880nm 5mm array

[stein09163]

hi

I designed a small array.  I'm using a 24V 4A AC/DC adapter as a power supply.  The array is series parallel  16x4 and uses 64 880nm, 5mm, discrete LED's and 4  1/4W, 1 ohm resistors.  I used a test light on the array and found that the brightness from one led to another was different.  The positive end of the array is brighter then the negative end.  What is causing the brightness to change.

I thought maybe the voltage being supplied was to low so I changed the array using the same 24V adapter for power.   The array is series parallel 12x4 with the same LED's but using 4 each 1/2W, 82 ohm resistors.  I got the same results.

Both times I used the series parallel wizard to design the arrays.

What is causing the brightness problem?

Rick

[Benta]

Too little info, but 1 ohm is ridiculously low.
Don't use wizards before you can do it with paper and pencil.

Just to start: which LEDs? What's your target LED current? How exactly are they wired? I could go on.

A picture (in this case a schematic) is worth 1000 words.

[stein09163]

The led's are discrete led's......infared  880nm  5mm  75 mW    64 of them   16x4  series parallel.........source voltage  24  ,  Forward voltage 1.5,  forward current 50 (mA),  number of led's in array  64 

         +   >->->->->->->->->->->->->->->->   -   1 ohm Resister
          +  >->->->->->->->->->->->->->->->   -   1 ohm resister
24V    +  >->->->->->->->->->->->->->->->   -   1 ohm resister
          +  >->->->->->->->->->->->->->->->   -   1 ohm resister

[Benta]

Forward voltage 1.5,  forward current 50 (mA)

This is your problem. The forward voltage is typically 1.5 V, but can be 1.3...1.8 V, and you don't know which.

The best solution would be shorter LED chains and constant-current sinks.

[edavid]

"I used a test light on the array" - what does that mean?

Most likely your brightness measurement was off somehow.