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LED Array question

Irozato

Jul 21, 2022
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Hello, I'm rather new to the field of electronics apart from some Arduino basics. But I'm a "never tried before so I guess I should be able to" kinda guy and I like to set myself a challenge to learn new skills.

Currently I'm working on an Arduino controlled flea circus. I designed and 3d-printed all the mechanics and now I'm starting on the electronics part. I decided to start with the 'easy' bit of making a circle of LED's.

I made an array of 17 Flickering (yellow) LED's. 2.2V, 20mA with a switch and a 9V battery.
The calculator came up with a Parallel setup of 5 times 3 LED's in SERIES with a resistor and a single branch with 2 LED's + resistor.

And after testing, soldering everything, testing again and pruning all the sticky out bits, testing again, fixating everything with hot glue ...Riding to my holiday location I just realized I messed up (wanted to get it done before leaving on holiday ... Never a good idea ) and put the LED's in the branches in parallel to each other (but still in series with the resistor).

Is this going to pose a problem and what would the effects be?
My basic understanding of Ohm's Law makes me guess each LED will get more Voltage than needed and less current. Resulting in less brightness, which isn't a problem since it's meant to be decorative mellow fire anyway.
So maybe it's a happy accident. But maybe I'm missing something important and I'd rather be safe than sorry.
(And I'd prefer not having to pull everything back out, buy new LED's and start over if it isn't needed).

Thanks in advance for your kind advice.

If needed I'll try to post up some schematics, bit difficult to do on my phone at the moment.
 

Irozato

Jul 21, 2022
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That is indeed my basic understanding. So it'll work just fine?
From what I know: in theory the parallel should make for slightly brighter LED's and my battery will probably run out faster ... Correct?
In that case I should take the time and cost into account of replacing the battery more often or stripping everything out and buying new materials and rebuilding everything.

From reading up on the forum and other sites the main reason for the series in LED arrays is efficiency and making all the LED's have the same brightness (the one with the lowest voltage drop drawing the higher current) Which for my application (flickering flame like movement) shouldn't be an issue in comparison with say an LED screen.
 
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Nanren888

Nov 8, 2015
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in theory the parallel should make for slightly brighter LED's and my battery will probably run out faster ... Correct?
yes, but, ...
not so clearly ok.

I don't know how flickering LEDs work.

LEDs will not actually have the same voltage drop.
Also the voltage tends to go down with temperature, so if one gets more current, hence gets warmer, its junction voltage will drop and will get more of the current.
Someone with more experience will be able to tell you whether you will get away with this for small leds.
 

Martaine2005

May 12, 2015
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What is the max forward current of each LED?
If 20mA - 30mA, then 56mA will destroy them.
You have two choices, rewire the circuit or change the resistor. But parallel circuits require a resistor for each LED. Series circuits can use one Resistor. A 9V (pp3) battery won’t last long at all. You’d be much better off using an AC-DC adapter or lithium cells.
EDIT: LED x 3 = 18-19mA.

Martin
 
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danadak

Feb 19, 2021
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No "right" answer here because :

1) If LEDs all from safe lot they could have some level of matching.
2) LEDs various lots would be worse case, possibly resulting in 1 LED
taking way too much current and burning out or shorting. Then that cascades\
into next parallel LED with a lower Vf hogging all the current and ......
3) The reason one "gets away" with paralleling LEDs is they are lousy diodes
with relatively high dynamic R, but one cannot rely on absolute values of this
or threshold V.

iu


I would match up parallel LEDs, eg. remove, test, rewire.


Regards, Dana.
 

Irozato

Jul 21, 2022
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This is the schematic I used. But obviously mistakenly placing the LED's. So using Bertus' math I should still be in the 'safe' range for the max forward current (18 vs 20mA max).

That said: Dana's reply makes me worry a bit. Guess my next order will include replacement LED's ... And I'll replace them after getting everything else to work.

The performance should last about 10 minutes. If a LED goes, it isn't that big a worry. And the first few times we'll see how long the 9V lasts, AC/DC isn't an option since it needs to be entirely portable and self contained. But I will look into the Lithium cells. I need the 9V for another effect I can't get done without blowing up the Arduino, but it should work on the Lithium as well.
 

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Martaine2005

May 12, 2015
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Very unlikely or very lucky to find matching LEDs. That is why a resistor should be used to limit current on each parallel LED. If one LED fails, the rest will follow.


Martin
 

danadak

Feb 19, 2021
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Irozato

Jul 21, 2022
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A lot of that reading was a bit too complicated for me. But interesting nonetheless. I gather there might be an overheating problem and the 9V isn't the best choice. 3 hours if I'm lucky, in my current setup even only 1. Looks like I'll be looking at an alternative power supply.

Oh well, new problems to overcome and some learning to do In the process! Thanks for all your input!
 

Nanren888

Nov 8, 2015
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Summary:
parallel LEDs without individual resistors are often not a good idea as one gets most of the current, blows up, then the next one gets most of the current and blows up , until there are none left.
.
If each LED can take the max current, then no blow, up, but they will have dfiffernet intensities.
 

danadak

Feb 19, 2021
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A rough look at matching. The V source offset in D1 leg swept from -.1V to +.1V.
Basically simulating a nominal diode Vf from +1.6 to +1.8V.

As you can see dramatic sharing unbalance at extremes of 100 mV between diodes.

upload_2022-7-23_8-47-26.png

The sum of current also shows T swept from 20 C to 50 C

Regards, Dana.
 
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