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marciot

Victorian Wallpaper Shadow Sconce

by marciot Jun 19, 2015
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Do you think the Cree XP-E2 would be ok powered by a USB 5v power supply without the transformers?
Alternatively, could the base be carved out to make room for three AA or AAA batteries?

You would not want to connect the Cree XP-E2 directly to your USB port. It is a diode and would short out your USB port. Either the LED would burn out, or your USB port would burn out, which ever came first.

You would need to have some sort of current limiting. A resistor would be the easiest way to make it work. I have attached the spec sheet for the XP-E2:

http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Discrete-Directional/XLamp-XPE2

The key specification here is the Typical Forward Voltage, which is 2.9 V White @ 350 mA. So you would have to choose a resistor to put in series with the XP-E2 such that 350 mA would flow through the circuit. There would be a 2.9V voltage drop across the LED, and a 2.1V voltage drop across the resistor in that situation. While a USB port can handle the required wattage, you would need at least a 1W resistor to dissipate the heat generated.

You would have a similar problem with AA or AAA batteries. If you simply connect three batteries directly without current limiting, you would blow out your LED or overheat the batteries, so you need to choose a resistance properly to keep the current at or below 350 mA.

If you don't know how to do this, I recommend getting a basic electronics book, such as Forrest Mims's "Getting Started in Electronics" to get a basic familiarity with how to power LEDs.

And if you do experiment with this, start out by using a cheap USB charger, as you don't want to fry the USB ports on your PC if you make a mistake :)

Thanks for the lessons! I was intending to use a USB charger from an old phone. Can I ask how your method adds a resistor? Does the DC transformer take care of it?

No, but the voltage is adjustable on my DC-to-DC converter, which sits between the power brick and the LED. So the output voltage is tuned to approximately 2.9V, which gives the correct operating current for the LED.

The repurposed transformer (power brick) provides an intermediate voltage (typically 6V to 12V) and the DC-to-DC converter steps it down precisely to 2.9V.

A resistor can be used in place of the DC-to-DC converter, but is non adjustable and needs to be selected for a particular input voltage. The DC-to-DC converter is adjustable and more flexible, but is more expensive. It may have difficulty operating with an input voltage of 5V, but if you find one that works with that, you could use it from USB power. No resistor or power brick needed, as long as you tune the output of the DC-to-DC converter to less than 2.9V.