Leds in parallel?

picbits · Oct 13, 2013

Bear in mind that document has your details (A.M , your gmail address and the date and time created embedded within it)

If you're still trying to stay anonymous, you're going to fail by posting documents with these details contained within them ........

Flyback · Oct 13, 2013

My email address is not in that document. How would you find this out? Its not my computer that I am using by the way, its my housemates.

Anyway, supposing I use a series parallel array and no series resistors to limit the current, then the LEDs should be from the same Vf batch and the same production batch. But how do you know they are from the same production batch?
A distributor may tell you that all the leds he supplies you are from the same production batch, but are they? How can you be sure?

picbits · Oct 13, 2013

Editing so Google doesn't cache this post

The document has the person An* ***ey with an email address of **y2@gm*** associated with it.

Stars above don't indicate the number of digits/characters by the way but you'll see that it's easy enough to trace the person that software is registered to and the time/date the document was written or amended.

Flyback · Oct 13, 2013

Sorry but that isn't anything like my name..i don't know where the computer I am using came from though.
Do you object to the report?

picbits · Oct 13, 2013

Not at all - just letting you know the report has a signature with someones email address in that file that you put up on the first post

I'm not getting involved with the parallel / series connection arguments on here

tcmtech · Oct 13, 2013

what is the minimum length of series led strings that can be paralleled?....or do you say that even single leds can be paralleled without series limiting resistors.?

Two in series or two in parallel or four in a combination of the two configurations being that obviously you obviously can't parallel or series connect anything less than two devices in eitter fashion.

Really you can't figure this sort of basic concept out on your own?

ronsimpson · Oct 13, 2013

I included a data sheet for a white 30mA LED. C503d-WAN
Vf is 3.2V typical, 4.0 max, the min is not rated but maybe 2.2V
Paralleling two LEDs is like paralleling two Zeners. Not done in good engineering.
The graph shows voltage vs current for the led. The three lines are for a min, typ, and max LED. The min curve is at the far left corner of the graph.
While it is not likely you will find a min and a max LED in parallel. It is much more likely you will find a 25% and a 75% LED in parallel. A 25% and a 75% LEDs will act just like a typical and a max LED in parallel.

Lets place a 40mA current source driving these two LEDs in parallel. The typical LED will take all 40mA and have 3.5V across it. While 3.5V will cause only uA in the 4.0V LED.

If we increase the current to 60mA the 3.2V LED will have 3.8V across it and most of the 60mA. The 4.0V LED, with 3.8Vf, will have 2mA.

Someone here thinks that if you parallel 100 LEDs this problem goes away. I think using 100 LEDs increase the likely hood of having
differences in Vf and thus huge differences in current.

Flyback · Oct 14, 2013

yes, I think the lots-of-paralleled-leds argument has a lot to answer for.
And also it relies on having all the leds from the same production batch, not just the same vf batch.
I cannot see how anyone (other than the led foundry itself) can ascertain that a load of leds ordered from a distributor is definitely all from the same batch.

tvtech · Oct 14, 2013

wow

ronsimpson said:
I included a data sheet for a white 30mA LED. C503d-WAN
Vf is 3.2V typical, 4.0 max, the min is not rated but maybe 2.2V
Paralleling two LEDs is like paralleling two Zeners. Not done in good engineering.
The graph shows voltage vs current for the led. The three lines are for a min, typ, and max LED. The min curve is at the far left corner of the graph.
While it is not likely you will find a min and a max LED in parallel. It is much more likely you will find a 25% and a 75% LED in parallel. A 25% and a 75% LEDs will act just like a typical and a max LED in parallel.

Lets place a 40mA current source driving these two LEDs in parallel. The typical LED will take all 40mA and have 3.5V across it. While 3.5V will cause only uA in the 4.0V LED.

If we increase the current to 60mA the 3.2V LED will have 3.8V across it and most of the 60mA. The 4.0V LED, with 3.8Vf, will have 2mA.

Someone here thinks that if you parallel 100 LEDs this problem goes away. I think using 100 LEDs increase the likely hood of having
differences in Vf and thus huge differences in current.

Amen. Thank you Ron. You explain things far better than I can...or have time to do

Everything boils down to Basics understood. Properly.

Regards,
tvtech

Flyback · Oct 14, 2013

OK,
but these guys don't agree with us...
**broken link removed**

...Will I ask them why they think they can do parallel strings of eleven LEDs in series, with no limiting resistors?
Are these guys chancing it?...or have they hit on a great way of driving loads of leds from just one current source driver? They are an official "Cree solutions provider", and maybe (?) that means that cree ship them special leds all from the same production batch, specially processed to have ultra similar vf.?

Nigel Goodwin · Oct 14, 2013

As I've long since explained to you, paralleling series LED's is OK, and I've seen Philips datasheets documenting it.

Why do you keep repeating the same old posts all the time?.

tvtech · Oct 14, 2013

Ag no. Flyback you STILL don't get it...

See the teaser...ELEVEN LEDS IN SERIES.
See PARALLEL strings.

I am putting you on my ignore list. You are making me age daily.

tvtech.

Flyback · Oct 14, 2013

As I've long since explained to you, paralleling series LED's is OK, and I've seen Philips datasheets documenting it.

We already discussed this, that Philips app note actually recommended adding series resistances.

See the teaser...ELEVEN LEDS IN SERIES.
See PARALLEL strings.

..we ordered one, -a sample, and its a series parallel array with no limiting resistors.

tvtech · Oct 14, 2013

Flyback...STOP LIKING MY POSTS.

Are you thick or what??

Bah

tcmtech · Oct 14, 2013

..we ordered one, -a sample, and its a series parallel array with no limiting resistors.

So what exactly is your perceived problem with it then?

Does it work or not?

BTW. They make LED's with built in resistors so the odds are the companies making the LED lighting systems with large numbers of LED's that appear to be in parallel without any current limiting or balancing resistors probably are using LED's with the resistor built in.

See for yourself.

https://www.mouser.com/new/kingbright/kingbright-resistor-LEDs/

ronsimpson · Oct 14, 2013

Paralleling LEDs and (paralleling series LEDs) is not the same thing.

ELEVEN LEDS IN SERIES. See PARALLEL strings.

The Vf of 10 LEDs in series, you would think, would tend toward the average.
(10 in series)//(10 in series) 10 series in parallel with 10 series would be much safer.
Example; All LEDs are average except two. One on the right string his Vf=max and one on the left has Vf=min. In this case one string has 80mV more voltage than expected and the other is short 80mV. That's fine.

I am against parallel LEDs. I am OK with long series in parallel. (I have done that in production).

Flyback · Oct 14, 2013

They make LED's with built in resistors

..yes but they are ~150R resistors, and this doesn't happen with power leds.

Does it work or not?

It depends whether or not any given production batch of the led product contains rogue leds from a different led foundry batch...then they could be in serious problems. Also, what if thise leds were spaced further apart?....would the thermal coupling be good enough.?

The Vf of 10 LEDs in series, you would think, would tend toward the average.

...if each led were taken randomly from the same production batch yes..but thet are not, the next led taken is always the next one on the reel, not any led on the reel....this kind of sampling misdemeanour ruins that long series parallel array theory.

tcmtech · Oct 14, 2013

So you don't actually have any LED arrays that are giving you problems?

All of this is just semantics based on theoretical what if 'ing the concept to death?

I'm done here as well.

Flyback · Oct 14, 2013

So you don't actually have any LED arrays that are giving you problems?

I wish to evaluate it before doing it....the downside is horrendous, one rogue led batch could runi 1000's of products,

we all know that series parallel led arrays without limiting resistors must use vf matched leds....

but the remaining two unanswered questions are:
1..Do the leds all have to be from the same production batch aswell?
2..if so, then how do you really know that they are all from the same batch?

ronsimpson · Oct 14, 2013

tcmtech said:
So you don't actually have any LED arrays that are giving you problems?
All of this is just semantics based on theoretical.

I do have parallel LEDs in production and they are sorted to with in 100mV. With out sorting they mostly work but some times burn out. The margen is so small that I can not afford a dead unit on the assembly line. If one unite dies in the field it will take 200 unites to pay for the loss.

I really hate parallel LEDs and the trouble it causes. (in production and on this forum)

Leds in parallel?

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