Wiring an LED series circuit requires a constant current (CC) driver where the driver's output voltage range encompasses the sum of the forward voltages (Vf) of all LEDs in the string, while maintaining a strict, unvarying current. Unlike parallel circuits where voltage is constant and current divides, a series string forces the exact same current through every diode, ensuring uniform brightness and eliminating the need for individual current-limiting resistors on each chip.
The Physics of an LED Series String
In a series topology, voltages add up while current remains constant. If you wire ten high-power LEDs in series, and each has a forward voltage (Vf) of 3.2V at 700mA, your total string voltage is 32V. Your CC driver must be rated to output at least 32V, but ideally, you select a driver with an output range of 24V to 40V to account for thermal drift and manufacturing tolerances in the LED Vf.
The primary advantage of the LED series configuration is current matching. Because LEDs are highly sensitive to current variations—which cause exponential shifts in light output and junction temperature—forcing identical current through the entire string guarantees uniform lumen output. The tradeoff is reliability: if a single LED in a basic series string fails open, the entire circuit breaks and the whole fixture goes dark. Commercial fixtures mitigate this by using bypass ICs across each LED or by utilizing series-parallel hybrid topologies.
Sizing the Driver and Dimmer for Your Fixture Count
Choosing the right driver and dimmer for a specific fixture count requires checking three parameters: voltage headroom, minimum load, and phase-cut compatibility.
Driver Sizing and Inrush Math
When calculating the driver size, multiply the typical Vf by your fixture count, then add a 10% to 15% voltage overhead. For a 6-fixture count of 36V nominal LEDs, you need a driver with a maximum output of at least 40V.
Cheap CC drivers lacking an internal NTC thermistor can exhibit inrush currents 50x to 100x their steady-state draw. A 150W driver might pull 60A for 200 microseconds upon switch-on. If you wire eight of these drivers to a single 20A branch circuit, the combined instantaneous surge can exceed 400A, tripping the magnetic trip mechanism of your breaker. Always check the driver datasheet for 'Inrush Current' (often measured at 240VAC, 90° turn-on) and stagger the startup via smart relays if combining high-wattage drivers on one circuit.
Power Factor (PF) Impact: A driver with a poor PF of 0.65 drawing 100W of real power actually pulls 153VA of apparent power. At 120VAC, that is 1.27A of current instead of the expected 0.83A. For commercial LED series arrays, always specify drivers with an active PF correction circuit (PF > 0.9) to prevent overloading the neutral conductor and avoiding utility penalty tariffs.
Dimmer Compatibility and Minimum Load
Dimming an LED series string on mains voltage requires a trailing-edge (ELV) dimmer, not a leading-edge (Triac) dimmer. Trailing-edge dimmers use MOSFETs to chop the back half of the AC sine wave, which aligns with the capacitive input filters of modern LED drivers.
The most common failure point in residential LED dimming is ignoring the minimum load requirement. A standard Lutron Skylark SELV-300P trailing-edge dimmer requires a minimum load of 15W to keep its internal circuitry powered. If your LED series string consists of three 4W modules (12W total), the dimmer will drop out, causing severe flickering or failure to turn on. Always verify that the total connected LED wattage exceeds the dimmer's published minimum load threshold.
Efficacy, Heat, and Enclosure Constraints
Pushing maximum current through an LED series string generates significant junction heat. Efficacy (lumens per watt) drops as the junction temperature (Tj) rises. The U.S. Department of Energy's Solid-State Lighting program highlights that thermal management is the primary determinant of LED lifespan.
| Module Wattage | Nominal Lumens | Efficacy (lm/W) | Thermal Waste (BTU/hr) | Typical Application |
|---|---|---|---|---|
| 9W | 1,080 lm | 120 lm/W | 10.2 BTU/hr | Recessed downlights |
| 18W | 2,340 lm | 130 lm/W | 20.5 BTU/hr | Track lighting / Pendants |
| 35W | 4,900 lm | 140 lm/W | 34.1 BTU/hr | High-bay / Flood arrays |
| 60W | 7,800 lm | 130 lm/W | 68.2 BTU/hr | Architectural linear runs |
Note: Efficacy peaks at lower drive currents. Overdriving a 35W module to 50W will increase lumens but drastically reduce lm/W efficacy and spike junction temperatures.
Enclosure Constraints: When sealing an LED series array inside an IP65-rated enclosure, ambient air cannot circulate. The internal ambient temperature can easily rise 20°C to 30°C above the room temperature. According to DesignLights Consortium (DLC) testing standards, the internal driver temperature must not exceed its rated Tc (case temperature) point, typically 85°C or 90°C. For every 10°C the electrolytic capacitors inside the driver operate above their rated temperature, the driver's operational lifespan is cut in half. Always mount the driver outside the sealed optical chamber, or use remote-mounted drivers connected via low-voltage DC wiring.
LED Series Circuit FAQ
Why does my LED series string flicker on a dimmer, and how do I fix it?
Flicker in an LED series circuit usually stems from one of two issues: a leading-edge/trailing-edge mismatch, or failing to meet the dimmer's minimum load. If the dimmer is leading-edge (Triac), it will clash with the driver's capacitive filter, causing a 120Hz strobe effect. The fix: Swap to an ELV trailing-edge dimmer. If you are already using a trailing-edge dimmer and it flickers specifically at the low-end (below 20% brightness), the driver's internal bleeder circuit isn't drawing enough holding current to keep the dimmer's MOSFETs latched. The fix: Wire a 10W / 120V dummy load resistor in parallel with the primary AC side of the driver, or upgrade to a 0-10V DC dimming topology which bypasses phase-cut issues entirely.
Which dimmer and driver should I use for a 6-fixture LED series count?
First, calculate the total wattage. If each fixture is 12W, your total load is 72W. For the driver, select a constant current model with an output voltage range that covers 6x the LED's Vf, and ensure it supports 0-10V or PWM dimming for the cleanest results. For the dimmer, if you must use phase-cut, choose a high-capacity trailing-edge model like the Lutron Diva DVCLV-153P. Because your 72W load easily exceeds the typical 15W minimum load threshold of this dimmer, it will operate smoothly without dropout. Always verify the dimmer's maximum LED load rating, which is often much lower than its incandescent rating (e.g., 150W LED max vs. 600W incandescent max).
What are the heat and enclosure constraints for high-density LED series arrays?
High-density arrays (like COB modules wired in series) generate intense localized heat. The primary constraint is the LED's maximum junction temperature (Tj), usually rated at 85°C or 105°C by manufacturers like Cree or Lumileds. If Tj exceeds this, the phosphor layer degrades, causing color shift and permanent lumen depreciation. You cannot mount high-density series arrays on FR4 fiberglass PCBs; you must use Aluminum Core PCBs (MCPCBs) with a thermal conductivity of at least 2.0 W/m·K, bonded directly to an extruded aluminum heatsink using a phase-change thermal interface material (TIM). In sealed enclosures, you must derate the drive current by 15% to 20% to compensate for the lack of convective cooling.
