Understanding LED strip lights

I bought a 12 volt white LED strip light on ebay. After some testing and further research, I figured the luminous intensity was overrated. This post should be helpful for understanding LED strip lights.

LED strip light package

An LED strip is basically a package of Surface-Mount-Device (SMD) LEDs with current limiting resistors. The strips are flexible and easy to install with a self-adhesive backing. Waterproof strips have a clear, silicon-like layer over the electronics.

Section of ebay LED strip light (both sides). The strip is 12 mm wide. There are two groups of three LEDs (yellow). The SMD resistors are black. A splice can be seen in the lower left. The strip can be cut and new leads soldered at the copper tabs. Contrary to the logo on the back, the adhesive back is not genuine 3M quality.

Examining my strip light, I see repeated groups of three LEDs, each with one 39 ohm resistor. The wiring is series within-groups and parallel between-groups.

LED strip light wiring. R = resistor.

An LED strip is a passive device. It contains no electronics to regulate the power and brightness. If the supply voltage drops, then brightness drops and vice versa.

Currents are highest near the supply end. Firstly, the current for all the LEDs passes through the supply end. Second, the copper is thin and there could be substantial voltage losses along the strip, meaning that LEDs nearer the supply operate with higher voltages and currents.

You can feel that the strip is warm at the supply end because more heat is produced at higher currents.

Strip light specifications

Here are the specifications for the White LED (WLED) strip I bought:

LED 5630 SMD Colour temp. (K) 5000–5500 Viewing angle (deg.) 120 LED density (LEDs/m) 60 Intensity (lumens/m) 840–900 Current (A/m) 1

Specifications for ebay 5630 LED strip light.

The following table compares the ebay LED with specifications for two other 5630 LEDs from Samsung and Philips.

ebay Samsung Philips Intensity (lm) 14.5 20 32 Current (mA) 50 50 100 Voltage (V) 1.95 3 3.1 lm/mA 0.29 0.40 0.32

Comparison of 5630 SMD LEDs (single LEDs). I calculated the ebay specification as follows: current = 1/20 A (remember that the LEDs are in groups of three), intensity = 870/60 lm. Voltage was calculated by subtracting the drop across the current limiting resistor from the supply voltage (12 – 39 × 0.05) and dividing by three. Other LED data from Samsung SPMWHT5225D5WAR0S0 and Philips LUXEON 5630 datasheets.

WLEDs come in different colour temperatures. “Natural” or “daylight” LEDs (5000–5500 K) are effectively brighter than “warm white” (3200 K). This is because human colour vision is more sensitive to wavelengths in the middle of the daylight spectrum. The ebay listing did not include the Colour Rendering Index (CRI). My impression is that the LEDs are rather blue compared to 5000-5500 K daylight.

Mounting

SMD LEDs cast light across a very wide angle (viewing angle typically 120 degrees). If installed inside fixtures, the fixture should be shallow to achieve a wide floodlight. Avoid vertical mounting (e.g. wall mounting), because up to half of the light will be directed upwards.

It is best to have the SMD LEDs horizontal and facing down