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Help making constant current source

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I have tried over and over again but nothing works. What I want to do is make a constant current source for my bi-color LEDs. The problem that I'm having is that the LEDs are all common anode, the only help google has given is a common cathode current source.

Can someone please help me with my rather anoying problem?
 
I don't have a schematic yet, nothing worked.

hi DT,
Couple of options using transistors.
Fig #1 is the best option, Fig #2 requires the +V supply to be regulated.
 

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Thanks Eric, this is what I made that works:

**broken link removed**

I tried every single possible combination of transistor arrays to get the common anode constant current source, but with no luck.
 
The circuit should work if you connected the leftmost resistor to +Vcc.
 
I use an opamp to drive a Mosfet via a current sense resistor, the current is set via 10 turn POT. You can find the circuit here on the forums it's been posted in at least two threads including a full spice schematic.
 
The combination you apparently didn't try was to convert the NPNs to PNPs. You can then add an NPN to control it with a ground referenced signal which gives a constant-current source that can drive grounded cathode LEDs. See figure.

CC Source.GIF
 
Thanks Eric, this is what I made that works:

**broken link removed**

I tried every single possible combination of transistor arrays to get the common anode constant current source, but with no luck.
This is a current sink, which is what you need for common anode LEDs. You need to connect one to each cathode.

Why would you want to drive the all the anodes of a common anode unit with a single current source?
 
The combination you apparently didn't try was to convert the NPNs to PNPs. You can then add an NPN to control it with a ground referenced signal which gives a constant-current source that can drive grounded cathode LEDs. See figure.

I did try to convert the NPN to PNP transistors, I now know that the mistake that I made was that I hadn't tried to use 3 transistors (like you used) I just tried to get it to work with only 2.

This is a current sink, which is what you need for common anode LEDs. You need to connect one to each cathode.

Why would you want to drive the all the anodes of a common anode unit with a single current source?

I was doing my best to avoid making the exact same circuit for every single cathode of the LED, that's why I was looking for the common anode CC source.

Sceadwain, was your circuit an oscilator? That's the only possible way that I can think of where something like that would work.
 
hi DT,
I guess you know this version already, LM317L [TO92 100ma].
 

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hi DT,
I guess you know this version already, LM317L [TO92 100ma].

No I didn't, thank you for that schematic.

Although I have been making some of these and testing them (using a smimple variable vreg) the LEDs are still noticably dimming...

Is there something that I'm doing wrong?
 
No I didn't, thank you for that schematic.

Although I have been making some of these and testing them (using a smimple variable vreg) the LEDs are still noticably dimming...

Is there something that I'm doing wrong?
Post a schematic of what you are doing, including the LEDs.
 
This is the schematic that I am working on right now, I think that the mistake that I made is that I didn't remover R5 from the circuit. I don't know if that's the problem or not, I'll try removing it the first chance I get.

When I was simulating a "dying" battery, it was just a 3904 held in a simple Pot voltage divider. (emitter follower circuit)
 

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I was doing my best to avoid making the exact same circuit for every single cathode of the LED, that's why I was looking for the common anode CC source.
You can't avoid that. Each series string of LEDs needs it's own dedicated current limiter (resistor or constant-current source).
 
Crutshow, the circuit that you posted seems to work the best, which is weird because I had already tried that same configuration of LEDs but with no luck.

The LEDs seem to start loosing their brightness around 3V, but since the battery is cutt off at 6V then there is nothing to worry about.
 
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