# A simple one with LED strip

Discussion in 'LEDs and Optoelectronics' started by Lol999, Feb 17, 2017.

1. ### Lol999

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Feb 16, 2017
Hi folks and thanks for reading. I want to make an edge lit polycarbonate nightlight for my new grandson's nursery and the best way would be to use led tape and a suitable driver or psu. Drivers tend to be expensive (I might plan to sell some of these if the first goes okay on our local market, PAT tested etc) so I found a suitable wall wart off ebay for about £3.

However, I want to maximise the life of the led's and I have read about using a resistor in line to tailor the applied current for maximum lifespan but don't know how to go about calculating it. The PSU I was looking at outputs 12v 1A and the strip would probably be for example 10 led's long.

Could someone help me out with the math's please? I'm good with maths but don't know where to start with this one.

Many thanks, Lol

2. ### BobK

7,645
1,663
Jan 5, 2010
LED strips typically have the resistors built in and are designed to run off a fixed voltage.

All you need is a power supply of the correct voltage and a higher current rating than the strip needs.

Bob

3. ### Audioguru

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Sep 24, 2016
The quality of an LED strip determines how long it lasts. A cheap no-name-brand LED strip made with a manufacturer's rejected LEDs and sold on ebay might last one week. A good one will last for 50000 hours which is 11.4 years worth of nights.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,222
2,696
Jan 21, 2010
Running them at maybe 50% of their "rated" current will make them last a lot longer.

Because of the non-linear nature of LEDs, this may require only a small change in voltage. It is most easily done by placing a low value resistor in series with the strip.

If you look on the strips, there will be a single resistor for each group of (frequently) 3 LEDs. If you count the number of resistors in the strip you're planning to use, and note the value, the resistor you want to place in series should have an approximate resistance equal to the existing value divided by the number of them.

Let's say there are 10 resistors, each having a value of 68Ω. The additional series resistor should be 68/10 = 6.8Ω. The resistor will probably end up having less than 1V across it, so a power rating of 1²/R = 1/6.8 (in this case) ≈ 0.15W. I would use a 1/2W resistor (a 1/4W resistor would probably be fine, but I chose a power rating at least double that required).

5. ### oz93666

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Dec 7, 2014
The cheap strips from ebay work fine , I've had one running for years on 12V .... they still emit light even at 8V ... I would first buy the strip ...look around for a suitable power supply you have already , an old cordless drill charger , ....any charger ... if necessary you can adjust the voltage(either up or down) by a regulator then you can vary the output , you'll only need a few watts at most... this one for 99p could be conected to an old mobile phone charger http://www.ebay.co.uk/itm/XL6009-DC...140345?hash=item3f5dc52339:g:P94AAOSwr7ZW2Ss0 ... make sure the level you run these at is not disturbing to the eyes , @ 12 v it's unpleasant (damaging?) to look directly at these... better to reflect off a surface ...even better to run at around 9 or 10 V

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,222
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Jan 21, 2010
*Some* cheap strips from eBay work fine.

7. ### Lol999

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Feb 16, 2017
Interesting about the "cheap" strips. I bought a 10m strip with driver in brilliant white for £18 off ebay to install in a jewellery display stand. Now 2 years later with less than 1000 hours on them they have mainly failed. A recommendation for a good supplier/brand would be welcome.

8. ### Audioguru

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Sep 24, 2016
There is a huge city in China where they make only fake electronics. Most is sold on ebay, Alibaba and Amazon.
How do you know if something there is real, fake or poor quality?
Digikey and Newark do not sell that junk.

9. ### oz93666

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Dec 7, 2014
I'm not convinced the quality of the chips , which make up these strips ...have a good or bad quality ....the smaller the chip the more efficient they are , by a factor of about 25% (light /W input) ... https://en.wikipedia.org/wiki/SMD_LED_Module .....5050 are the least efficient and used in the cheapest strip ...
A 'bad' strip will be running the chips at high power ,hot, which will give more light per chip , inefficiently, short life ....
These strips are only a few pounds for 5meters ...easy to replace ... if you want to avoid changing them and run more efficiently halve the running current and have 2 strips ..

I would agree with audiguru when it comes to 18650 li-ion cells ... all types ,low weight indicates the can is not full !! ... so you have the choice of buying a deceptive cell advertised at 4.4AHr which only has 1.5AHr for 2 pounds ...or buying a correctly rated panasonic 2.8AHr for 6 pounds .... still the cheap fake is much better value .. But you have to wade through a few Youtube videos from people who have tested them to find a good brand.

A quick look at digikeys site shows they are a complete rip off ... half a meter strip costs over \$10 !!! , no evidence the quality is any different from what's being sold on eBay for less than one tenth the price

Last edited: Feb 19, 2017
10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,222
2,696
Jan 21, 2010
Bwahahahahahahahaha!!!!

11. ### oz93666

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Dec 7, 2014
Not only that , but nowhere in digikey specs , not even in the data sheet do they say what chips they use in the strips .... if they were using the efficient ones (3528) I'm sure they would say so ...
Another company praying on the uninformed general public ...

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,222
2,696
Jan 21, 2010
This is the first one I looked at. Seems to have far more information than I have seen from a cheap seller on eBay.

Bwahahahahahahaha

Do you know what 5050 and 3528 mean?

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Feb 16, 2017
14. ### oz93666

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Dec 7, 2014
Nowhere on the site do they give chip size ... just "product may vary according to manufacturer"

Every ebay listing gives chip size .. that's the most important information

It's the size of the small led chips that make up the strip .... 5050 is 5xx5mm ......3528 is 3,5x 2.8 mm .... generally the smaller the chip the more efficient ...3528 give 30% more light than 5050 for the same electrical input , that's a big difference ... https://en.wikipedia.org/wiki/SMD_LED_Module

15. ### oz93666

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Dec 7, 2014
Here's the identical item on eBay(one third the price) 6.99 no vat no delivery charge ... I'd be willing to bet they came of the same production line ....ultraleds can offer a warranty, they know very few people will bring them back after a year ..... I'd also be willing to bet if you ran them both at 12v, after 2,000hrs you will have at least one drop out on each strip (the leds are usually in bunches of 3 , so if one led goes down , only three go dark)
For long life , and greater efficiency run at half normal current.

16. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,222
2,696
Jan 21, 2010
That Wikipedia article is very misleading.

It also reinforces the fact that you don't understand the difference between 5050 and 3528. All these are are the dimensions of the package the led is in. It has nothing to do with the intensity or efficiency of the LEDs themselves.

The basic specs given by digikey (without resorting to the datasheet) include voltage, test current, brightness (at the test current) lumens per length, lumens per watt, and test temperature.

Even a cursory glance at the short form datasheet will give you the number of LEDs per metre, and enough information to track down the datasheet on the LEDs used (if you're that concerned).

Both sites offer warranty (5yrs or allegedly 3). Both sites charge freight which is eliminated for large orders. Neither are nameless sellers on eBay.

17. ### oz93666

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Dec 7, 2014
The numbers refer to are dimensions of the led chip ....the dimension of the light emitting area ...the smaller it is , the less heat build up so it runs cooler and more efficiently.

.... the wikipedia article lists the "efficacy" range (efficiency ) of each chip in lm/W the 3528 are 30% more efficient than 5050

Last edited: Feb 20, 2017