Control High power LEDs for my project - Page 1
Forum section: electronics / projects

author: djlorenz
author_rank: Contributor
timestamp: July 09, 2013, 01:45:33 pm
content: Hi guys!wanna to ask you some infos about led control.i bought 10x COB led from china: these are the specifications:Type:COB LEDModel Number:KGL-MCT-005Luminous Efficacy:650-700lmPower:7WColor Rendering Index(Ra):>72Color Temperature:W/WW/CWForward Voltage(Vf)21-24VForward Current(IF)350mACertificate:CE & RoHSLife:>50000HrChip brand:EpistarBeam Angle:140 degreeOperating Temperature:-30~+90°CStorage Temperature:-40~+100°Ci've tried them with power supply for creating a led lamp but i've already burned one of them...i wanna create a LED Matrix (like this:) and i found a nice DMX driver with 27channels (9xRGB) with 1A output each channel.These are specs:Power input:DC 7V- 24V max 15AOutput: 1A each channel max 15AChannel: 27channel 9 groupController: DMX512 XRL 3PWork temperatire:-20~50i wanna ask you, need i driver between board and led? led works to 21-24V, so i'm thinking to power the board with 24V PSU, but how can i keep the current under 350mA?hope you understand the problem, for any info i'm here!Thank you for your HelpLorenzo

author: KTP
author_rank: Frequent Contributor
timestamp: July 09, 2013, 03:48:50 pm
content: Here is a very crude current limiting circuit that uses 5 common parts. There are tons of ways to do current limiting, but this is certainly something you could build from almost any junkbox.I would guesstimate that as drawn the current would actually end up being about 200mA. The very worst it could be is around 0.8V across R1 which would be 270mA. You might be able to reduce R1 to 2.5 ohms or even 2 ohms if you want to live dangerous and go right near the 350mA max led current.You could get a lot fancier and more robust, but maybe this could get you up and running. I am assuming your DMX drive output would be around 24V, and you would feed that into this limiter circuit. D3 is the actual LED you are trying to drive.

author: djlorenz
author_rank: Contributor
timestamp: July 09, 2013, 04:14:47 pm
content: that's a good start! thanks!so should i create a board like this with 25 in and 25 outs...i wanna show you the device, you can find in different stores around, here is very well explained:http://www.holidaycoro.com/27-Channel-DMX-Controller-for-RGB-Lights-12v-DC-p/24.htmthe thing i'm thinking is: maybe this transistor on the board are transistors for limiting current?the thing i cant understand is if the input is 7-24V the output should be PWM of 7-24V or it's standard (maybe 12V) output... i cant see this information anywere

author: KTP
author_rank: Frequent Contributor
timestamp: July 09, 2013, 04:46:49 pm
content: Where is the data sheet on the device? The schematic?The website just says it can output 2 amps per channel, 15 amps maximum...no mention if this is sourcing or sinking current, no mention of output voltage.I would *guess* the 27 devices are high side switches, probably p-channel mosfets, each capable of over 2 amps but the gate driver chip for them has a maximum package power dissipation that would limit the overall output of the board to 15 amps. I certainly don't see anything on the board that would regulate the 7 to 24 volt input down to 12V at 15 amps...that would be a helluva linear regulator or it would be a hefty switcher. Based on this I would guess that whatever raw voltage you supply the board is what gets pwm switched by the 27 transistors and is presented to your leds.So long story short it does look like you do need some sort of external current limiting.Sheez...no data sheet? for reals?

author: djlorenz
author_rank: Contributor
timestamp: July 09, 2013, 05:05:28 pm
content: Quote from: KTP on July 09, 2013, 04:46:49 pmyes man... no datasheet, no schematics... nothing...on some website you find only input voltage - max current... stop!that's a crappy chines board but that's the unique i can find useful for me... i spent some time on pic for dmx decoding but i don't have much time now...i found a picture of the bottom:input connector goes directly to the output... so i think yes.. if i put 24V input i will get 27x 24V PWM modulated output....about the current limiting... a simple resistance?

author: KTP
author_rank: Frequent Contributor
timestamp: July 09, 2013, 05:20:40 pm
content: Yes maybe just start with a simple resistor for current limiting. You don't even really know that these are high side switches...could be pwm'ing the low side using n-channel fets.If you know that your leds have a drop in the range of 21V to 24V and you are providing a well regulated 24V supply, then you probably can use something like a 10 ohm 1/2 watt (or better, 1 watt) resistor in series with each led before connecting it to one of the 27 ouputs of the dmx device.at 21V across the leds with a 24V regulated supply, the resistor would limit the current to 3V/10ohm = 300mA. Realistically though the leds probably drop closer to 23V at 300mA which would mean you actually need a 3 to 4 ohm resistor to get 300mA from a 24V regulated supply. The 10 ohm would be a lot safer bet for testing, even though it may limit the current to something lower like 150 to 200mA.If your led voltage drop was unknown, or you didn't have a regulated 24V supply, then the circuit I posted would be better, since it would limit the current to under 350mA for voltages in the range of 0V to well over 40V (transistor would probably smoke at 40V though)Just grab some 1 watt 10 ohm resistors and start having fun!

author: djlorenz
author_rank: Contributor
timestamp: July 09, 2013, 05:33:36 pm
content: Quote from: KTP on July 09, 2013, 05:20:40 pmok man! thank you so much for your help!we will think about that now (if it work we wanna build some of them for our stage) and maybe we can also think about 10W leds... we wanna a bright effect....the idea is to get something like this (maybe you don't care, maybe you should be interested to see that):i'll buy one of this board and try some experiments with my 7w leds i have home...i'll start with 10ohm then i'll will go down... i think they are 350mA with continuous output so for our "strobe" and blink effects i probably can drive them near 300mAi'll post some experiments! cheers!

author: mariush
author_rank: Super Contributor
timestamp: July 09, 2013, 07:37:55 pm
content: If the price is not a big issue, these chips would be excellent:http://uk.farnell.com/diodes-inc/ap8803wtg-7/ic-led-driver-buck-1a-5tsot23/dp/1825359RLWorks with 8-30v, you can set the current using a resistor up to 1A (so you could use one of these for let's say 4 leds so each would get 250mA, and it has a control pin where you can send a pwm signal to adjust the brightness of the output.Doesn't need much else besides a capacitor, an inductor and a shottky diode.

author: David_AVD
author_rank: Super Contributor
timestamp: July 09, 2013, 10:37:25 pm
content: 99% of boards like this use low side switching via N-CH MOSFETS. Looking at the bottom layer confirms that's the case with this 27 channel one.

author: KTP
author_rank: Frequent Contributor
timestamp: July 09, 2013, 11:01:48 pm
content: Quote from: David_AVD on July 09, 2013, 10:37:25 pmI suspect you are correct, in which case the OP should probably start with 10 ohm series current limiting resistors to get up and running with a prototype, then decide how elaborate a current limiting circuit he wants. Could use the one I posted rearranged with a pnp transistor instead of npn...

author: David_AVD
author_rank: Super Contributor
timestamp: July 10, 2013, 12:07:50 am
content: No need to rearrange the current limiter. It makes no difference whether the controller uses high or low side switching.

author: KTP
author_rank: Frequent Contributor
timestamp: July 10, 2013, 12:27:24 am
content: Quote from: David_AVD on July 10, 2013, 12:07:50 amCorrect. Brain fart...I was puzzling over propane connections in my rv build when I wrote that...honest!You would just need to connect the pwm lines to where ground is drawn and the 24V positive rail to where it says dmx drive

author: djlorenz
author_rank: Contributor
timestamp: July 10, 2013, 01:52:15 pm
content: you are so kind, so i ask another qestioni found another kind of led that should be nice, 18V - 600mA max, that' perfect for my fit: that's exactly 15A for 25 diodes (max board support)i was thinking to still buy a 24V 350W Switching PSU that's has Vadj potentiometer,can this kind of PSU work at lower voltage? about 18-20V? so i need less power Resistor on single output... or it can be dangerous?thanks

author: KTP
author_rank: Frequent Contributor
timestamp: July 10, 2013, 02:01:56 pm
content: Quote from: djlorenz on July 10, 2013, 01:52:15 pmVery hard to know without data sheet/model number. I have some powerone linear 24V supplies that can be adjusted down to 18V.

author: David_AVD
author_rank: Super Contributor
timestamp: July 10, 2013, 11:05:13 pm
content: Using a fixed voltage and low value resistor is not a good way to go. The Vf of the LED will vary significantly (with temperature and LED batches) compared to the resistor voltage drop. This can lead to thermal runaway and destruction of the LED. Constant current is pretty much essential with higher powered LEDs.

author: djlorenz
author_rank: Contributor
timestamp: July 11, 2013, 07:23:54 am
content: Quote from: David_AVD on July 10, 2013, 11:05:13 pmnice to know, i've tested one of my 7W yesterday, used with 2x9ohm-1W in parallel @24v it seems to be quite stable at 320mA, 1V voltage drop to the resistor, 23V to the led...but if you say this i will think about... i'll probably create a little board with 25 drivers, or there are some drivers with multiple output?

author: David_AVD
author_rank: Super Contributor
timestamp: July 11, 2013, 07:57:05 am
content: I don't think you'll find a ready rolled solution.

author: KTP
author_rank: Frequent Contributor
timestamp: July 11, 2013, 01:10:58 pm
content: See, here is the problem and what David pointed out. You decided to go close to the max current so you had to use 4.5 ohms of resistance with only 1V drop (I suspect some problem in your measurement since 1v across 4.5 ohms would be 220mA)If the voltage drop across the leds change as they heat up (it does) then you could have a swing of +/-500mV or so across your resistor, meaning your current draw might land anywhere in the range of 110mA to 330mA in steady thermal state. If a certain LED is out of spec, it could be even worse.

author: djlorenz
author_rank: Contributor
timestamp: July 11, 2013, 01:14:18 pm
content: understood,i'll think about it... i'm waiting for a response from a colleague that wants this led panels.... when he will confirm i'll start developmentthank you so muchLorenzo