Simple current source calculation?

[FriedMule]

I have understood that this current source is both ignoring voltage ripples, if voltage is above X and it is not changing much with temperature.
It has 4 resistors and some unknown BJTs.

Can any of you please tell me how to find out what to use / select for a determined value?

[ledtester]

To a first approximation the voltage at the left voltage divider (and also the base of the PNP) will be the same at the voltage at the emitter or the NPN. So the sinking current will be V/R3 where V is the voltage set by the voltage divider.

But the exact values will depend on the transistors used, the stiffness of the voltage divider and R2.

One issue is that the voltage divider itself is connected to the voltage source which presumably will be fluctuating. It should be replaced by a more stable voltage reference.

[FriedMule]

Thanks for your reply! I like this current source since it, as I understand it, easily can be made to ignore ripples, but I am told it is only usable for a very low current.
I would like a simple current source where I easily can adjust current.

[ledtester]

I would use a TL431. Check page 30 of the datasheet:

https://www.ti.com/lit/ds/symlink/tl431.pdf

and perhaps this web page: http://www.bristolwatch.com/ccs/TL431A3.htm

[FriedMule]

Looks really great, I have looked at page 30 + some more and the link you send. And I love the component!!
But I am not totally sure how to make the best schematic, it looks like it demands a sort of current source and the TL43x is then make it perfect?

[ledtester]

Just follow the schematic on the bristolwatch page... the load goes between "Iout" and GND.

Or watch this video: https://youtu.be/3DT1Ou9Pgag

[FriedMule]

Thank you a lot, I think I can calculate it now! :-)