EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: CheekyRobot on November 22, 2020, 11:38:28 pm
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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??? |O
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Your calculation checks out. 470R should give you about 18.5 mA from 12v - 3.3v.
Any chance you bridged your 470R SMD while soldering? :-//
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Your calculation checks out. 470R should give you about 18.5 mA from 12v - 3.3v.
Any chance you bridged your 470R SMD while soldering? :-//
Thanks for the second pair of eyes on that. Did a continuity check across both resistors and all tracks. No shorts - so unless it shorted magically AFTERwards... but I also cleaned it all up nicely to avoid solder balls, flakes of copper etc. Even inspected under the lab microscope. Clean as a whistle. Unless... this is some odd burn-in property of UV leds that I dont know about!
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> All simple SMD parts,
SMD resistor marking "470" means "47 Ohms" not "470 Ohms", would be the most likely mistake to make, 200mA instead of 20mA would indeed let out magic smoke.
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> All simple SMD parts,
SMD resistor marking "470" means "47 Ohms" not "470 Ohms", would be the most likely mistake to make, 200mA instead of 20mA would indeed let out magic smoke.
That's a good idea. Happly, I've got all my parts nicely labelled and in fancy little draws. So 100% sure they're 470r... I even checked a sample as well
But it's solved now. Occam's razor being the sharpest tool in the box and Paul was right! Even though I'd checked it all out nicely and examined under a mic, after I'd carefully enclosed the wire-end in shrink-wrap, a TINY LITTLE bit of wire had gotten loose and had indeed secretly shorted out the resistor under the wrap! BAH!!!! S@D'S LAW!!!
Ah well, the other LED was working so I'll just get some more. Thanks all for suggestions- glad it was something STUPID in the end.