Designing a Simple Flashing LED Circuit for 24V Lighting

Scaling a Simple Flashing LED Circuit for Architectural Lighting

To scale a simple flashing LED circuit from a single 20mA breadboard indicator to a 100W+ architectural lighting load, you must replace the direct LED connection with a logic-level N-channel MOSFET acting as a low-side switch, driven by a constant-voltage 24V DC LED power supply. The classic NE555 astable multivibrator works, but swapping it for a CMOS LMC555 ensures a rail-to-rail output swing, fully enhancing the MOSFET gate without needing a secondary gate driver IC.

This guide bridges hobbyist DC timing circuits with professional AC-to-DC lighting requirements. We will calculate exact driver sizing, address inrush current limitations on branch circuits, and solve the flicker issues that occur when mixing low-frequency flashing circuits with high-frequency switched-mode power supplies.

Lumens, Watts, and Efficacy: Sizing the LED Load

Before selecting a driver or MOSFET, you must quantify the optical and electrical load. Architectural 24V LED strips are categorized by their chip-on-board (COB) or surface-mount (SMD) density. Efficacy (lumens per watt) is not a static number; it suffers from thermal droop as drive current increases.

Driver Selection and Circuit Impact Math

Let us size a driver for a specific fixture count: three 4-meter runs of Premium COB strip (12W/m). Total load = 12 meters × 12W/m = 144W. Applying the standard 20% headroom rule for continuous operation, we need a driver rated for at least 172.8W. The Lighting Design Lab recommends sizing constant-voltage drivers between 80% and 90% of their maximum rated load for optimal thermal performance and power factor stability.

The Pick: Mean Well HLG-185-24 (185W, 24V, 7.7A).

Circuit Impact: Inrush and Power Factor

When you flash high-wattage LED loads, the AC branch circuit experiences repetitive stress. The HLG-185-24 features active power factor correction (PFC) and large internal bulk capacitors.

• Inrush Current: At 230VAC, cold inrush is 65A for roughly 350µs. At 120VAC, it is approximately 35A. If your flashing circuit operates at a high frequency (e.g., 2Hz) and the driver's internal capacitors fully discharge between flashes, you will hammer the AC breaker. Use a Type C or Type D curve miniature circuit breaker (MCB) rather than a standard Type B, which will nuisance-trip under repetitive magnetic inrush spikes.
• Power Factor (PF): At the 144W load, the HLG-185 maintains a PF > 0.95. However, if your flashing circuit uses pulse-width modulation (PWM) to dim the light during the 'on' phase, dropping the apparent load below 75W, the driver's PFC circuit may disengage, dropping the PF to 0.7 and increasing total harmonic distortion (THD). Keep the DC-side flashing strictly binary (100% on / 100% off) to maintain grid compliance.

Dimmer Compatibility and Flicker Mitigation

Integrating a wall dimmer into a flashing circuit introduces severe compatibility risks. If you place an AC dimmer upstream of a dimmable LED driver (like the Mean Well PWM-170-24), you must adhere to strict trailing-edge criteria.

Trailing Edge and Minimum Load

Leading-edge (TRIAC) dimmers chop the front of the AC sine wave, creating massive dI/dt inrush spikes that will eventually destroy the driver's input bridge rectifier. You must use a trailing-edge (ELV) dimmer, such as the Lutron DVELV-300P. Crucially, this dimmer requires a 15W minimum load to keep its internal MOSFETs commutating correctly. If your flashing circuit drops the total system draw below 15W during a specific state, the dimmer will chatter, buzz, or reset.

Why Flicker Happens (and the Fix)

According to the U.S. Department of Energy's Solid-State Lighting guidelines, visible flicker often results from 'beating'—the interference pattern created when two different frequencies overlap. If your simple flashing LED circuit operates at a 1.5Hz astable rate, but the LED driver's internal smoothing operates at 1kHz, the interaction with the AC phase-cut waveform can create a visible 120Hz stroboscopic ripple during the 'on' state.

The Fix: Do not dim on the AC primary side. Set the wall switch to a standard relay (or set the dimmer to 100% full-on) and let your DC-side LMC555 and MOSFET handle 100% of the flashing modulation. This isolates the AC power factor from the DC timing logic, eliminating beating and flicker entirely.

Thermal Constraints and Enclosure Sizing

Heat kills both timing ICs and power supplies. When switching 144W of 24V LEDs, the DC current is 6A. Using an IRLZ44N logic-level MOSFET (Rds(on) = 0.022Ω at Vgs=5V), the conduction loss is calculated as:

P = I² × R = (6A)² × 0.022Ω = 0.79W

At under 1W, a bare TO-220 package can dissipate the heat into ambient air if the flash rate is slow (e.g., 1 flash per second). However, if you modify the circuit for a fast strobe effect (>100Hz), switching losses during the Miller plateau region will spike the dissipation past 3W. In that scenario, a 20°C/W clip-on heatsink is mandatory to prevent the MOSFET from entering thermal shutdown.

Enclosure Sizing: The 185W driver operates at roughly 90% efficiency at full load, meaning it dissipates 18.5W as heat. For an unvented NEMA-1 polycarbonate enclosure, the standard rule of thumb is 10 square inches of external surface area per watt of dissipated heat. Your enclosure must have a minimum surface area of 185 square inches (e.g., a 10" × 6" × 4" box) to keep internal ambient temperatures below the driver's 45°C derating threshold.

The Decision Path: Selecting Your Final Components

Use this decision matrix to lock in your bill of materials based on your total calculated LED wattage.