# LED Array Problems

Thread Starter

#### Protactinium

Joined Nov 28, 2011
2
Hello,

I am fairly new to electronics and I attempted to build an LED array to attach it to a timing circuit.

However after building the array and trying to attach it to the 12V power source directly to test it, and none of the leds worked. Its 36 leds in 4 parrel circuits with resistors. When I used an LED out of the same bag to test the timing circuit everything seem to function properly.

I honestly have no idea what is wrong with the array so can someone please give me advice on how to troubleshoot it in order for me to identify my problem. I own an multimeter but thats all the diagnostic tools I have.

Thank in advance!

#### SgtWookie

Joined Jul 17, 2007
22,220
Well, we really need to see how you wired it up, and the manufacturer's specifications on the LEDs.

What is your intended application? What is the "timing circuit"?

Post a schematic image; it needs to be complete.

Thread Starter

#### Protactinium

Joined Nov 28, 2011
2
I am trying to make a flashing LED.

The LED array is 9 leds with a resistor with 4 parrel circuits.

R-Led-Led-Led-Led-Led-Led-Led-Led-Led
| ~~~~~~~~~~~~~~~~~~~~~~~|
R-Led-Led-Led-Led-Led-Led-Led-Led-Led
| ~~~~~~~~~~~~~~~~~~~~~~~|
R-Led-Led-Led-Led-Led-Led-Led-Led-Led
| ~~~~~~~~~~~~~~~~~~~~~~~|
R-Led-Led-Led-Led-Led-Led-Led-Led-Led
|~~~~~~~~~~~~~~~~~~~~~~~|
+~~~~~~~~~~~~~~~~~~~~~~~ -
12 Volt

Its kinda ghetto schematic. This is what I used to bench test it, and no leds turned on. ~ is for

But I am just curious if there is any kind of tests I can run in order to tell why none of the leds are ligting up. IE run a voltage test in order to tell if the circuit is running.

Last edited:

#### Wendy

Joined Mar 24, 2008
22,164
Every LED drops a specific voltage, referred to as Vf. The sum of the Vf in a chain must be less than the power supply voltage. You are showing 9 LEDs per chain, where 3 is the usual maximum allowable.

A tutorial...

LEDs, 555s, Flashers, and Light Chasers

Read Chapters 1 and the first ½ of chapter 2.

#### SgtWookie

Joined Jul 17, 2007
22,220
The LED array is 9 leds with a resistor with 4 parallel circuits.
It helps a lot if you compose your ASCII schematic using Notepad with Courier New as the font (monospaced), and copy/paste it in here with [ CODE] and [ /CODE] blocks around it to preserve the formatting.
Rich (BB code):
   R-Led-Led-Led-Led-Led-Led-Led-Led-Led
|                                   |
R-Led-Led-Led-Led-Led-Led-Led-Led-Led
|                                   |
R-Led-Led-Led-Led-Led-Led-Led-Led-Led
|                                   |
R-Led-Led-Led-Led-Led-Led-Led-Led-Led
|                                   |
+                                   -
12 Volt
You undoubtedly have attempted to place too many LEDs in series.

1) You say 12v is your supply voltage. Subtract .75v from that.

2) Divide the remainder by the typical Vf of the LEDs at the rated current. You should have been provided that information when you bought the LEDs. If not, you will have to measure them using a constant current circuit to find out what the Vf is.

3) Take the integer of the result of 11.25/Vf_LED. That is the maximum number of LEDs of the type you are using that you can operate in series.

4) Subtract the result of 3) multiplied by the LED_Vf from your supply voltage.

5) Calculate the resistor required by the result of 4) divided by the desired LED current. Use a standard value greater than or equal to that result.

What color and current rating are the LEDs?

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