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.
Examining my strip light, I see repeated groups of three LEDs, each with one 39 ohm resistor. I guess the wiring is series within-groups and parallel between-groups.
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. Long strips in series can melt.
Strip light specifications
Here are the specifications for the White LED (WLED) strip I bought:
|Specifications for ebay 5630 LED strip light. The intensity is more realistically about 290–320 lm/m at this current.|
The above lumens are overrated. The following table compares the ebay LED with specifications for two other 5630 LEDs from Samsung and Philips. The lumens/milliamp calculated for the ebay LED is impossibly high. The ebay seller seems to have calculated strip values using the typical LED intensity, but the LEDs are operating at a lower current.
|Comparison of 5630 SMD LEDs (single LEDs). I calculated the ebay specification as follows: current = 1/60 A, intensity = 870/60 lm. Voltage was calculated by subtracting the drop across the current limiting resistor from the supply voltage (12 – 39 × 0.017) and dividing by three. Voltage seems a bit high. Other LED data from Samsung SPMWHT5225D5WAR0S0 and Philips LUXEON 5630 datasheets.|
If I assume the Philips lumens/milliamp result, I estimate 0.32 lm/mA × 17 mA = 5.33 lm and 60 × 5.33 lm = 320 lm/m for the ebay strip light. Alternatively, I could assume the Samsung current and then estimate intensity as 14.5×17/50 = 4.83 lm (the datasheet showed that intensity is approximately proportional to current) and 60 × 4.83 lm = 290 lm. Both of these estimates seem reasonable in side-by-side comparisons with my 97 lumens Fenix HL21 headlamp.
Some reasons for running the LEDs at lower currents are: to reduce heat, reduce voltage losses and allow longer strips. Using the Samsung LED for comparison, at 60 LEDs/m the current is 60 × 20 lm = 3 A/m and a 5 A strip would be only 5/3 = 1.67 m long. Lower currents and less heat is safer and easier for home made strip lights.
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.
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.
Here are some cost-saving measures to expect from ebay LED strip lights:
- LEDs from low grade bins and mixed bins (variable intensity, colour, forward voltage).
- Thin copper (more resistance, more heat, shorter maximum strip length).
- Spliced strips (possible failure points).
- Poor quality adhesive backing (strip lights come unstuck after a short time).
- Low currents (see previous discussion).
Cheap ebay LED strip lights may be false economy if more LEDs are required to achieve satisfactory brightness.