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audiofan
audiofan vor 51 Minuten
Sorry for my late response. I was out during weekend. Think of following: The design is for a maximum of 10mA per transistor. It is allowed to assume a beta of 50 for small signal transistors at 10mA IC, I think. So IB will be 0.2 mA in linear operation. Worst case is the not connected situation. Here IB = 2.5V/249ohm = 10mA. So worst case demands roundabout 100mA drive from opamp. I perfectly agree that most opamps cannot continously deliver 100mA. This was not in my focus at first place. My focus was on the current sources, where a simple transistor for less tha 10 cents does the job of the expensive LT3092 almost as well. The variable voltage reference was just an "add on", which this circuit allows, almost for free. The obvious solution to this problem is an emitter-follower, i totally agree. But it can be simply jumpers shorting the unused outputs or trasistors with a higher beta (150 and worst case current drops to 30mA) also. As I wrote: TUN. Use whatever you have in your bench drawer. Simply adjust component values accordingly. You can even use PNP transistors supplying current from +BATT instead, if you also derive Vref from there. So this is intended as a quick and cheap approach as I wrote: "But it is not tested so far. It is a proposal and it may require some tweaking." Thank you DwayneR for pointing this out. I did it from scratch and you proposed the right improvement. It is really encouraging that there are people left, who do not only consume schematics and expect them to work right out of the box. klausb
10-fold LED constant current source Variable.
DwayneR
DwayneR vor 1 Tag
Depends on how much current the op-amp can supply.  The op-amps that I used way back then could supply only a small output current.  The emitter-follower fixed that problem.
10-fold LED constant current source Variable.
pascal.tours
pascal.tours vor 1 Tag
Tank you Sebelec :)  
Repellent system for cats - Module anti-chats
Michel Grigaut
Michel Grigaut vor 2 Tagen
Hi Dwane, I don't see this. For me The output of the op-amp (called VRef) does not move as its value only depends on potentiometer P1. It is true however that even when no led are present, all the transistors are forward biased. But when you plug a led Vref does not change (nor VBAtt theorically). Did I messed something? Regards, Michel
10-fold LED constant current source Variable.
Sebelec
Sebelec vor 2 Tagen
Really good project Pascal, quick, cheap and ... usefull. Poor cats!
Repellent system for cats - Module anti-chats