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Simple 5X5 LED array not working. Flickering and high voltage problems, pictures included? Answered

Hello Instructables!

I want to make a 5X5 5mm LED array/display looking like this:

http://oi44.tinypic.com/wuh3f4.jpg.

The information is the following:

Source voltage: 12
LED forward voltage: 3.5
LED forward current: 20 (mA) 
Total LED's: 25.
Resistor value: 270 ohm (closest to needed value)

Calculated resistance needed for each vertical row:
20 (mA) x 5 LEDs = 100 (mA)
=> 100 (mA). Using Ohms law, I found optimal resistance at 120 Ohms.

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I tried to transfer this circuit onto a stripboard and theres a number of problems:

1: I have to use 24 volts (from a wall charger) for all the LED's to light up. ( Is it possible to make it work with 12 volts? )
2: The resistors get hot, which I find strange. They are 150 Ohms above recommended level, so it seems strange.
3: The LED's got hot, and burned out even with the above.
4: The LEDs "flickers/flashes" extremely fast, almost not being visible to the eye. This shouldn't happen.

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What am I doing wrong? Why is this happening? I have tried troubleshooting this for 3 hours and I am giving up. 

Thank you in advance,
Ardavan.

(Ignore the CAP in the real photo. Added as a desperate action after everything self destructed)

3 Replies

user
ArdavanAbtahi (author)2013-07-28

- I see that with the 12V I have an 1x25 option. Does this eventually mean that I could build that 5x5 if I just connect those 25 in series all the way with one resistor (Like a snake)? Would it work just as good/bright as a 5x5 using 24v?

Thanks!

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user
ArdavanAbtahi (author)2013-07-28

Thank you very much for your exemplary answer! Your question answered my questions while being very accurate, thorough and specific. Thanks!

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user
bwrussell (author)2013-07-22

The issue is that 12V is not enough for that array. I like to use the array wizard HERE when I do LED arrays. It's accurate and gives you several options.

Each leg has equal voltage drop. To calculate the minimum voltage source needed multiply the number of LED's in a row by their forward voltage, in this case 5x3.3V = 16.5V min. If you want to use 12V your options are an 8x3 with one extra, a 2x12 with one extra, or a 1x25.

The resistor and LEDs are having trouble because @ 24V you need a minimum of a 330 Ohm resistor in a 5x5 configuration.

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