Okay, this is doing my head in. I thought I KNEW how to calculate the resistor value for a simple LED.
Excuse the crude text-schematic here, but nothing special going on...
+12v o--->|--[470r]---o gnd
All I wanted to do was run an LED off a 12v supply rail. Powered by a 4s LIPO through a 12v linear regulator.
LED is UK Farnell SKU 2900671 UV led with Vf of 3.3v(typ) Ir(typ) of 30mA.
The usual led resistor calculators say 435r at 20mA.
Slapped in a 470r because I've got loads soldered to quickly made red-neck PCB. Checked it, buzzed it all out with a meter, no shorts (esp across the res). All simple SMD parts, nothing I've not done hundreds of times before.
Connected it up, plugged in and the LED flares up and dims down like it's burned out.
After some head-scratching, I just hooked it up to the 5V supply out of frustration and (there's two of these on the board) the LEDs light up at uneven brightness... just liike they're damaged.
12v supply is okay because it's running other more delicate electronics just fine (and some other LEDs!).
SO.... what did I miss???