Why does brightness decrease along a long LED strip?
Why does brightness decrease along a long LED strip?
A bright beginning and a dim far end usually point to voltage drop. The power supply may be producing the correct voltage, but resistance in the wiring, connectors and strip's copper tracks leaves less voltage available farther along the installation.
The pattern matters. A gradual fade along the strip, a sudden change after a connector and dimming that develops as the installation warms up are different clues. This guide focuses on low-voltage, constant-voltage LED strips, typically rated for 12 V or 24 V DC. Mains-voltage strips and specialist constant-current systems require their own installation guidance.
Start with the way the fault appears
| What you see | What to investigate | Useful comparison |
|---|---|---|
| Brightness falls progressively toward the far end | Voltage loss along the strip and excessive length supplied from one point. | Compare the operating voltage at the beginning and end. |
| Everything beyond one joint is noticeably dimmer | A poor contact, unsuitable connector or defective solder joint. | Compare voltages before and after the joint; inspect it with power disconnected. |
| The entire strip is dim | Incorrect supply voltage, loss in the feed cable, a dimmer setting or insufficient supply capacity. | Compare supply output and strip input under load. |
| Output drops after several minutes | Temperature effects or protection in the supply or controller. | Compare cold and warm operation against the manufacturers' temperature limits. |
| Mixed RGB white changes colour along the run | Power distribution, channel connections and controller limits. | Compare individual colours with the white setting. |
| Only a small section is dark or weak | Local damage to an LED group, resistor, solder joint or copper track. | Inspect the affected section and its boundaries with power disconnected. |
These observations narrow the investigation; they do not identify a failed component by themselves. If the supply output is unstable, consult the guide to identifying a faulty adapter. Temperature-related symptoms also warrant a look at adapter heating.
The copper tracks are part of the circuit
Copper has electrical resistance. On a strip fed from one end, the first section of track carries the combined current needed by the sections farther along. Each part of that path contributes some voltage loss.
This is why voltage and current capacity must be considered separately. A supply can have ample current capacity while the voltage available at the far end remains too low.
Common resistor-based strips contain repeated LED groups connected across shared supply rails. Their response depends on LED operating characteristics and the resistors that limit current. A reduction in supply voltage can therefore cause a substantial change in light output. A 10% voltage loss does not imply exactly 10% less brightness.
The supply cable can introduce a separate loss before power reaches the strip. Read about voltage drop in long cables and how to calculate voltage drop. For a two-wire DC circuit, the resistance of both the outgoing and return conductors matters.
Locate the loss with three voltage readings
With suitable equipment and the necessary competence, compare voltage at the supply output, strip input and far end. Work only on safely accessible low-voltage DC points. Measure across the local positive and negative supply connections at each location while the strip operates at the same setting.
Review DC voltage measurement and the purpose of the COM, V and current input sockets first. Avoid bridging adjacent strip pads with a probe. Readings on PWM-controlled outputs need to be interpreted in light of the controller's operation.
An illustrative set of readings: Suppose a 24 V installation measures 24.0 V at the supply, 23.7 V at the strip input and 21.8 V at the far end. The feed path accounts for a 0.3 V difference, while the strip and its intermediate joints account for a further 1.9 V. That points the investigation toward distribution along the strip. These figures demonstrate the method; they are not universal pass/fail limits.
A no-load reading is insufficient because the loss changes with current. Also, current measurement uses a different connection: never put a meter set to amperes directly across the supply. Perform resistance measurements and continuity checks with power disconnected. A continuity beep does not prove that a connection can carry the operating current reliably.
Why a larger power supply may make no difference
Replacing an undersized supply can help if its output is collapsing under load. It does not remove resistance from the strip. For instance, 8 metres of strip rated at 12 W per metre has a nominal demand of 96 W. That figure helps size the supply, but says nothing about whether all 8 metres can be fed from one end.
Use the guide to LED strip power supply sizing and, for a 12 V installation, the required current rating. Allow for the manufacturer's operating margin, temperature conditions and controller limits.
The basics of matching voltage and current ratings still apply. A higher-current adapter does not increase the current rating of the strip tracks or connectors. Raising the output voltage to brighten the far end can overdrive the LEDs near the supply.
Improve the distribution, not just the supply rating
Where the product permits it, divide a long installation into shorter runs with separate feed cables from the same supply. If a particular strip allows a maximum 5-metre run from one feed point, a 10-metre installation could be arranged as two separately fed 5-metre branches. The supply must support their combined load, and each branch must remain within its wiring and connection limits.
Compatible single-colour strips can also use feeds at both ends or approved intermediate points. There is no universal spacing for these connections. Check the manufacturer's maximum run length, permitted feed arrangement and connection ratings. Select wiring for both current-carrying capacity and acceptable voltage drop, with branch protection appropriate to the conductors and installation.
Multiple feed cables from one supply are not the same as connecting separate supplies together. Do not parallel outputs unless the manufacturers explicitly support that arrangement. With multiple supplies, separate the powered sections as required and follow the controller's instructions for any shared reference connection.
Check every joint in the current path
A small connector or poor joint can introduce a large local loss. For distribution wiring, compare terminal and connection methods. For soldered strip joints, review cold solder joint symptoms. A connection that becomes unusually hot needs investigation; the article on electrical cable heating explains the related losses.
If the installation uses a barrel connector, verify connector polarity and the fit of 5.5 × 2.1 mm and 5.5 × 2.5 mm plugs. Adding another feed farther along the strip does not fix an undersized connector that still carries the installation's total current.
What changes when the system runs at 24 V?
For the same electrical power, a higher supply voltage requires less current. A nominal 72 W load corresponds to 6 A at 12 V or 3 A at 24 V. The explanations of watts and kilowatts and converting watts to amperes provide the calculation.
| Calculated quantity | 12 V example at 6 A | 24 V example at 3 A |
|---|---|---|
| Cable voltage drop: I × R | 1.2 V | 0.6 V |
| Drop relative to nominal voltage | 10% | 2.5% |
| Cable heating loss: I² × R | 7.2 W | 1.8 W |
This comparison assumes the stated currents are flowing. Actual strip current changes with operating conditions, and the table excludes losses inside the strip. It illustrates one reason to consider 24 V rather than 12 V strips when planning a new installation. It does not make a 12 V strip compatible with 24 V: see the consequences of using the wrong supply voltage.
RGB controllers and addressable strips need different wiring
RGB, RGBW and addressable strips do not share one universal connection scheme. Analogue colour strips have a common connection and separate colour channels. Check both the controller's per-channel rating and its total rating. Activating several channels together can reveal a limitation that is not apparent when testing one colour.
Any additional feed must preserve the controller's intended switching arrangement. Connecting an uncontrolled supply directly to a controlled output can bypass dimming or create unintended current paths. The explanation of pulse-width modulation covers the underlying principle. With addressable strips, treat power distribution and data direction separately: extra power connections will not repair a broken data path.
Do not overlook temperature and ageing
If the strip starts evenly and loses output after warming up, check its mounting surface, ventilation and the supply's temperature limits. A suitable aluminium channel can help remove heat, but it cannot eliminate voltage loss in an inadequate feed. Do not leave a strip operating while tightly wound on its reel. The broader relationship between temperature and service life is covered in LED heating and early failure.
Neither SMD nor COB construction makes a strip immune to distribution losses. When comparing replacement products, distinguish electrical consumption from light output using lumens versus watts.