USB LED without a resistor?

[tylernt]

I dissected a dollar store USB keyboard light that had failed (infinite resistance at the LED) after a few minutes of use. When I took everything apart, including the USB plug, I found absolutely no current limiting resistor anywhere. The 5mm white LED looks normal -- I can't see any evidence of a built-in resistor inside the package.

I know it's from a dollar store, but really? No resistor? I did seem awfully bright for the few minutes it worked...

 

[AnAppleSnail]

I dissected a dollar store USB keyboard light that had failed (infinite resistance at the LED) after a few minutes of use. When I took everything apart, including the USB plug, I found absolutely no current limiting resistor anywhere. The 5mm white LED looks normal -- I can't see any evidence of a built-in resistor inside the package.

I know it's from a dollar store, but really? No resistor? I did seem awfully bright for the few minutes it worked...

Now you know why it toasted! It's a shame when someone will waste about $0.20 on building the darn thing and not put in a $0.01 resistor to make it work correctly. I bet if you look down the LED housing you'll see the crispy blackened phosphor.

 

[tylernt]

Looks like the bond wire melted, actually. Capitalism at it's best, eh? :ohgeez:

 

[MikeAusC]

You can just see the Cloner looking at a working light - "Hey, this little black thing on the board doesn't give off any light at all, I'll save money by leaving it off "

 

[VidPro]

ohh gee, manufacturing made 18,000 of these incorrectally, look, plug it in and it shines bright but then dies.
oh great, now what are we going to do with them :devil:

 

[Ray_of_Light]

I come across one of them, too. I paid 2 Euro for an USB light at flea market, without the resistor. I paid, for the exactly same looking light with a brand name, ten euros in a computer store. The only difference was the presence of a 6.8 ohm resistor in the latter.

It happened to me with some other small Chinese LED lighting accessories.

Virtually all dollar store and flea market LED items are factory rejects. As rule of thumb, you need to open them and install a limiting resistor.

Regards

Anthony

 

[JohnR66]

To limit current to around 20 ma for a 5mm type white LED, the light should have a 100 Ohm resistor in it for the 5 volt supply.

I bought one of these at the dollar store. It had a 22 Ohm resistor in it. The LED lasted for a day or two before it got really dim. The LED was was one of those cheapo white ones with a lot of violet tint. Complete junk.

 

[qwertyydude]

Apparently the Chinese manufacturers think more ohms costs more $$$ so only low value resistors or none at all. I should be selling them 1 mega-ohm resistors. Think of the profits :party: