LED Driver - Page 1
Forum section: electronics / projects

author: thomst77
author_rank: Contributor
timestamp: July 10, 2013, 09:10:13 am
content: I am new to the world of LEDs. I just bought some Ebay cheapies to experiment with. The specs are 1w DC Forward Voltage: Typical:3.4V Max:3.8V DC Forward Current: 350mA mounted on a star base. My first project will be to make a Light Bulb of sorts. Maybe to illuminate the desk. I am going to be using a 12v Wall Wart which by the way when I test it reads over 16v. I am not sure if I may need to add a voltage regulator or not. I am looking for an easy to build LED Driver I think I would like to drive 1-3 of these leds. they are 90lm a piece. I may hook one up and if it is not bright enough I will add another and another until I get the brightness I want. I will also probably put a diffuser of some sort over the leds. If it needs it. As I said I am not at all familiar with these More powerful leds. I have messed around with some 5mm Superbright white leds. They are easy. Just connect a little resistor and your set. But with the larger leds for efficiency I think I might need some LED Driver. I want to build it my self to learn and because they are expensive.Thank You all in advance for your inputTom

author: mariush
author_rank: Super Contributor
timestamp: July 10, 2013, 09:36:15 am
content: You don't need a voltage regulator, you need a current regulator.LEDs are current devices, they only need more than 3.4-3.8v to actually start working and then you have to limit the current under 350mA so they won't burn up.You can do tests with a simple resistor to limit the current. Assuming you'll power the led from 5v and want 300mA through the led :V = I x R5v - 3.8v (led forward voltage) = 0.3A X R => R = 1.2 / 0.3 = 4 ohm so you could use a 3.9 ohm resistor that has a standard value.It will have to be rated for 0.3ax0.3ax3.9ohm = 0.351 watts, so at least 0.5w but preferable 1w.With 12v, you have 12v-3.8v = 0.3 x R => R = 8.2/0.3=27 ohms with power dissipation of 0.3x0.3x27 = 2.43w so at least 3w rated resistor.But at this point, you should already look at least into using a LM317 or something similar as a current regulator, if you're not going to use a proper led driver.

author: thomst77
author_rank: Contributor
timestamp: July 10, 2013, 11:09:36 am
content: I guess what I am looking for is something like a 350mA BuckToot. Does anyone have a schematic/documentation for a bucktoothttp://www.ledsupply.com/bucktoot.phpor similar? I am assuming it would be more efficient, And I like the broad input range. In fact I would be interested in the schematics for the 350mA MicroPuck LED Driverhttp://www.ledsupply.com/02009a.phpor similar as well. As it would make a nice flashlight driver. I think this is what I am looking for. What do you think?ThanksTom

author: mariush
author_rank: Super Contributor
timestamp: July 10, 2013, 11:18:13 am
content: That seems like quite expensive for what it does.Just yesterday I pointed someone to a led driver chip that's quite easy to use and cheaper:http://uk.farnell.com/diodes-inc/ap8803wtg-7/ic-led-driver-buck-1a-5tsot23/dp/1825359RLadd a resistor to set the current, a shottky diode, an inductor and you can put several leds in series up to 1A of current, takes 8-30v input....And there's also a control pin to which you can send a pwm signal to adjust the brightness.There are similar such led drivers which are much cheaper than those thinigs and the datasheets are generally informative enough to be able to make your own for 2-3$, not 9-10$ plus shipping.

author: KTP
author_rank: Frequent Contributor
timestamp: July 10, 2013, 11:34:20 am
content: The micropuck he linked is a boost dc-dc converter driver.I bought some of the Flexblock modules from the same manufacturer and distributor...10v to 30v in, 0 to 48v out at up to 700mA with a current set/dim input line. I am measuring them to be 94% efficient in boost mode. I couldn't build one for the $17 they cost, even if I paid myself half minimum wage. Quality products.

author: croberts
author_rank: Regular Contributor
timestamp: July 10, 2013, 12:52:56 pm
content: Hello thomst77I light my house with 24 LED lights I designed and built over the last two years and I thought I would pass along a few rules of thumb.My lights are about 1200lm and it takes 4 lights per 100sq ft for adequate brightness and even light. The lighting effect is very cumulative. In other words lighting one light in a space makes you feel as though it will not be enough light when more is added but the light from each source overlaps and reinforces the others to fill the space. The light exits the LED in a more or less conical shape so the light is brighter and the light spot smaller the closer you are to the LED.To get maximum life from the LED you need to operate them at or below the maximum temperature given by the manufacturer. This maximum temperature is usually between 70 and 85 degrees C. Most white LEDs have a life of about 50000 hours at rated output and then is derated to 80% or so of rated output at 70000 hours. To keep the LEDs running at or below that maximum temperature you will need a heat sink. For 1/8" thick aluminum about 9 square inches per W of LED dissipation is a good rule of thumb. For my LED lights I use an 1.75"W x 4.5"H x 6"L aluminum extrusion that also doubles as the body of the light. As long as the ends of the extrusion are not blocked to the flow of free air the inside and outside surface area add to give about 144 sq in. Mount the LEDs with heat grease or thermal epoxy for good heat transfer.I have attached a few photos. I hope the information is helpful.

author: thomst77
author_rank: Contributor
timestamp: July 11, 2013, 02:15:52 am
content: That is a very nice looking setup. Thank You for sharing.