Constant Current LED Driver - BJT - Page 1
Forum section: electronics / projects

author: bullonwheels
author_rank: Contributor
timestamp: July 11, 2019, 02:05:45 am
content: Hello All,I am designing a constant current LED driver for driving multiple LED's each having the following circuit:Any issues I would face or any changes required?VG1 = 2.4VVG = 4.2VD1 would have around 0.65V drop similar to VBE across drop.

author: FenTiger
author_rank: Regular Contributor
timestamp: July 11, 2019, 02:30:53 am
content: That's a lot of parts per LED. How many are you driving? Would you be better off putting them into longer series strings?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2019, 02:32:17 am
content: What's R2 for?Tim

author: OM222O
author_rank: Frequent Contributor
timestamp: July 11, 2019, 07:32:35 am
content: how about using an op amp + a BJT / mosfet in order to do that? you just need a high voltage supply.you can also use constant current LED driver ICs (both linear and switching) which usually ends up being cheaper actually.

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2019, 08:18:54 am
content: Quote from: T3sl4co1l on July 11, 2019, 02:32:17 amTo reduce the power being dissipated in the BJT maybe?

author: bullonwheels
author_rank: Contributor
timestamp: July 12, 2019, 05:44:22 am
content: R2 is LED drive resistor.I saw the constant current IC's are more expensive then the BTJ version one.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 12, 2019, 06:04:09 am
content: Quote from: Zero999 on July 11, 2019, 08:18:54 amwith R2 470 ohm, the max current is 6mA through (Vled 2V give or take) worst case bjt shorted is 5 x 6mA = 30mW (in case nothing existed but the bjt), assuming with the load balancing the bjt is essentially 0W, but is it worth it? 0.03W dissipation? and then there's R3 to take some of the heat. VG2 worths few more components. for a single led? i think we are not seeing the big picture here.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2019, 08:14:31 am
content: The transistor is set for ~5.2mA, so it's rather peculiar. I suspect this is overcomplicated, or indeed the "big picture" is not nearly explained here.Tim

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: July 12, 2019, 08:33:07 am
content: Look at this tread, I think it's the cheapest option!as a bones it can be run from mains toohttps://www.eevblog.com/forum/projects/do-you-recognize-this-part/

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 12, 2019, 10:23:57 am
content: there is also IC WS3441A driving many strings of led worth of standard 3' flourescent tube size. but you can only get that with mains potential.

author: zitt
author_rank: Regular Contributor
timestamp: July 12, 2019, 10:19:22 pm
content: I usually use one LED + Resistor + BJT in diode mode to set the LED current... then use only a BJT to drive the remaining LEDs.IF there is enough supply voltage on the BJT side; I use two or more LEDs in series to the collector and attach directly to V+.That said; I do tend to put a current limiting resistor on the final BJTs just to limit the Power dissipation in that BJT.http://pinball-mods.com/blogs/?p=452

author: Benta
author_rank: Super Contributor
timestamp: July 13, 2019, 06:31:02 pm
content: Remove R2, replace R1 with a second 1N4148. You'll have ~0.65 V over R3.

author: mariush
author_rank: Super Contributor
timestamp: July 13, 2019, 06:46:42 pm
content: My question is why, just to learn or for an actual application?There's loads of simple led drivers out there, from super simple to very advanced ones.Here's one of the simplest, NSI45015WT1G:www.farnell.com/datasheets/1878366.pdf(others with current preset at various values:https://uk.farnell.com/search?st=NSI* )

author: Benta
author_rank: Super Contributor
timestamp: July 13, 2019, 07:03:43 pm
content: Well, the OP's costs around 5 cents...

author: mariush
author_rank: Super Contributor
timestamp: July 13, 2019, 08:08:18 pm
content: Quote from: Benta on July 13, 2019, 07:03:43 pmAnd how much in PCB real estate and pick and place machine time or human time to solder the parts... how much time to bin the transistors for same hFe / other factors?

author: strawberry
author_rank: Super Contributor
timestamp: July 13, 2019, 09:03:19 pm
content: from infineon BCR405U all in one BJT constant current source for LED

author: Benta
author_rank: Super Contributor
timestamp: July 14, 2019, 07:14:29 am
content: Quote from: mariush on July 13, 2019, 08:08:18 pmWhy on earth would you want to do that? Completely unnecessary.