LED Strip PCB: Voltage Drop, Segments and Thermal Design

An LED strip is a long, narrow circuit board with LEDs mounted at regular intervals along its length. The design looks simple, and that is the trap. Over a length of several metres the copper resistance accumulates, the voltage at the far end sags, and the brightness and colour of the LEDs change along the strip. Managing that voltage drop is the central design problem, and it interacts with the choice of drive scheme and the thermal design.

This article covers how a strip is built, why the segments are the length they are, and how the substrate and copper choices affect the result.

How an LED Strip Is Built

The substrate is usually a flexible circuit, either a simple single-sided polyimide with a coverlay or, for higher power, a thin metal core board that spreads heat along the strip. Copper thickness ranges from one to three ounces, and the width of the strip is set by the LED pitch and the mechanical mounting rather than by electrical considerations.

LEDs are placed in series groups with a current-limiting resistor, repeated at regular intervals along the strip. The repetition defines the cut points, and it also defines how the strip behaves electrically: each segment is a separate load connected across the same pair of power rails.

Series Segments and Cut Points

Each segment contains a fixed number of LEDs in series, usually three for a 12 V strip and six for a 24 V strip, together with one or more resistors. Because the segments are identical and connected in parallel along the strip, the strip can be cut at the marked points without affecting the remaining part.

The series count is set by the supply voltage and the forward voltage of the LEDs. Too few in series and the resistor has to dissipate a large share of the power; too many and the supply has no headroom to compensate for the drop along the strip. The 12 V and 24 V conventions exist because they balance those two effects, and the higher voltage is preferred for long runs precisely because there is more headroom.

Voltage Drop Along the Strip

Every segment draws its current through the copper trace, and the trace has resistance. The current in the trace increases in steps as each segment is passed, so the voltage drop also increases in steps, and the segment at the far end receives the least voltage. Since the current through an LED is a steep function of the voltage across it, a small drop produces a visible change in brightness.

The remedy is copper. Wider traces, thicker copper or a second layer of copper connected in parallel all reduce the resistance. The trace width and current calculation gives the geometry needed for a chosen drop, and the calculation should be run for the worst case, which is a strip fed from one end at full brightness. Feeding the strip from both ends halves the effective length and reduces the drop by a factor of four, which is usually the cheapest fix available.

Constant Voltage and Constant Current

A constant-voltage strip regulates the supply and relies on the series resistor to set the LED current. It is simple, cheap and the standard approach, but the current depends on the LED forward voltage, which varies with temperature and between production batches. As the strip warms, the forward voltage falls, the current rises, and the strip draws more power.

A constant-current strip regulates the current directly, which removes that dependence and gives a more uniform output along the length. The cost is a driver circuit per segment or per strip, which is why this approach is common in professional lighting and rare in decorative products. Where the application requires accurate colour, constant current is the practical choice.

Thermal Management in the Strip

LEDs convert a significant fraction of their input into heat, and the light output and colour both depend on junction temperature. On a flexible strip the heat has nowhere to go except the thin copper and the air, so the temperature rises quickly when the strip is run at full power in still air.

The copper under and around the LED acts as the heatsink, so a wide copper area connected to the LED pads is the primary thermal measure. Where the power is high, a metal core substrate moves the heat along the strip far better than a polyimide one, and the insulated metal substrate construction is the usual choice for that reason. Derating the drive current is the other lever, and it is often the most effective one, because light output falls more slowly than the electrical power when the current is reduced below the rated value. Running a strip at three quarters of its rated current costs a few percent of the light and can cut the temperature rise by a quarter.

Substrate and Copper Choices

Polyimide with a coverlay is the standard flexible substrate for low and medium power strips. It bends easily, survives the flexing that occurs when the strip is unrolled and installed, and is inexpensive in volume. The coverlay protects the conductors and defines the solder pads, and the rules that govern its openings apply as described in the coverlay design guidance.

Where the strip is mounted on a metal channel, the substrate only has to provide electrical insulation and the thermal path is through the channel. In that case a thin polyimide strip with a thermally conductive adhesive layer performs better than a thick one, because the heat has less material to cross before it reaches the metal.

Bending and Mounting

A strip is bent during installation, and the bend radius matters. Copper cracks under repeated flexing, so the direction of the bend relative to the trace layout is important, and the strip should be specified with the tightest bend it has to survive. For an installation that forms the strip once, the requirement is modest; for a strip that moves, it is not.

Mounting adhesive is part of the thermal path. A strip attached with a thin, thermally conductive adhesive transfers heat into the mounting surface, while one attached with a thick foam tape is thermally isolated and will run hotter. That choice should be made with the current derating in mind, since the two interact directly.

FAQ

Why do my LEDs dim toward the end of the strip? The copper trace resistance drops the voltage progressively along the strip, and the LED current falls with it. Feeding the strip from both ends, using a higher supply voltage with more LEDs in series, or specifying thicker copper all reduce the effect.

What is the maximum length for a single 12 V strip? It depends on the copper weight and the current per segment. As a practical guide, a two-ounce strip at a moderate current can run several metres before the drop becomes visible, while a high-power strip may need to be fed every metre or two.

Can an LED strip be used outdoors? It can, with the right materials. The LEDs, the coverlay and the connectors all have to tolerate moisture and ultraviolet exposure, and the strip is usually mounted in a sealed channel rather than left exposed.