Large difference between specced values and real values for LED strips?

Question

Recently I bought some "300LED/5meter Waterproof 5630 SMD LED Flexable Strip" i.e. High brightness LED strips from China. In the specifications, it was said that the strip consumes 75W @ 12v, which equates to 6.25A @ 12v.

I first tested the LED strips with a 12v 2.5a power supply, seeing as it was the easiest to find. Later I found my 12v 5a power supply, and I was surprised to see very little difference between the two power supplies (in terms of perceptible brightness). Not having any proper test equipment on hand, I bought it over to my local hackerspace which is a little more suited towards doing things like this.

After setting up the LED strip so that it's connected to a 3~20v <10a bench power supply, a voltmeter and an ammeter, I took a video of the readouts from the two meters, whilst I increased the voltage: http://youtu.be/uX-3r6mrF9A

The results I obtained were... interesting :P Surprisingly, at 12v, I only got 3.44a (41.28w, 45% less than advertised value!). You can get the results here (google doc).

I then plotted watts against volts, and it turns out that I would need to power this strip at 14.14v in order to get 75w!

Basically, I'm not overly happy with the performance of this LED strip. In order to get the correct power output, you would have to overvolt it by 2.14v, which has the potential to significantly reduce the LED's lifetime. Unfortunately, I was unable to measure the exact voltage at the individual LED level (they were epoxied), but I suspect that they would need at least some heat-sinking if they're to operate for any period of time.

I know that the light output of LEDs isn't measured in watts, but in my mind, < watts specified = < luminous flux specified. I'm still not quite comfortable with all of these conversions, so I am probably wrong somewhere.

Aside from this, I suspect that the values indicated by the multimeters may be a bit off (both are cheap ones), and that there is some power loss from the way I set it up. The values on the PSU matched the values on the multimeters so there shouldn't be too many discrepancies.

Any ideas? I only get ~55% of the specced power consumption @12v, is it O.K to go any higher in terms of voltage?

Cheers,

Sasha

Answer 1

The 'specs' (pasted below) indicate 12V, not 12.0V. I'd guess that this product is made to be compatible with automotive 12V, which is usually in the 13.8 to 14.2V range. Your measurements somewhat confirm this.

Specs from the reseller's page:

Item Type: Strip
Place of Origin: Guangdong China (Mainland)
is_customized: Yes
Brand Name: G-Lighting
Average Life (hrs): 50000
LED Chip Brand: Edison
Warranty: 2
Wattage: 75 
Certification: CCC,CE,UL
Waterproof: Yes
Voltage: 12V 
Power Source: DC
LEDs Number/M: 60 pcs/m
Model Number: GL60-5630C
Power: 75W Per Pcs
waterproof: yes
Length: 5 m
Beam Angle(°): 120

Answer 2

Sasha, most Chinese LED strip manufacturers use very thin copper film in strip's FCB (Flexible Printed Board). They rate LED strip "theoretical" power without taking in to consideration copper resistance. When 12v applied to the LED strip wire leads, the voltage at the opposite end of the strip will be substantially lower because of the resistance loss in FCB copper. In order to get full 72 watt from those strips, run pair of 12 gauge wires parallel to that strip and connect those wires to every FCB stitch pint located 50cm apart.

Answer 3

I have tested 1m of 5630 led bar with 72 led/m and the voltage drop was about 0.5V for 1 meter. Imagine that you have a very high current in the first part of the strip and a big voltage drop until the end of it. Please tell us what value has the limiting SMD resistors, i can not see from the image. What I have tested has 39ohm resistors and draws 1.6Amps at 12.0V for 1 meter.

You can do the following simple test:

• take a 3 led module and test it;
• then multiply the results with the number of modules that you have (100).

Only then you will know the real maximum current (@ 12.0V) that can be drawn from this led strip.

Answer 4

My operating definition of the word "problem" is a violation of expectations. The problem was and is that you expect the advertised 12-volt rating to be correct from an engineering perspective. Experience selling products to less-well educated consumers than you have taught me that when they believe their automotive systems are "12 volt systems" that roughly 12.0 volts is expected continually. By comparison, in service nominal 12-volt systems display voltages from about 9.6 volts during starting to as high as 14.8 volts in some systems during charging. Design cut-off peak values have varied with OEM battery chemistry, so while 14.8 volts is uncommonly high, aftermarket product vendors experience defective product claims if they can't tolerate the highest sustained voltage common nominally 12-volt systems deliver. If you were selling products to typical Americans which you expect to be powered by these automotive electrical system, you would quickly learn that providing rather complete engineering data kills potential buyer interest. If a message can't fit on a bumper sticker and be read from 50 feet, it's probably too complex.

You are correct. They are also correct. You just need to decode their claims based on market expectations.

Answer 5

The answer to the power disparity from LED strip manufacturers is simple. The majority of strips are made for automotive operation from 12 volt electrical systems. that's because the engine starting systems use 12 volt batteries.

Once the engine is running, the battery charging system float charges the battery at about 14.2 volts at the battery terminals. I've verified this on several late model automobiles.

The 5630 LEDs flexible strips use 3 LEDs connected in series and 1 current limiting resistor across the 12 volt power bus. I've confirmed measurements at 12 volts and 14.2 volts from about 6 different GREEN and cool WHITE colored LEDs and the results are about the same - the actual wattage of the advertised strip wattage is achieved around the 14 volt region, close to the battery charging voltage of the running engine.

But there are no 14 volt auto batteries. Only 6 and 8 v8lt golf cart batteries connected in series. The other options is to use a 12VDC to 14VDC boost converter to make up the extra missing 2 volts needed to achieve the wattage and brightness specified.

Brightness will increase about double the 12 volt lumens, and this will result in about a 40% increase in spot brightness in lighting with all other conditions kept the same.

I am just finishing development for underwater LED fishing and shrimping lights to be sold under the brand name of LEDgreenie Lights, LLC, so this topic is of great interest to me.

Hope this helps you. You're safe at 14.2 volts, but take care you can handle the doubling of the power dissipation from 12 to 14 volts.

BTW, I'm a 75 year old BSEE from Drexel University.

Answer 6

I'm hoping this will be helpful. If you look online you will find switching power supplies that although rated for 12 V have a potentiometer on the circuit board that will allow for adjustments in output voltage. This potentiometers commonly located outside of the circuitboard enclosure near the input and output strip and is intended to be utilized to fine-tune your voltage requirements. Maybe this will help anyone who's looking to drive the strips at the voltage necessary to provide lighting at the maximum wattage capability. One example is a 12 V 30 amp switching power supply at approximately $23 US. As of November 2015.

Answer 7

This is a normal effect.

1. LED stripes from china are normally overrated 25-50% from specs.

2. Voltage drop over 5 m thin stripe is enormous, you should connect on multiple even length parts at least 4 times over 5 m stripe to get better results.

3. A little over-voltage as of 12 - 14 V is OK as long as you care for cooling by mounting the stripe on alu-profile. This shouldn't get warmer than 40 deg. Celsius.

4. Real wattage of one 5630 LED is around 0.17 W (specs are around 0.25 W).

5. If you apply less power you get better efficiency light output! (But less luminous flux of cause)

Answer 8

Firstly please note those strip lights are powered by constant voltage PSU (DC5V, DC12V or DC24V). Take a DC12V 300xSMD5630 led strip light for an instance, it has usually 3 LEDs, 1 resistor as a segment (in serial) and then totally 100 segments in parallel to the circuit. At present a typical SMD5630 has its forward current as 60ma @3.1-3.2V, it is easy to tell theoretically its total power consumption is 0.06A*12V*(300/3)=72W. If we add the consumption of your power, it will reach about 75W.

In practice, if adjust the resistor with a higher value, current will drop (say about 40ma). Then you can get the power consumption as 0.035A*12V*(300/3)=42W (with a less brightness output).

As for a voltage drop, it is common to see to low-voltage products, and this is also why most manufacturers set the length of strip light less than 5m due to a visual light decay if longer.

Answer 9

Any ideas?

the stripes are of automotive grade -> they have a nominal rating of 12v just as the batteries in cars.

you probably can indeed run it at 14v constant.

BTW, your experiment is a great public service. I have been saying for years that high power leds behave more like resistors (of low resistance) than diodes and your experiment provided another empirical data point on that.