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I would like to drive a lot of LEDs with the same current, so I decided to use a current regulator.

I am not sure which circuit is better for uniform current in all branches.

Should I use two separate R resistors or one common R/2 resistor?

As an example, I would have 4 current regulators with 12 leds together.

enter image description here

TonyM
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Daniel Šebík
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  • What's the min and max forward voltage of your LED and how closely do you need the currents to match each other? – winny Sep 23 '21 at 14:43
  • Its purpose is emmiting part of line follower. So it would be fine to be equal, maybe it is more important to have stable and repeatable currents. I wanted to know if one common resistor is ok for this purpose and I will not overlook any mistake with that. – Daniel Šebík Sep 23 '21 at 15:03
  • And max is 1.5V and min 1.0V, and I have 3 LEDs in a serie. Supply voltage is made of two lipos so 6 - 8,4V. – Daniel Šebík Sep 23 '21 at 15:08
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    I think those resistors need to be separate, but you could almost certainly share the pair of reference diodes connected to the base of the transistors. The resistor from the diodes to ground would probably need to be decreased proportionally to the number of transistors sharing it. – brhans Sep 23 '21 at 15:22
  • those two diodes and PNP is in a IC as compact current source, so I cannot use just two common diodes :) but I also think that the resistors should be separated. – Daniel Šebík Sep 23 '21 at 15:27
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    I started a simulation for you to simplify your circuit but I need to address the elephant in the room first. How come you need to run several strings in parallel? Why can't you just string all LEDs in series and use one boost converter? – winny Sep 23 '21 at 16:47
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    @DanielŠebík The modern way is to use some boutique IC, as winny suggests, for a boost design that adjusts voltage on the basis of a measured current through the LEDs. But you can see a horse-and-carriage way [here](https://electronics.stackexchange.com/a/579727/38098) using BJTs. The current mirror complexity shown there ***is*** required as it is possible that one or more LED strings are broken or disconnected and in that case the mirror BJT for that string would otherwise upset the other mirror strings. – jonk Sep 23 '21 at 18:27
  • @jonk thank you very much for that link, it is interesting to read. As you both have mentioned, boost design could be the option. I had not considered it. I will try to think about that, but I am little bit afraid of added complexity and costs. I will take a look, thank you. – Daniel Šebík Sep 23 '21 at 19:52
  • @DanielŠebík The IC designers have done a good job in producing specialized ICs that are as simple to understand and use as is possible. Perhaps the main thing will be in selecting an inductor if you aren't familiar with that process. (You will need at least one inductor.) After you do some searching, a reasonable next question to ask would be about which of the ICs you found would be more appropriate for your needs (and why.) Once you nail one of them down, another question may be about selecting that inductor. – jonk Sep 23 '21 at 20:08
  • @jonk One more problem here is that I need to switch LEDs on and off at frequency about 20kHz (25us on, 25us off for filtering purposes and ambient light detection), but I hope that can probably be solved by large capacitor. I think I will try the parallel design first because it is easier to implement (as you say computing those inductors and capacitors, it would take quite a lot of time. I am SW engineer not HW), and later I will see how it acts and maybe change circuit if it would not work properly. Thank you for your time. – Daniel Šebík Sep 23 '21 at 20:51
  • @DanielŠebík That circuit I provided allows each string to be independently activated with an I/O pin. So they can be PWM'd. Up to a point. I didn't design them for high speed. But 20 kHz is probably within their range. – jonk Sep 23 '21 at 20:59
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    That _"switch LEDs on and off at frequency about 20kHz (25us on, 25us off"_ is vital information to be edited into the original question. – winny Sep 24 '21 at 09:17

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