STUMPED! 3528 SMD LED always flickering out

[EasyGoing1]

I've been having a hell of a time driving these LEDs so I broke down and bought a cheap buck CC/CV power supply and I configured it for 12.5 volts output (input is 15 volts) at 35mA.

I have two rows of these 3528 LEDs, each row has four of them in series, but the rows are in parallel.

When I connect them to the power supply (monitoring the current with my Fluke meter) the current rides a VERY STEADY 35mA yet after they are lit for a minute or so, they just flicker out and eventually all of them turn off while still drawing current.

I have 100 of these LEDs so this time around, I used brand new LEDs so that I knew I wasn't using any that had been over driven yet I am having this problem...

Any ideas?

(I attached a quick and dirty schematic just in case)

[budm]

Re: STUMPED! 3528 SMD LED always flickering out

You have two LED connected directly in parallel so the 35mA will be splitted up and not likely that the current will be equal for each string due to variation in the LED, one may have 10mA, another string may have 25mA.
Try having just one string connected first, then add may be something like 10 Ohms resistor in series with each string and then hook them up as see.
https://www.ledsupply.com/blog/wirin...its-explained/
'The downfall of multiple LED strings:

'One thing to keep in mind about running parallel and series/parallel circuits is that if a string or LED burns out, the LED/string will then be cut out of the circuit so the extra current load that was going to that LED will then be distributed to the rest. This isn’t a huge issue with larger arrays as the current will be dispersed at smaller amounts but what about a circuit with just 2 leds/strings? The current would then be doubled for the left over LED/string which might be a higher load than the LED can handle resulting in a burn out and ruining your LED! Make sure you always keep this in mind and try to have a set up that wouldn’t ruin all your LEDs if one happened to burn out.

Another potential issue is that even when LEDs come from the same production batch (same binning) the forward voltage can still have a 20% tolerance. Varying voltages across separate strings results in the current not being divided equally. When one string draws more current than another, the LEDs being overdriven will heat up and their forward voltages will change more, resulting in more unequal current sharing; this is called thermal runaway. We have seen many circuits set-up like this work well, but caution is required. For more information on this concept and ways to help avoid it (current mirror) there is a great article here within LEDmagazine.com.'

[EasyGoing1]

Re: STUMPED! 3528 SMD LED always flickering out

Try having just one string connected first.

Seems no matter how thorough I think I am when I make a post, I always leave something out ...

I paid $7 for 100 of these things ... I have about 30 left and ordered another 100 today (but different LED this time) ... ive tried everything under the sun ... single series of 3 then single series of 4, ... I actually do manage to keep a single LED working just fine without going out ... but as soon I stack em up its like they dont like to party or something .... stupid LEDs ...

before I bought this power supply, I built four different (supposedly) constant current power sources and was convinced it was my piss pour resistors or china voltage regulators that was causing the problem but then I get this PS and its respected and well reviewed and i'm still in the same position facing the same problem ... at some point i'm going to have to concede and realize that its not me. lol

forgot to mention ... I did check the current in each leg and they were spot on identical.

ALSO, When I let the LEDs sit for a few minutes without being powered .... they will repeat the cycle ... they will light up for a while ... then blink out ... I checked the temp both with my fingers and my infrared thermometer and they never go even one degree above ambient room temp.

.. im looking over the rest of your post now...

Thank you,

Mike

[eccerr0r]

Re: STUMPED! 3528 SMD LED always flickering out

May need a small resistor for each string in parallel to eat voltage drop differences between the series strings, if you don't want to manually bin match all your LEDs!

LEDs are fickle... on the other hand I'm shocked that there are companies that seem to go ahead and match LEDs so that they can make devices with a zillion of them in parallel and it works (like all those multi-LED flashlights!).

[budm]

Re: STUMPED! 3528 SMD LED always flickering out

"forgot to mention ... I did check the current in each leg and they were spot on identical." So each string show 35mA/2 =17.50mA on each string when you measure each one when both of them are connected in parallel?

[stj]

Re: STUMPED! 3528 SMD LED always flickering out

got the datasheet?

[sam_sam_sam]

Re: STUMPED! 3528 SMD LED always flickering out

I've been having a hell of a time driving these LEDs so I broke down and bought a cheap buck CC/CV power supply and I configured it for 12.5 volts output (input is 15 volts) at 35mA.

I have two rows of these 3528 LEDs, each row has four of them in series, but the rows are in parallel.

When I connect them to the power supply (monitoring the current with my Fluke meter) the current rides a VERY STEADY 35mA yet after they are lit for a minute or so, they just flicker out and eventually all of them turn off while still drawing current.

I have 100 of these LEDs so this time around, I used brand new LEDs so that I knew I wasn't using any that had been over driven yet I am having this problem...

Any ideas?

(I attached a quick and dirty schematic just in case)

Can you post the link where you are buying them from thanks

[EasyGoing1]

Re: STUMPED! 3528 SMD LED always flickering out

"forgot to mention ... I did check the current in each leg and they were spot on identical." So each string show 35mA/2 =17.50mA on each string when you measure each one when both of them are connected in parallel?

Give or take a few µAmps (I'm sure ... my fluke lacks that resolution so I'm assuming) ... yes.

And is my assumption correct when I assume that is none of those LEDs ever sees more than its rated amperage, then the flickering I am witnessing is not being caused because they are damaged???

[EasyGoing1]

Re: STUMPED! 3528 SMD LED always flickering out

got the datasheet?

Here is the datasheet

I am specifically using the 4002-1 (White) model of the 3528.

[CapLeaker]

Re: STUMPED! 3528 SMD LED always flickering out

why aren't you using a proper led driver like an NSI45020 or similar? Perfect for those small strings.

[budm]

Re: STUMPED! 3528 SMD LED always flickering out

"they just flicker out and eventually all of them turn off while still drawing current." This does not make any sense that it still draws the same amount of current but none of the 4 LED's light up.

[stj]

Re: STUMPED! 3528 SMD LED always flickering out

datasheet says you need to take thermal disipation into account - what are they soldered to?

[stj]

Re: STUMPED! 3528 SMD LED always flickering out

"they just flicker out and eventually all of them turn off while still drawing current." This does not make any sense that it still draws the same amount of current but none of the 4 LED's light up.

it does if they contain a bypass element often used in stuff like signage and streetlamps.
although that's not mentioned in the datasheet.

[budm]

Re: STUMPED! 3528 SMD LED always flickering out

it does if they contain a bypass element often used in stuff like signage and streetlamps.
although that's not mentioned in the datasheet.

That is right there is no protection device (protection diode connected in anti-parallel with the LED) shown in that data sheet.
I sure like to see his LED assembly.

[sam_sam_sam]

Re: STUMPED! 3528 SMD LED always flickering out

These could be very poor quality LED
Here try something use just one LED @ 20 milliamperes and see how long it will stay light
I have seen this before in poor quality LEDs where not every LED has the same current requirement so when you try to use more than one together not all of the LEDs are the same brightness or after they have run for awhile some will feel warm than other ones

[EasyGoing1]

Re: STUMPED! 3528 SMD LED always flickering out

Just to make this post complete, I am posting the solution that I came up with that works well in my application.

I decided against the LEDs that I was trying to make work, although I do believe this circuit would power them perfectly. But I did order a bunch of 5730's which are much more resilient since they are rated at 150mA instead of 20mA. And I went with this circuit using an LM317 in a constant current source configuration ... those darn things are pretty stable and so cheap!

I am only driving each branch at 80mA because I didn't want to push my luck and drive them at max amperage, and they are so bright, 80mA was more than enough to illuminate what I was trying to illuminate.

The formula to calculate the resistor value is 1.25 / R1 = Iout so in my case I needed 160mA so transposing Iout with R1 gives me 1.25 / .016 which is approximately 80Ω I grabbed the nearest value I had which I cant remember what it was ... and it worked like a champ!

Here is the circuit I ended up using:

[budm]

Re: STUMPED! 3528 SMD LED always flickering out

0.0016A = 1.6mA
0.0160A = 16mA!
0.1600A = 160mA
1.6000A = 1600mA
http://www.reuk.co.uk/wordpress/elec...nt-calculator/