Introduction: The Super-efficient USB Light
Lights that plug into the USB port have been very popular and I bought one for use at night.
The first thing I noticed was that it was not too bright, and when one of the LEDs quickly died, it was time to do some upgrading.
Prying the cover apart revealed 3 white LEDs in parallel, with a 30-ohm resistor to limit current to about 60mA.
I want more light, but the 1/10-watt resistor was already putting out major heat.
Step 1: The Upgrade
We know that 3.2-volts is going to the LEDs, and we can measure it with a voltmeter across the LED's wires, but what about the other 1.8-volts supplied by the 5-volt USB?
It's making lots of useless heat in the resistor!
That is what the top circuit shows - of all the power the USB is delivering, a full 36% is being wasted! And 1.8-volt represents 55% of the power going to the LEDs!
That is to say, if we can somehow eliminate the resistor, we'll be able to improve output by 55%! That's like getting an extra 16-miles per gallon from your 30-miles-per-gallon car!
(In EU terms, that's going from 7-l/100km to 5-l/100km!)
The bottom circuit show how we achieve this - we pair each white led with an amber (or yellow) LED.
Amber LEDs, with a Vf of 2-volts, will raise the Vf of the pair to about 5-volts - exactly what the USB delivers!
Step 2: Seeing the Light
With this change, and both circuits drawing about 60mA from the 5-volt USB, the results are startling.
First, with the old design: barely usable.
And then, with the improved design.
Both pictures are taken with the exact exposure and the lights placed an identical distance from the keyboard.
This is the difference 55% makes! Much more usable light, with a warmer (warm-white) glow from the amber LEDs as well.
The 3rd image shows the new stylish "Easter Egg" design, with a less-than-stylish lampshade around it.
With amber LEDs selling on eBay for under 10c each, it's time to upgrade your USB lights!
You can see more of my LED projects here.