resistor draws enough current to keep the TRIAC latched. When the dimmer is OFF, the resistor safely bleeds off the leakage current that causes ghosting.
Tools, Materials, and Resistor Sizing Chart
Do not use standard carbon-film or carbon-composition resistors for this application; they will overheat and catch fire at these wattages. You must use a wirewound, aluminum-housed, chassis-mount resistor (e.g., Vishay RH050 series). The aluminum housing acts as a heatsink and must be bolted to a metal junction box to dissipate thermal energy.
Required Tools and Materials
- Resistor: 50W Aluminum Chassis-Mount Wirewound Resistor (576Ω for 25W loads, 960Ω for 15W loads).
- Wire: 14 AWG THHN (rated 600V, 90°C) in Black, Red, White, and Green/Bare for pigtails.
- Connectors: UL-listed twist-on wire connectors (e.g., Ideal Yellow Wire-Nuts) rated for 14 AWG solid/stranded combinations.
- Hardware: #6-32 or #8-32 machine screws and nuts to mount the resistor to the metal box.
- Test Equipment: CAT III/IV Digital Multimeter and Non-Contact Voltage Tester.
- Hand Tools: Wire strippers (calibrated for 14 AWG), Lineman's pliers, Phillips/Flathead screwdrivers, torque screwdriver.
Resistor Sizing for 120V AC LED Dimmer Loads
The table below dictates the exact resistor value and wattage rating required based on your dimmer's minimum load specification. Assumptions: 120V AC nominal (114V-126V actual), 60Hz, copper conductors. Always select a resistor with a wattage rating at least double the calculated dissipation to account for trapped heat inside a closed junction box.
| Dimmer Type | Min. Load Req. | Resistor Wattage (Derated) | Target Resistance (at 120V) | Thermal / Mounting Note |
|---|---|---|---|---|
| Standard TRIAC (Incandescent) | 25W | 50W | ~576Ω | Must bolt to metal box; high heat. |
| ELV (Electronic Low Voltage) | 15W | 30W or 50W | ~960Ω | Moderate heat; metal box preferred. |
| MLV (Magnetic Low Voltage) | 20W | 40W or 50W | ~720Ω | High heat; requires metal box mounting. |
| Modern LED (CL / ELV-LED) | 2W - 5W | N/A (No Resistor) | N/A | Upgrade the dimmer switch instead. |
Reference: Resistance calculated using P = V² / R. For deeper compatibility data, consult the Lutron LUT-MLC Installation Guide or your specific dimmer manufacturer's LED compatibility matrix.
Step-by-Step: Wiring the LED Resistor at the Dimmer Switch
This procedure assumes a modern NEC-compliant 3-wire switch loop (14/3 NM-B or THHN in conduit) where Black is Line (Always Hot), Red is Load (Switched Hot), White is Neutral, and Bare is Ground. If you have an older 2-wire switch loop, identify the re-identified white wire as your Line.
- De-energize and Verify: Turn off the breaker. Test the Black (Line) and Red (Load) wires with your multimeter (set to VAC) to confirm 0.0V.
- Mount the Resistor: Apply a thin layer of thermal compound to the back of the aluminum-housed resistor. Bolt it directly to the interior back or side of the metal junction box using machine screws. Ensure the resistor body does not touch any wire insulation.
- Connect the Ground: Splice the Bare Copper ground wires from the wall, the dimmer's Green pigtail, and a 14 AWG Bare pigtail to the metal box's green grounding screw. Tighten to 1.2 Nm (10 in-lbs).
- Wire the Line (Hot): Connect the wall's Black (Line) wire, the dimmer's Black (Line) wire (or Brass screw terminal), and Resistor Lead 1 (Black high-temp wire) together using a yellow wire nut. Twist until the wires torque tightly and no bare copper is exposed.
- Wire the Load (Switched Hot): Connect the wall's Red (Load) wire, the dimmer's Red (Load) wire, and Resistor Lead 2 (White high-temp wire) together with a second yellow wire nut. Note: The resistor is non-polarized, so Lead 1 and Lead 2 are interchangeable, but maintaining Black-to-Line and White-to-Load color consistency aids future troubleshooting.
- Cap the Neutral: If your dimmer does not require a neutral (common for 2-wire TRIACs), cap the wall's White (Neutral) wire with a wire nut and tuck it into the back of the box. If it is a smart dimmer requiring neutral, connect the wall's White to the dimmer's White.
- Secure and Dress: Carefully fold the wires into the box. Ensure the resistor's high-temp leads are not pinched against the sharp edges of the metal box knockout holes. Mount the dimmer switch and install the wall plate.
Verification, Meter Testing, and the Most Common Botch
Never skip the verification step. A miswired resistor will either fail to stop the flicker or create a severe fire hazard.
Expected Meter Readings
Restore power at the breaker. Set your multimeter to AC Voltage (VAC) and carefully place the probes across the two wire nuts connecting the resistor (Line and Load).
- With Dimmer Switch OFF: The meter should read ~120V AC (nominal range 114V-126V). The resistor is actively drawing leakage current to prevent ghosting. It will begin to warm up.
- With Dimmer Switch ON (100% Brightness): The meter should read < 2.0V AC. Because the switch is closed, the Line and Load are at the same potential. The resistor stops dissipating heat, saving energy and protecting the junction box from thermal overload.
- With Dimmer Switch ON (Dimmed to 20%): The meter will read a fluctuating RMS voltage (typically 40V-70V). The resistor will draw partial current, assisting the TRIAC in maintaining its latch. The LEDs should remain completely flicker-free.
The Most Common Botch: Wiring in Series
The Mistake: Cutting the Black (Hot) wire and wiring the resistor in series (inline) between the panel and the fixture, treating it like a current-limiting resistor for a raw 12V DC LED strip.
The Symptom: The LEDs will either not turn on at all, or they will glow very dimly. The resistor will instantly overheat, potentially melting the wire nuts or tripping the breaker.
The Physics: In a series circuit, the resistor acts as a massive voltage drop. If you use a 576Ω resistor in series with a 9W LED (which has an internal driver impedance), the resistor will drop nearly the entire 120V, dissipating 25W of heat continuously while starving the LED driver of the voltage it needs to operate. Dummy load resistors for AC dimmers must always be wired in parallel across the Line and Load to provide an alternate current path without interrupting the voltage reaching the fixture.
By correctly sizing the wirewound resistor and wiring it in parallel across the Line and Load, you satisfy the TRIAC's holding current requirements, eliminate ghosting, and ensure your LED retrofit operates smoothly for years to come. For comprehensive grounding and switch installation standards, always refer to NFPA 70 (National Electrical Code) guidelines.
