Question regarding superbright LED problem

[Mach7]

Hell All,

Over the years I have converted my grimes indicator lamps to LED's. Specifically using 5mm superbright yellow, white, green and reds for my simpit, (BAe146).

I have had no issues with any of the 180 lamps that I have converted up to this date except this one particular one that..when power is supplied it will not illuminate right away, rather will remain dead then slowly flicker after a minute or so and then come on to full bright.

...but it gets stranger...

All my leds lamp units are wired in pairs of two (two LEDs per warning lamp), and in series. (see attachment).

Therefore, If one LED becomes inoperative or burns out, then the other should not illuminate.

In this case, the first LED in the package always comes on at full bright and the other is totally dark, (or for what I can see anyway). after a minute or two, the right bulb begins to flicker and then comes on at full brilliance. It is also interesting to note that the level of illumination does not change with the first bulb during this transformation...that is to say it does not illuminate super bright then dim down to normal when the other bulb comes on line.

I think I have a bum LED...only because there must be current going through that bulb in order to complete the connection to illuminate the first bulb.

The bulbs are connected as such (k) anode on first LED connected to ground aircraft ground, (a) anode on first LED connected to (k) anode on second LED then (a) anode on second LED connected to aircraft power.

I am using 12 volts and a 380 ohm resistor for two superbright white LED's wired in series. (Although it calls for a 270 ohm resistor in this case...assuming a Vf of 3 volts @ 20 ma..., I have utilized higher resistor values to stretch LED life).


any thoughts?

-Jim

[Trevor Hale]

Hi Jim, I have seen that on my setup, where the one LED requires more Current to light then I am giving the rest.  I could be out to lunch here, but I personally have seen different color LEDS and Different manufacturers LED's requiring differences.  As a shot in the dark have you replaced the Troublesome LED and Resistor?

Off topic, that is one of the most beautiful MIPS with Wiring I have ever seen.  Nicely done.

Trev

[Mach7]

Thanks Trevor,

Both LEDs come from the same manufacturer, and they both are in series so share the same single resistor (380 ohms).

I have since inspected the wiring and led and found no hidden shorts or anything obvious that would attribute to this problem.

What is peculiar is that the LED will function normally after a couple of minutes with no issues turning it on or off...only when it sits for a long period,(couple of days), will it begin this cycle all over again...as if it needs to warm up in order to function.

you did bring up an interesting point ...quite possibly the second LED may not be the problem and it could be that the first LED is taking all the current for a short period of time before relinquishing it to the second LED...who knows...shot in the dark.

I have never come across this scenario before.

For now I think I will leave it as is as it would be a pain to disassemble and change out the LED cluster. My panels are hardwired into the sim as my thoughts were that the LEDs should last a long time and would be years before they needed to be replaced.

-Jim

[kurt-olsson]

sorry for no ideas on the LED but man... Gorgeous MIP!