Or to put in another way: "this is better than using a resistor". It's more consistent, more efficient, and more flexible. It's ideal for High-power LED's especially, and can be used for any number and configuration of normal or high-power LED's with any type of power supply.
As a simple project, i've built the driver circuit and connected it to a high-power LED and a power-brick, making a plug-in light. Power LED's are now around $3, so this is a very inexpensive project with many uses, and you can easily change it to use more LED's, batteries, etc.
i've got several other power-LED instructables too, check those out for other notes & ideas
This article is brought to you by MonkeyLectric and the Monkey Light bike light.
Step 1: What you need
R1: approximately 100k-ohm resistor (such as: Yageo CFR-25JB series)
R3: current set resistor - see below
Q1: small NPN transistor (such as: Fairchild 2N5088BU)
Q2: large N-channel FET (such as: Fairchild FQP50N06L)
LED: power LED (such as: Luxeon 1-watt white star LXHL-MWEC)
power source: I used an old "wall wart" transformer, or you could use batteries. to power a single LED anything between 4 and 6 volts with enough current will be fine. that's why this circuit is convenient! you can use a wide variety of power sources and it will always light up exactly the same.
heat sinks: here i'm building a simple light with no heatsink at all. that limits us to about 200mA LED current. for more current you need to put the LED and Q2 on a heatsink (see my notes in other power-led instructables i've done).
prototyping-boards: i didn't use a proto-board initially, but i built a second one after on a proto-board, there's some photos of that at the end if you want to use a proto-board.
The circuit is a constant-current source, the value of R3 sets the current.
- LED current is set by R3, it is approximately equal to: 0.5 / R3
- R3 power: the power dissipated by the resistor is approximately: 0.25 / R3
I set the LED current to 225mA by using R3 of 2.2 ohms. R3 power is 0.1 watt, so a standard 1/4 watt resistor is fine.
where to get the parts:
all the parts except the LED's are available from http://www.digikey.com, you can search for the part numbers given. the LED's are from Future electronics, their pricing ($3 per LED) is far better than anyone else currently.