LED's are great display tools. Their prices have decreased to a point where they are replacing more conventional light sources. In one sense their characteristic need for low voltages is an advantage e.g compatibility with I.C drives, but this voltage requirement can also be a disadvantage. I wanted to light some LED's in a simple sign application, nothing fancy, but I needed to decrease the utility line voltage to a LED-compatible value for this application.
The line voltage can be reduced in various ways, via dropping resistor, a transformer, a "brick" power supply, a capacitor etc. A dropping resistor wastes considerable power and generates heat. A "brick" power supply must be adapted to the particular design needs, i.e a specific voltage and current and also may have an expense associated with it that is undesirable for a simple circuit. A transformer can be bulky, weigh a lot and may be expensive.
source: William Rynone Ph.D, P.E
Read more at:
http://www.electronics-lab.com/projects/motor_light/037/index.html 