Author Topic: Constant Current LED Driver - BJT  (Read 3288 times)

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Offline bullonwheelsTopic starter

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Constant Current LED Driver - BJT
« on: July 11, 2019, 02:05:45 am »
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 = 2.4V
VG = 4.2V
D1 would have around 0.65V drop similar to VBE across drop.
 

Offline FenTiger

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Re: Constant Current LED Driver - BJT
« Reply #1 on: July 11, 2019, 02:30:53 am »
That's a lot of parts per LED. How many are you driving? Would you be better off putting them into longer series strings?
 

Offline T3sl4co1l

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Re: Constant Current LED Driver - BJT
« Reply #2 on: July 11, 2019, 02:32:17 am »
What's R2 for?

Tim
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Offline OM222O

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Re: Constant Current LED Driver - BJT
« Reply #3 on: July 11, 2019, 07:32:35 am »
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.
 

Offline Zero999

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Re: Constant Current LED Driver - BJT
« Reply #4 on: July 11, 2019, 08:18:54 am »
What's R2 for?

Tim
To reduce the power being dissipated in the BJT maybe?
 

Offline bullonwheelsTopic starter

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Re: Constant Current LED Driver - BJT
« Reply #5 on: July 12, 2019, 05:44:22 am »
R2 is LED drive resistor.

I saw the constant current IC's are more expensive then the BTJ version one.
 

Offline Mechatrommer

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Re: Constant Current LED Driver - BJT
« Reply #6 on: July 12, 2019, 06:04:09 am »
What's R2 for?
Tim
To reduce the power being dissipated in the BJT maybe?
with 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.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline T3sl4co1l

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Re: Constant Current LED Driver - BJT
« Reply #7 on: July 12, 2019, 08:14:31 am »
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
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Online ali_asadzadeh

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Re: Constant Current LED Driver - BJT
« Reply #8 on: July 12, 2019, 08:33:07 am »
Look at this tread, I think it's the cheapest option! ;) as a bones it can be run from mains too :D
https://www.eevblog.com/forum/projects/do-you-recognize-this-part/
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I'm a Digital Expert from 8-bits to 64-bits
 

Offline Mechatrommer

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Re: Constant Current LED Driver - BJT
« Reply #9 on: July 12, 2019, 10:23:57 am »
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.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline zitt

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Re: Constant Current LED Driver - BJT
« Reply #10 on: July 12, 2019, 10:19:22 pm »
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
 

Offline Benta

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Re: Constant Current LED Driver - BJT
« Reply #11 on: July 13, 2019, 06:31:02 pm »
Remove R2, replace R1 with a second 1N4148. You'll have ~0.65 V over R3.

 

Offline mariush

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Re: Constant Current LED Driver - BJT
« Reply #12 on: July 13, 2019, 06:46:42 pm »
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* )

 

Offline Benta

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Re: Constant Current LED Driver - BJT
« Reply #13 on: July 13, 2019, 07:03:43 pm »
Well, the OP's costs around 5 cents...
 

Offline mariush

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Re: Constant Current LED Driver - BJT
« Reply #14 on: July 13, 2019, 08:08:18 pm »
Well, the OP's costs around 5 cents...
And 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?
 

Offline strawberry

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Re: Constant Current LED Driver - BJT
« Reply #15 on: July 13, 2019, 09:03:19 pm »
from infineon BCR405U all in one BJT constant current source for LED
 

Offline Benta

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Re: Constant Current LED Driver - BJT
« Reply #16 on: July 14, 2019, 07:14:29 am »
... how much time to bin the transistors for same hFe / other factors?

Why on earth would you want to do that? Completely unnecessary.

 


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