Using Darlington array to drive multiple LEDs - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: September 26, 2014, 03:07:40 pm
content: Hello all,I would like to know whether I can drive multiple LED arrays using a Darlington array IC from TI.As you can see, on the left is my current setup and the way I drive my LEDs, where I get my PWM signal from an NE555, put it through a 1k Resistor to the base of the NPN. My LED string is connected to VCC and the Sense resistor (RSense) limits the current of the transistor. Emitter is then connected to GND.If I run multiple arrays, I need multiple transistors, resistors, etc. So, I started looking for a transistor array to see whether I can do this. I found the ULN200xx series Darlington array from TI.The datasheet:ULN2002AIn the datasheet, they specify that the ULQ2004A should be used for a 6V - 16V CMOS (and I thing the NE555 is exactly that, granted it works from 4.5V - 16V, but hey), and that, if you check the internal schematic, it already has a 10.5k resistor at the base input.From that, I take it that I can then feed my PWM signal directly to the Input pins (1 - 7) without the need of another base resistor.Now, how do I regulate the current that I want from my transistor? Do I need to connect a sense resistor (RSense) to every output pin and LED array? Or can I use the Common pin (Pin 9) and just use one resistor? Will this regulate current for the entire array?On my drawing, Ideal Setup has an RSense for every Output.IDEALLY, of course, would be if I can just use one RSense connected to pin 8 (Comm).Or does the E pin 8 (is that emitter) just get sunk to ground along with Pin 9 Comm ??

author: Marco
author_rank: Super Contributor
timestamp: September 26, 2014, 05:36:25 pm
content: You're not supposed to ground COM, you're shorting everything to ground through it at the momentIt's a freewheeling diode for inductive loads, leave it open or connect it to VCC. If you want to use this IC you will need a current limiting resistor per LED string (because you aren't supposed to parallel BJTs).QuoteThis isn't true, you can just use a single switch (FET or darlington). As for whether the LED strings are well matched enough to use a single shared current limiting resistor, why even take the chance? Resistors are cheap ... burning a LED string is annoying.http://www.ledsmagazine.com/articles/print/volume-6/issue-2/features/led-design-forum-avoiding-thermal-runaway-when-driving-multiple-led-strings-magazine.htmlThey had thermal runaway even with a small per string resistor (with larger resistors and thus a larger voltage drop it can work fine).

author: made2hack
author_rank: Frequent Contributor
timestamp: September 26, 2014, 06:03:20 pm
content: Ok, so to recap, COM (Pin 9) needs to float or connect to VCC.I need 1 Sense resistor for every OUT Pin in order to set the current (that's fine, cost is not the issue, I just wanted to know whether it could be made with even less parts).And E (Pinis connected to GND.Am I missing anything?

author: Marco
author_rank: Super Contributor
timestamp: September 26, 2014, 06:55:30 pm
content: Well, as I said you can just use a single FET or Darlington instead of that IC (or even a BJT, the 555 can source quite a bit of current). The IC only makes sense if you need to selectively turn on the strings.

author: made2hack
author_rank: Frequent Contributor
timestamp: September 26, 2014, 09:19:40 pm
content: I'm not sure what you mean about selectively turning on the strings?I want to have up to 7 strings of LEDs running, as opposed to 7 different BJT's to run them.Actually, 4 strings running @ 100mA, with 8 LED's per string (@ 24V).

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2014, 10:38:20 pm
content: Quote from: trs80 on September 26, 2014, 09:19:40 pmThere is no reason to use a separate transistor for each string. One single larger transistor can switch all of them connected in parrallel if they each have a separate current limiting resistor.

author: made2hack
author_rank: Frequent Contributor
timestamp: September 27, 2014, 09:45:39 am
content: @Dave Hess Are you talking about something like this?What if one of the strips dies out, what happens to the rest? Anything bad?Or does the fact they are all isolated and on their own resistor, not matter? The NPN just spits out 100mA per strip?Thanks@wilfred - Yeah, thanks for that, it does look like what they are doing is quite similar!

author: David Hess
author_rank: Super Contributor
timestamp: September 27, 2014, 10:14:18 am
content: That is exactly what I was thinking about although the other end of the LED strings should go to the positive supply and not ground. If one of the strings goes open, it will not affect the others. If one LED shorts, that string will draw more current. If an entire string shorts, then its resistor will probably go open.LED string drivers which use ballast resistors for each string use the same configuration and they do not have a problem with it.

author: made2hack
author_rank: Frequent Contributor
timestamp: September 27, 2014, 11:26:32 am
content: @Dave Hess, yeah, lol I was in a hurry, it should connect to VCC