Before we run any wire, we need to clear up a massive misconception: standard 120V or 230V household LED bulbs are never wired in series. They are wired in parallel across your mains voltage. If you wire two 120V bulbs in series on a 120V circuit, each gets 60V, they will barely glow, and if one fails open, the entire circuit dies.
However, when we talk about LED lights wired in series in professional lighting, landscape design, or DIY PCB builds, we are referring to bare LED emitters (SMD/COB chips) or constant-current LED strips driven by a DC constant-current power supply. In a DC series circuit, the current remains identical through every emitter, while the forward voltage ($V_f$) adds up. This is the most efficient way to drive high-power LEDs because it guarantees uniform brightness and prevents thermal runaway, provided you use the right driver.
Lumens, Watts, and Efficacy in Series Strings
When designing a series string, you are usually selecting bare emitters rather than pre-packaged bulbs. The critical metric here is not just raw lumens, but luminous efficacy (lumens per watt, or lm/W). Pushing an LED past its peak efficacy curve generates waste heat that your heatsink must dissipate.
The table below provides real-world bench data for common high-power emitters used in series arrays. Note that efficacy drops significantly as drive current increases.
| Emitter Model | Forward Voltage ($V_f$) | Drive Current | Watts | Output (Lumens) | Efficacy (lm/W) |
|---|---|---|---|---|---|
| Cree XP-G3 | 2.85V | 350 mA | 1.0W | 165 lm | 165 lm/W |
| Cree XP-G3 | 3.05V | 1050 mA | 3.2W | 340 lm | 106 lm/W |
| Bridgelux V-Series 13mm COB | 36.0V | 360 mA | 13.0W | 1850 lm | 142 lm/W |
| Lumileds Luxeon CoB 1204 | 35.5V | 700 mA | 24.8W | 2700 lm | 108 lm/W |
| Osram Oslon Square | 2.90V | 700 mA | 2.0W | 230 lm | 115 lm/W |
Circuit Impact Math: Inrush, Power Factor, and Driver Selection
Sizing a driver for LED lights wired in series requires strict adherence to constant-current (CC) math. Unlike constant-voltage (CV) LED strips where you just add up the wattage, a CC series string requires you to add up the forward voltages.
The Core Formulas
- Total Voltage ($V_{total}$): $V_{f1} + V_{f2} + ... + V_{fn}$
- Total Current ($I_{total}$): $I_{emitter}$ (Current is identical across all series components)
Let us calculate a driver for a fixture containing 12 Bridgelux V-Series COBs wired in series at 360mA.
$V_{total} = 12 \times 36.0V = 432V$.
This is a high-voltage DC string. You need a specialized high-voltage CC driver, such as a Mean Well HLG-600H-C0360 (which outputs up to 500V at 360mA).
Inrush Current and Power Factor (PF)
When you flip the switch on a high-wattage LED driver, the internal bulk electrolytic capacitors look like a dead short to the AC mains for the first few milliseconds. This inrush current can be 40 to 80 times the steady-state operating current. If you are wiring multiple series-string drivers to a single 15A branch circuit, the cumulative inrush can trip a standard thermal-magnetic breaker or weld the contacts of a standard relay.
The Fix: Use drivers with built-in active Power Factor Correction (PFC) and internal NTC thermistors to limit inrush. According to the US Department of Energy Solid-State Lighting guidelines, commercial drivers should maintain a PF > 0.90 to minimize reactive power draw and reduce the load on upstream wiring. If your driver lacks inrush limiting, install an external NTC inrush current limiter (like an Ametherm MS35) on the AC line side.
Dimmer Compatibility and the Minimum Load Trap
Dimming LED lights wired in series is where most DIY builds fail. How you dim depends entirely on where the dimming occurs: on the AC mains side (phase-cut) or on the DC secondary side (PWM/0-10V).
AC Phase-Cut Dimming (Mains Side)
If your constant-current driver is labeled 'TRIAC Dimmable', it reads the chopped AC sine wave and adjusts its DC output accordingly. Standard incandescent dimmers use Leading-Edge (TRIAC) technology, which chops the front of the sine wave. LEDs despise this; it causes buzzing and dropout. You must use a Trailing-Edge (ELV) dimmer, such as the Lutron Diva DVELV-300P or Leviton IP10-1LZ.
The Minimum Load Trap: Trailing-edge dimmers require a minimum wattage to keep their internal MOSFETs latched. A common minimum load is 15W to 20W. If you wire a series string of three 3W emitters (9W total) to a 15W minimum-load dimmer, the circuit will strobe or fail to turn on. The Fix: Check the dimmer's spec sheet for minimum load. If your series string falls below it, either add more emitters to the string or install a dummy load resistor (like a Lutron LUT-MLC) in parallel at the fixture to satisfy the dimmer's minimum current draw.
DC Side Dimming (Secondary Side)
For raw series strings, the most reliable dimming method is Pulse Width Modulation (PWM) on the DC output. You place a PWM controller (like a Mean Well PWM-60) between the driver and the LED string. This avoids AC phase-cut issues entirely and allows for 0.1% dimming without flicker.
Why Flicker Happens (And How to Fix It)
Flicker in series-wired LEDs is rarely the fault of the emitters themselves; it is almost always a driver or dimmer fault. The Illuminating Engineering Society (IES) and IEEE PAR1789 standard define acceptable flicker limits based on modulation frequency and depth.
- Low-Frequency PWM Dimming: If using a DC PWM dimmer, ensure the frequency is above 1,250 Hz. Anything below 200 Hz will cause visible flicker and potential eye strain. Check your controller's dip switches or datasheet.
- Driver Ripple Current: Cheap constant-current drivers use inadequate output filtering. If the driver's output ripple current exceeds 15% of the DC setpoint, the LEDs will pulse at 120Hz (twice the 60Hz mains frequency). Measure the DC output with an oscilloscope; if you see massive 120Hz sawtooth waves, replace the driver with a low-ripple model (ripple < 5%).
- Incompatible Phase-Cut: If using an AC dimmer, the driver's internal microcontroller may be misreading the trailing edge of the chopped sine wave. Swap to a 0-10V analog dimming signal, which is immune to AC waveform chopping errors.
Heat Dissipation and Enclosure Constraints
When wiring LEDs in series, the total system voltage increases, but the heat generated is localized at each individual emitter. Heat management is the single biggest factor in the lifespan of your series string.
The Thermal Runaway Danger
LEDs have a negative temperature coefficient for forward voltage. As an emitter gets hotter, its $V_f$ drops. If you were to power a series string with a constant-voltage power supply (a catastrophic mistake), the dropping $V_f$ would cause the driver to push more current to maintain the voltage. More current equals more heat, which drops $V_f$ further, until the emitters melt. This is why constant-current drivers are mandatory for series strings; they lock the current, preventing runaway regardless of temperature shifts.
Sizing the Heatsink
Every emitter must be mounted to a Metal Core PCB (MCPCB) and thermally bonded to an aluminum extrusion. To calculate your required heatsink thermal resistance ($R_{th(s-a)}$), use this formula:
$$R_{th(s-a)} = \frac{T_{j(max)} - T_{ambient}}{P_{dissipated}} - R_{th(j-c)} - R_{th(c-s)}$$
- $T_{j(max)}$: Maximum junction temperature (usually 105°C for standard white emitters, but keep it under 85°C for longevity).
- $R_{th(j-c)}$: Junction-to-case thermal resistance (found in the emitter datasheet, e.g., 2.5°C/W).
- $R_{th(c-s)}$: Case-to-sink resistance (depends on your Thermal Interface Material; a high-quality ceramic TIM pad is ~0.5°C/W).
If your math shows you need a heatsink with a thermal resistance of 1.2°C/W, you must select an aluminum extrusion profile that meets or beats that number in natural convection. If the series string is enclosed in a sealed IP65 fixture, ambient temperature inside the enclosure will rise significantly. In enclosed fixtures, you must derate the drive current by 20% to 30% to compensate for the trapped heat, or integrate passive ventilation chimneys to allow convective airflow.
