[AL]

Instead of a resistor, you should try a three terminal regulator. They couldn’t be simpler. With three pins (left to right), input, ground, output. They are internally set to output 12.6 volts. Search eBay for LM7812 and you’ll get lots of hits. One seller has them for $.2.88 for 10 pieces (28 cents each).

The device is rated at 1.5A which is more than enough for most LED strips up to 15 feet. Turn it on and see if the package gets hot. Warm is OK. If it gets too hot to touch, attach it to a piece of metal with a small screw to dissipate the heat. There are larger versions too.

Resistors are problematic. You have to get a size that will yield the current appropriate for your string, at the brightness you want. Then, measure the current and use ohm’s law to determine the wattage that will be required for the part. Yes, it will work, but no single size resistor will work for all light string lengths.

The other option is the LM338T which is adjustable. With this, you can add just a small pot and set the brightness to your liking! It’s also rated at 5 Amps which is huge for an LED application. see http://www.ti.com/lit/ds/symlink/lm138.pdf and http://www.futurlec.com/ICLinearRegulators.shtml

There’s a lot you can do with a soldering iron and a few simple parts!