[roger1030]

My question is that I see many USB powered lamps on the market that use more than one LED. Many of them use several LED’s. When a USB only outputs 5 volt and an LED is a little over 3 volt how are they powering five LED’s at a time off one USB port? Even in parallel I cannot get more then one to light up. What am I missing? Is there a special LED out there that they are using?

Thank you,

[ebaines]

It's not the voltage that's the limiting factor for LEDs, as you can wire the LEDs in parallel so they all see 5 volts. What's important is the current draw - LEDs typically require between 20 - 50 mA, and a USB can put out as much as 500 mA @ 5 volts (2.5 watts), so there's plenty of power to light multiple LEDs. You would use resistors to control the current through the LEDs, and hence their brightness.

Here's an article from TI that may be of interest to you:
http://www.google.com/url?sa=t&rct=j...9BYRfe_Sb7STEg