Instructables

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

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)

ArdavanAbtahi (author) 12 months ago
- 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!
ArdavanAbtahi (author) 12 months ago
Thank you very much for your exemplary answer! Your question answered my questions while being very accurate, thorough and specific. Thanks!
bwrussell1 year ago
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.