An LED light chaser circuit sequences illumination across multiple nodes to create motion effects, commonly used in architectural cove lighting, marquee signs, and runway indicators. While sequencing low-voltage addressable pixels (like WS2812B) is trivial, building a high-power chaser for 12V/24V COB strips or 120V AC fixtures requires bridging DC logic with constant-current LED drivers. The direct answer for a robust build: use a microcontroller or CD4017 logic to trigger logic-level MOSFETs on the DC secondary side, but size your AC mains driver for the inrush current of the switching channels, not just the steady-state wattage.
Circuit Impact Math: Inrush Current and Power Factor
When a chaser circuit switches channels rapidly, the repetitive charging and discharging of the LED driver's input capacitors creates massive inrush current spikes. If you use mechanical relays to switch the AC mains side of the drivers, the contacts will weld shut within a few thousand cycles.
The inrush current ($I_{inrush}$) is dictated by the peak AC voltage and the Equivalent Series Resistance (ESR) of the driver's input filter capacitors:
$I_{inrush} = \frac{V_{peak}}{R_{ESR}}$
For a 120V AC branch, $V_{peak}$ is roughly 170V. A typical 150W industrial driver (like the Mean Well HLG-150H) might have an internal ESR of 2.5 ohms at turn-on, resulting in a momentary 68A spike. According to the Mean Well inrush current technical note, repetitive inrush degrades upstream breakers and solid-state relays (SSRs) if not managed.
Power Factor (PF) Impact: Active Power Factor Correction (PFC) circuits in LED drivers are optimized for 75-100% load. In a 4-channel chaser, when only one channel is active, the driver operates at 25% load. At this level, the PF of a standard 150W driver can drop from 0.95 down to 0.65. On a 20A branch circuit, this poor PF increases apparent power (VA) and heats up the neutral conductor in 3-phase commercial wye systems. Always calculate branch circuit loading using the lowest expected PF if the chaser dwells on single channels.
Lumens, Watts, and Efficacy in Chaser Arrays
Because chaser circuits rely on persistence of vision and rapid duty cycling, the perceived brightness of the array is different from continuous illumination. You must select LEDs based on efficacy (lumens per watt) to minimize the thermal load on the heatsinks during the 'on' portion of the chase cycle.
| LED Type | Nominal Wattage | Typical Lumens | Efficacy (lm/W) | Chaser Application Context |
|---|---|---|---|---|
| Standard SMD 2835 Strip | 14.4W/m | 1,300 lm/m | 90 lm/W | Indoor cove lighting; low thermal mass allows fast chasing without overheating. |
| High-Density COB Strip | 20W/m | 2,200 lm/m | 110 lm/W | Dot-free marquee runs; requires aluminum extrusion for heat sinking during high-duty cycles. |
| Architectural Spot Module | 7W per node | 850 lm | 121 lm/W | Outdoor runway or facade chasing; high efficacy reduces enclosure thermal buildup. |
| Vintage Edison Filament | 4W per bulb | 150 lm | 37 lm/W | Aesthetic marquees; extremely low efficacy means 80% of input energy becomes heat. |
Note: Efficacy drops as junction temperature rises. A COB strip driven at 20W/m will lose up to 15% of its lumen output if the aluminum channel exceeds 60°C.
Dimmer Compatibility and Driver Selection
A common failure in commercial chaser installs is attempting to put the entire master AC bus on a standard wall dimmer. Which dimmer and driver should you use for a multi-fixture count? The answer depends entirely on the minimum load requirement of the dimmer and the dimming topology of the driver.
For phase-cut dimming, you must use a trailing-edge (ELV) dimmer. Leading-edge (TRIAC) dimmers cause audible buzzing in LED drivers and fail to trigger the internal PFC circuits correctly. However, trailing-edge dimmers have strict minimum load requirements. For example, the Lutron Diva DVELV-300P requires a 15W minimum load to maintain the internal MOSFET's gate drive.
If your chaser circuit powers four 50W channels (200W total), you might think a 300W dimmer is perfect. But if the chaser logic drops the overall duty cycle to 10% for a slow 'fade' effect, the effective RMS load presented to the dimmer falls below 15W. The dimmer will drop out, causing the chaser to stall or strobe unpredictably. The Lutron LED compatibility guidelines explicitly warn against dynamic loads falling below the minimum threshold.
Thermal Management and Enclosure Constraints
Chasing inherently reduces the average thermal load on the LEDs because they are off for a percentage of the cycle. However, the LED driver runs at 100% continuous duty. Heat and enclosure constraints are dictated by the driver, not the LEDs.
If you are mounting the drivers inside a sealed NEMA 4X polycarbonate enclosure for an outdoor facade chaser, you must apply thermal derating. A driver rated for 150W at 40°C ambient must be derated by roughly 40% if the internal enclosure ambient reaches 60°C due to solar loading. This means a 150W driver can only safely output 90W continuously in that environment. Always mount drivers outside the sealed optical enclosure, or use a NEMA-rated enclosure with a breather valve and a sun shield to keep the internal ambient below 45°C.
LED Light Chaser Circuit FAQ
Why does my LED light chaser circuit flicker at low speeds?
Flicker in an LED light chaser circuit at low transition speeds is almost always caused by a frequency mismatch between the DC PWM logic and the AC phase-cut dimmer, or by a driver's internal output capacitor discharging too slowly. If you are using a 555 timer or microcontroller to PWM the DC side at 200Hz, and the AC side is on a 120Hz phase-cut dimmer, the beat frequencies will cause visible strobing. The fix: Increase your DC PWM frequency to at least 1kHz to 5kHz, well above the AC ripple frequency, and ensure your LED driver is rated for 'flicker-free' or 'high-frequency' PWM dimming. Alternatively, switch to constant-current analog dimming (0-10V) which eliminates PWM ripple entirely.
Can I use a standard 555 timer and CD4017 for a 120V AC LED light chaser circuit?
Yes, but you must isolate the low-voltage DC logic from the 120V AC mains. The All About Circuits 555 timer guide details the astable multivibrator setup needed to generate the clock pulse for the CD4017 decade counter. The CD4017 outputs can only source about 10mA at 5V-9V. You cannot connect this directly to 120V AC fixtures. You must use the CD4017 outputs to trigger the gate of a logic-level MOSFET (for 12V/24V DC LED strips) or an opto-isolated triac/SSR (for 120V AC bulbs). Never share the ground reference between your 120V AC load circuit and your 9V DC logic circuit.
How do I calculate the wire size for a 50-foot outdoor LED light chaser circuit run?
Wire sizing for an LED light chaser circuit depends on the total DC current and acceptable voltage drop (usually 3% for lighting). If you are running a 24V DC chaser array drawing 12A total over a 50-foot one-way run (100 feet total round-trip wire length), use the voltage drop formula: $VD = \frac{2 \times K \times I \times L}{CM}$. Using copper ($K=12.9$), 12A, and 100ft, 14 AWG wire (4110 circular mils) yields a 0.75V drop (3.1%), which is marginal. Upgrading to 12 AWG wire (6530 circular mils) drops the loss to 0.47V (1.9%), which is ideal. Always size the overcurrent protection (fuse or breaker) to match the wire ampacity (e.g., 20A fuse for 12 AWG), not just the load current.
