High power LEDs - are drivers really necessary? - Page 1
Forum section: electronics / beginners

author: algorhythm
author_rank: Contributor
timestamp: December 25, 2015, 02:56:04 am
content: Hello eevblog.I'm an artist who uses electronics in my work, and with no ee training, I have a question about high power leds:Are constant current driver boards really necessary, and why can't I just use a current limiting resistor.More specifically, let's say I want to light up a 7x7 grid of high power leds (5w or 10w). The grid may light up in different locations to display characters, animation, etc... Aren't these high power leds like normal leds? Will current limiting resistors work in my grid, or should I be using constant current driver boards for each led. Why or why not?Thanks in advance...

author: pmbrunelle
author_rank: Regular Contributor
timestamp: December 25, 2015, 05:56:23 am
content: The series resistor scheme can work, but for current-limiting that is not too finicky, a large fraction (half or more) of the supply voltage must be dropped across the resistor.So whatever power your LEDs need, double it, and that's what your power supply must provide. So I don't know if this is feasible or not for you.If the power wasted by the resistors is too much, then look at something more sophisticated.If the display doesn't need to be on continuously, you could use a car battery as a (temporary) high power supply.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 25, 2015, 06:10:59 am
content: Dissipation of heat would likely be a major problem. The LEDs alone will need a lot of thermal management, adding a bunch of lossy and hot resistance adds to the challenge.Sent from mobile device.... Keeping it short and mis-spelled

author: Seekonk
author_rank: Super Contributor
timestamp: December 25, 2015, 11:34:25 am
content: A LED is a diode and the forward voltage reduces as it heats up. Operating at constant voltage can result in thermal run away. Anytime you operate a LED at the maximum power you have to be careful. I make a lot of custom lighting for my camp and use high power LED for the ease of mounting. These are run at a small fraction of rated power and are mounted with just hot melt glue. Current could double and they still are in a safe range. LED are more efficient at low power. So a LED can be driven by simple methods. Remember the power rating of most LED are greatly exaggerated.

author: LukeW
author_rank: Frequent Contributor
timestamp: December 25, 2015, 01:42:30 pm
content: Basically, as others have said, it's the size of the resistors, heat dissipation in the resistors, and the energy efficiency of the system that are the limiting factors here.You can play with the familiar kind of If= (Vs- Vf)/R and Pr= (Vs- Vf)2/R sort of maths here to get a feel for this.

author: dom0
author_rank: Super Contributor
timestamp: December 25, 2015, 02:51:29 pm
content: Quote from: Seekonk on December 25, 2015, 11:34:25 amThis is the same story over and over again. It's not an exaggerated rating, it's simply that power dissipation for almost any semiconductor is specified for Tj = Tjmax, Tc = 25 °C (or 20 °C). Hence Pd  = (Tjmax - 25 °C) / Rthjc [K/W] . Obviously, keeping any semiconductor case at 25 °C while sinking power is usually not possible.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: December 25, 2015, 03:41:08 pm
content: If you have the space, thermal management need not be an issue...Here is the power resistor of a project I did:

author: Seekonk
author_rank: Super Contributor
timestamp: December 25, 2015, 08:07:49 pm
content: I must be having trouble with the New Math. It looks like they are more efficient at lower power. From your data sheet lm/W = 155@175mA, 137@350mA, 120@500mA, 112@700mA. 38% more efficient at low power or 13% more loss each time the current is increased about 200mA.The OP is an artist and talking about such esoteric details doesn't help him. Considering the artwork probably isn't a big aluminum heat sink, give the OP some advice he can actually use.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 25, 2015, 08:48:28 pm
content: With LEDs the rated power is usually less than that for maximum junction temperature. It's more close to the point where light output is close to maxmimum, efficiency has dopped to much to be usefull or where lifetime detoriates too much, whichever is lower.So yes efficiency is usually better at lower current and it's a good idea to use a little of derating, e.g. only 75% of the rated current. This eases cooling demands.It is possible to use resistors, but as allready said this results in quite some extra power loss. So the constant current driver may be the better choice. If not used often the resistors might be still a viable option, but you also need a higher power supply.

author: mrpackethead
author_rank: Super Contributor
timestamp: December 25, 2015, 09:15:39 pm
content: Its not just a matter of power loss either.With High Power Leds, Vfd varies as current goes up and down, and also as temperture changes. Small changes in Vfd, using a resistor as a current limiting device could result in large current changes.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 25, 2015, 09:25:58 pm
content: If you don't care about efficiency and just want to do it cheaply, resistors are an option, but you need to do it carefully to minimise heat.The key parameter is how much the LED forward voltage varies from part to part, and over temperature. For LEDs of the same type, 0.5V variation per die should be a fairly safe assumption.You need to choose a resistor value to cover at least this variation at the nominal LED current, and then trim the supply voltage down as low as possible.e.g. for a LED with nominal 3.5v forward voltage and 1A max current. Allow 0.5V for variation so trim a 5V supply down to 4V (note some PSUs go unstable if you trim them down too far)Resistor value 1.5v/1A = 1.5 ohms. Use resistors with at least 7W rating - you may need fan cooling on a bunch of them.Using LEDs with multiple die will be better as current will be lower, so wiring losses will be less of an issue.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 25, 2015, 09:31:11 pm
content: Something else to consider is that LED replacements for 12v halogen lamps are getting REALLY cheap nowadays, so you may find something off the shelf with inbuilt current regulation that you can just supply with switched 12V.

author: ocw
author_rank: Regular Contributor
timestamp: December 25, 2015, 11:55:14 pm
content: LED drivers are nice in raising power efficiency, but typically produce a lot of RF noise. One of mine wipes out the AM broadcast band and even adds noise on local FM broadcast reception. I can hear a neighbor's switching supply which is almost 200 meters away from my house.Below are some scope and spectrum analyzer views of the output of a loaded LED driver. That's typical of the eight LED drivers which I recently tested.For LED's being fed 350 ma or less, using a supply not much above the LED's voltage along with a self biased transistor current limiter such as a NSI50150ADT4G offers a nice way of eliminating noise generation and maintaining the current regulation while not dissipating too much power. There are obviously other models and brands available to do the same thing. They eliminate the noise problem while creating their own limitations.

author: Zero999
author_rank: Super Contributor
timestamp: December 26, 2015, 12:17:19 am
content: LED drivers don't have to be any more noisy than any other switched mode power supply.If you're going to use an SMPS anyway to get the voltage as close as possible to the VFof the LED, then you might as well use a constant current SMPS to power the LED and design it so it doesn't emit too much noise.

author: Brumby
author_rank: Unknown
timestamp: December 26, 2015, 01:12:34 am
content: If you want tosafelyget the most out any LED, especially with minimal losses (ie. heat), consider it a device that is current driven, not voltage driven. This is supported by the primary limiting factor being current with the forward voltage drop being, in essence, just a measured value.Resistors provide current limiting, but are of a fixed value. Variations in supply voltage and/or forward voltage drop across the LED means you have to choose a resistor value on the conservative side, which will usually result in the LED output being lower.This means, for optimal LED safety and output, use a constant current source - of whatever design.

author: algorhythm
author_rank: Contributor
timestamp: December 27, 2015, 06:08:05 am
content: Thanks for all the excellent info. It seems like constant current drivers is the way to go for this project.Does anyone know if these drivers can be modulated to control brightness? I.e. sink the driver through a transistor and pwm the base to "dim" the led?Quote from: Seekonk on December 25, 2015, 08:07:49 pmIronically, it is simply a giant aluminum heat sink: a lattice of 7 x 7 pieces of 1" x 1" x 1/8" square tube on ~ 6" centers, with 6" diameter cylinders around the junctions (see pictures). One led per junction. However, I don't want the thing to get too hot, so I may end up using drivers designed for 5w leds that provide ~600mA of current, instead of the 900mA, 10w ones I was looking at originally.Basically the circuit will consist of a microcontroller shifting data to 7 shift registers, and each shift register will control 7 transistors. Each transistor sinks power for one led driver, resulting in 7 "bars" comprised of 7 lights each, making a 7x7 grid of 49 lights. (see picture)I'd love to control overall brightness with a pwm signal to the output-enable pins of the registers, but I have no idea how the drivers will deal with this. I guess there's only one way to find out...Quote from: mikeselectricstuff on December 25, 2015, 09:31:11 pmMike's suggestion about 12v halogen replacements is excellent and might save me a ton of time. Thanks!Anyway, thanks again for all the thoughtful responses!Cheers.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 27, 2015, 10:05:53 am
content: Quote from: algorhythm on December 27, 2015, 06:08:05 amDepends very much on the driver - some are designed for dimming, some aren'tIf you have a big-ass chunk of aluminium, and don't care about effificncy (PSUs are cheap) then burning off power in metal-clad resistors bolted to it is probably a fairly viable approach, and will be much simpler and cheaper than proper constant-current drivers.Another possibility is to use simple LM317 based linear current regulators, again using the structure as the heatsink.

author: Seekonk
author_rank: Super Contributor
timestamp: December 27, 2015, 05:08:20 pm
content: A few days ago on TV they interviewed an artist that had a large light cube display near to Twin Towers site. There was a master light cube changing colors that a person would release their hands from when they liked the hue. The large array of light cubes overhead would ebb and flow in a color wave. It was quite an effort of software and hardware. It was quite obvious "the artist" had (expletive deleted) nothing to do with the design. Too bad the real creator will never get credited.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 27, 2015, 05:54:36 pm
content: I have known many artists with exceptional technical skills in many disciplines of engineering and/or sciences.Sent from my SAMSUNG-SM-G890A using Tapatalk

author: Seekonk
author_rank: Super Contributor
timestamp: December 27, 2015, 08:13:37 pm
content: This guy was definitely not one of them. He was the big name that got money to flow in. None of his other projects were anything like this, the work was contracted out. This was some hairy array math. Is it art? Even Michelangelo didn't do most of his works.

author: algorhythm
author_rank: Contributor
timestamp: December 28, 2015, 06:00:06 am
content: Quote from: mikeselectricstuff on December 27, 2015, 10:05:53 amThanks again Mike. I just ordered a couple of dimmable 12v G4 halogen replacements. I'll test and see how the dimming works for me. This should save me tons of time. If the replacements I found are too dim (instead of too bright) I may play around with a brighter cob and an LM317 circuit to get the lumens that I need.Quote from: Seekonk on December 27, 2015, 05:08:20 pmThis is definitely frustrating. In a past life I made electronics and software for artists to use in "their" work as well. It was irritating to know that these artists were showing work that they couldn't make themselves. However, I charged lots of money and realized that if I did a good job, other artists would hear about it and pay more money for other projects to get done. It paid the bills, but in the end I couldn't reconcile the amount of time I was spending on other peoples' projects when my own projects weren't getting done.In any case, it all reminds me of Jeff Koons, the stock broker turned modern artist who hires people to make giant sculptures, then signs his name to them.Here are a couple of his famous ones:

author: Seekonk
author_rank: Super Contributor
timestamp: December 28, 2015, 02:53:58 pm
content: If thinking 317, then consider those cheap 2596 switcher modules. Pin #5 ON/OFF is normally grounded, this can easily be lifted and pulsed at very high rates. Add an additional cap to the output.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 28, 2015, 03:04:10 pm
content: Quote from: rx8pilot on December 27, 2015, 05:54:36 pmI also know a few that take pride in learning only just enough to do the job, usually badly. Some nonsense about not diluting the integirity of their art or something I guess. Muppets.