connecting LEDs in parallel

Thread Starter

whitehaired novice

Joined Jul 15, 2017
283
I understand the problems with connecting LEDs in parallel as has been pointed out in many threads on this forum. But practice seems to indicate that it is not as catastrophic as it seems. I found 15 red LEDs in my parts box. One measured a forward voltage of 1.87. The highest of the 15 measured 1.91. All the rest were between 1.88 and 1.90. I was surprised to see this uniformity. I paralleled the lowest, the highest, and 8 of the ones in between. I put the paralleled LEDs in series with a fully charged LiPo and 20 ohms of resistance. They are all visually identical. So I added a yellow one with a forward voltage of 1.97--it is a little dim, but bright enough to be used as an indicator.

Now, I wouldn't try to sell a circuit which violated the rules like this--actually, I wouldn't try to sell any circuit. But to tell someone who wants to make a simple project using small LEDs that it should not be done, that the LEDs will self destruct, seems to me like bad advice.
 

wayneh

Joined Sep 9, 2010
16,114
I’ve had similar experience, but in my case and perhaps yours, the LEDs were all received in the same package and theoretically made the same way to the same specs. And maybe more importantly I wasn’t driving them anywhere near their maximum current rating. Maybe just 5mA.

You’re right that it’s not a huge no-no for the typical project, where one dim LED is no bother.

But you can hardly imagine some of the stuff we see here. Controllers for high power LEDs cost money and every noob gets that aha moment when they think they’ll save money by putting multiple LEDs in parallel on the same controller.
 

WBahn

Joined Mar 31, 2012
24,854
You don't provide enough information to make much comment, but I always applaud folks asking the questions and devising tests to explore them.

What was the voltage of the LiPo you were using? Was it a single 3.7 V cell? How much total current were you providing to the ten paralleled LEDs? How long did you let them sit?

If you used a single cell and let's say it had a voltage of 4.2 V. Then with an LED drop of 1.9 V you had 2.3 V across your 20 Ω resistor yielding a current of 115 mA. If the LEDs were rated at 20 mA, it is quite possible that they could survive for some time with 115 mA running though it. If they were reasonably closely matched, it is also likely that it would take some time for the mismatch to progress far enough to result in thermal runaway.

If you are still interested, I'd recommend first scoping out how much current it takes to get an LED to fail in a reasonably short amount of time. This is going to burn up some LEDs, so be sure you have plenty on hand. First put something like 100x the rated current through it and see how long it lasts. Then put 50x and measure it. Pick a few other points and plot them -- that plot itself is probably worth the effort.

Now pick a current that causes the LED to burn out in about 15 minutes and the current that caused it to burn out in a about a week. Picking numbers out of thin air, let's say that the first was 600 mA and the second was 70 mA. So double the first current (to 1200 mA) and divide by the second (to get 17 in this case). Put that many (or more) in parallel and set the current so that you get an average of the second current (70 mA) through them. Then leave them sit there for several days periodically noting how they are doing.

This is a test designed to see if the thermal runaway phenomenon is real or not in a fairly stressful environment. If it turns out it is, then further experiments can be done to see how impactful it is in practice for typical circuits. If it turns out it isn't, then you are in a fair position to argue that, in many practical circuits, it can be ignored. But keep in mind that, for professionally design products, it should never be ignored (though some designs, such as coin cell operated circuits, can and do address the issue by determining that, for their application, it is safe to ignore it).

Whether you choose to do the experiment with LEDs purchased at the same time from the same lot is up to you, depending on what question you are trying to answer. But the general question of whether it is a good idea argues for going out of your way to NOT pick them from the same lot, since most people (and companies) do not make any effort to store LEDs that way and, even more to the point, when on LED does go bad, it is going to be replaced with another LED that is unrelated to the rest of the ones in the group.
 

Thread Starter

whitehaired novice

Joined Jul 15, 2017
283
Well, it was a moderately used cell--about 3.8 v. The actual measured resistance was 20.8 ohms. The measured current started at around 80 ma and dropped over about an hour to about 60.

Actually, though your tests seem interesting, I don't feel I need to do them as I don't need the data and I suspect it exists elsewhere. I ran the test long enough to tell me that the average amateur circuit could well parallel a batch of LEDs with little risk. Lots of people ask for help on a simple circuit and it almost always devolves into recommending greater caution than necessary. Now, as has been pointed out, high wattage LEDs require caution. But someone's 30 or 50 low current LED circuit can surely run sets of the same color in parallel.
 

WBahn

Joined Mar 31, 2012
24,854
I fully understand not being interested in running more thorough tests -- it only makes sense to do so if the personal interest in the results are worth the personal effort involved. I might run them some day if I get a chance and write up the results here.

IF I can get my daughter interested in electronics, it's the kind of thing that would be interesting for her to do. Not holding my breath.

Almost any LED than can handle 20 mA continuously will handle 60 mA or even 80 mA for an hour will little apparent short term impact. Also, since there can be significant differences between currents -- factor of two or so -- among LEDs before the human eye can discern much of an effect, it is quite possible that there was a fair amount of mismatch building up in your test without you noticing. It's also possible there wasn't. It's an unknown.

I might be willing to allow that a few LEDs run in parallel are a reasonable risk for a hobbyist application -- but then I've noted before that with just a few in parallel the risk of anything catastrophic is fairly minimal. But I still wouldn't recommend putting 30 or 50 in parallel or let such a recommendation pass without comment.
 

ElectricSpidey

Joined Dec 2, 2017
739
Please watch the language.
No *^*&, but how do you determine whether the LEDs fail because of being in parallel or simply being overdriven?
 
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k7elp60

Joined Nov 4, 2008
555
Well, it was a moderately used cell--about 3.8 v. The actual measured resistance was 20.8 ohms. The measured current started at around 80 ma and dropped over about an hour to about 60.

Actually, though your tests seem interesting, I don't feel I need to do them as I don't need the data and I suspect it exists elsewhere. I ran the test long enough to tell me that the average amateur circuit could well parallel a batch of LEDs with little risk. Lots of people ask for help on a simple circuit and it almost always devolves into recommending greater caution than necessary. Now, as has been pointed out, high wattage LEDs require caution. But someone's 30 or 50 low current LED circuit can surely run sets of the same color in parallel.
I agree that multiple LED's of the same color can be put in parallel. What I do is match the Vf to within less than 50mV and use a small current. I find that a lot of the newer LED's are very bright with only 1.5Ma to 2.0Ma of current I have recently done some projects with LED's in parallel.
Here is a picture of a small cross with 10 5MM LED's in parallel.
WHITE CROSS.jpg
 

WBahn

Joined Mar 31, 2012
24,854
I agree that multiple LED's of the same color can be put in parallel. What I do is match the Vf to within less than 50mV and use a small current. I find that a lot of the newer LED's are very bright with only 1.5Ma to 2.0Ma of current I have recently done some projects with LED's in parallel.
Here is a picture of a small cross with 10 5MM LED's in parallel.
View attachment 143717
I would wager that most LEDs will be quite bright with even a good fraction of a Ma of current -- at least for their exceeding brief life span. :D
 

dl324

Joined Mar 30, 2015
9,155
Now, I wouldn't try to sell a circuit which violated the rules like this--actually, I wouldn't try to sell any circuit. But to tell someone who wants to make a simple project using small LEDs that it should not be done, that the LEDs will self destruct, seems to me like bad advice.
Parallel LEDs without current limiting resistors are typical in low cost LED flashlights; the kind powered by 3 AAA batteries and often freebies at Harbor Freight. When you're making a lot of them, every penny saved adds up; if you don't care about having repeat customers.

Most people who design circuits for a living won't do it because it's not a conservative, or accepted, design practice. In many cases, it won't cause problems; but how are novices supposed to know the cases where it could?

In cases where the total current in all LEDs is less than the maximum current allowed for a single LED, there's little risk. But if you have say 80-ish, as in a recent thread, and you're operating them at 20mA each; one failure could cause a cascading failure.

A high internal resistance in the voltage source, e.g. CR2032 batteries, is also a mitigating factor.

I wouldn't post a schematic with my name on it that paralleled LEDs without current limiting resistors for each string; unless it was an example of how not to do things. Damage to your reputation/credibility is cumulative.
 

mcgyvr

Joined Oct 15, 2009
5,394
Now, I wouldn't try to sell a circuit which violated the rules like this--actually, I wouldn't try to sell any circuit. But to tell someone who wants to make a simple project using small LEDs that it should not be done, that the LEDs will self destruct, seems to me like bad advice.
Not everyone that gets into a car after drinking kills someone.. But is it bad advice to tell people that drunk driving is bad? Absolutely not..
 

WBahn

Joined Mar 31, 2012
24,854
Are you implying that UNDER driving a LED shortens its life?
I don't know of many LEDs for which a million amperes would be considered underdriving them.

Just to be clear -- the emoticon was meant to indicate that I'm jokin' with you a bit.

1 Ma is 1 million 'a', whatever 'a' is. I actually looked and was unable to find any derived unit that uses 'a' as its symbol, so ampere is the only reasonable interpretation.

1 mA = 0.001 A
1 MA = 1,000,000 A

The case of the prefix in this case DOES matter because 'm' and 'M' are both proper SI prefixes that differ by 9 orders of magnitude. I suspect the committee that chose the prefixes was aware of the potential for confusion, but the prefixes were already well established by then and they likely realized that trying to use something different for one of them might put the widespread adoption of SI itself at risk.
 

dl324

Joined Mar 30, 2015
9,155
You are quite right--I carelessly misread "Ma" as ma. Sorry.
While we're at it, the proper abbreviation for ampere is A, so a milliamp[ere] is abbreviated mA.

The proper abbreviation for a thousandth of a meter is m, and the proper abbreviation for meter is m. So a 5 millimeter diameter LED would be 5mm, not 5MM (which would be 5 million 'M', whatever they are).

We can usually figure out what misspelled quantities are, but sometimes not. Case in point. I was taught that kiloohm was abbreviated KΩ and thought that all multipliers for prefixes were capitalized. Imagine my surprise, and chagrin, to learn that kiloohm was abbreviated kΩ. I still use both because it's hard to break old habits and people can usually figure it out because it would be highly unusual for Kelvin to be used in the context where a resistor value would be used.

Using improper abbreviations is akin to misspelling a word, so they should be caught when proofreading posts before and after posting...
 

WBahn

Joined Mar 31, 2012
24,854
While we're at it, the proper abbreviation for ampere is A, so a milliamp[ere] is abbreviated mA.
I'm glad you said it -- I didn't want to say it again.

Using improper abbreviations is akin to misspelling a word, so they should be caught when proofreading posts before and after posting...
And, it's only by having them pointed out that we learn that we are doing it wrong and can correct it going forward. Of course, that's not going to keep us from making and not catching the occasional typo, but even then the more they are pointed out to us the more aware we are of our penchant for making them and the more diligent we will be about catching them ourselves.
 
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