# Computing stuff tied to the physical world

## LED strip efficiency

In Hardware on Aug 29, 2010 at 00:01

LED lighting is efficient right? Oh, sure… the LEDs are, but the way they are connected isn’t necessarily!

I’ve always been puzzled by those resistors you see on RGB LED strips:

My impression is that these strips consists of segments which all have (essentially) the following schematic:

Measuring the 60-LED/meter RGB strip I have here, I get the following readings for 0.5 m, i.e. 10 of the above segments:

• RED draws ≈ 180 mA, the voltage over its resistor is ≈ 5.5V
• GREEN draws ≈ 170 mA, the voltage over its resistor is ≈ 2.2V
• BLUE draws ≈ 190 mA, the voltage over its resistor is ≈ 2.4V

Total current draw is roughly 540 mA, or 1.08 A/meter @ 12V. So the total power consumption is 13 watts for each meter of LED strip.

But…

• the red LED’s resistor consumes 0.18 x 5.5 = 0.99 W on each 0.5 m
• the green LED’s resistor consumes 0.17 x 2.2 = 0.37 W on each 0.5 m
• the blue LED’s resistor consumes 0.19 x 2.4 = 0.46 W on each 0.5 m

That’s 3.64 W of the 13 W pumped in per meter – turned into … heat!

Is LED lighting a good idea, in terms of efficiency? Yes, probably. A meter of RGB LEDs at full power draws 13 watts and looks a lot brighter to me than a classical lightbulb, halogen light, or even them new fluorescent “long-life” bulbs that are all the rage (but oh so ugly).

Note that I’m not talking of “power LEDs”, i.e. the 1W, 3W, or more LEDs that are used for very bright lights and which need te be mounted on a cooling fin (Infineon, etc). These are usually driven by a (more complex) constant-current source, and not a plain 12V supply. The reason for this is that at those power levels, you couldn’t possibly adjust the current draw via resistors – these would become scorching hot and cause lots of problems of their own!

The are a couple more figures to be gleaned from the above information, btw, but you can also measure these values directly on the RGB strip:

• the red LED resistors are 300 Ω each, each LED gets 2.2V
• the green LED resistors are 120 Ω each, each LED gets 3.3V
• the blue LED resistors are 120 Ω each, each LED gets 3.2V

That voltage is the real reason for all these resistors. If you were to feed exactly 6.6V to the 3 red LEDs (9.8V for green, 9.6V for blue), then you wouldn’t need any resistors to pick up the slack. But that’s not feasible in a practical / cheap way, so instead we drive these strips with 12V and have the resistors eat away 28% of excess energy in the form of generated heat. Note: it would be more accurate to say: “if you were to feed exactly 180 mA, etc” … because the voltage is a “side effect” for each type of LED, the current is what determines their brightness.

Another source of inefficieny is the 12V power supply – let’s assume it’s about 90% efficient. Then 72 watts of power going into the LEDs will draw a total of 111 watts from the power line.

The good news is that these ratios won’t change when the LEDs are dimmed via PWM. A 50% pulse reduces the total amount of energy used by the same percentage (again: 28% of that is gobbled up by resistors).

Sooo… if you’ve got 8 meters of RGB strip going at full power, then that’s about 100 watts – 28 of which are turned into heat before they even reach the LEDs. And the power supply used another 11 to do its job.

I still think LED strips are a good idea – if I can get the white balance right and if their color is very even.

1. I tried to tackle this thing a few years back by doing my own led strips. I used 20V power supply and maximum number of leds in one segment, leaving just few volts to be burned via resistor. Technically this works OK, although there are slight variations in led voltages as a function of e.g. temperature.

The approach is efficient, yet, nowadays with commercial led strips it just does not make any sense to make strips yourself. Took a lot of time and the result cost way more than present strips.

• Interesting. And the way the strips are wired up (every 3rd to “+”), you can’t easily bypass the resistors and string more than 3 of them in series with separate RGB’s.