Current Sink for LEDs

[nickagian1]

Hi everybody!

I want to design a project with some LEDs, something like a signboard and I need to use a current sink, which can drive up to approximately 500-600mA. I have thought of using a NPN transistor (for example 2N2222),  but I do not know what the exact circuit should look like. I have simply connected the E to the negative supply (ground), the C to the negative lead of the LED (of course the + of the LED is connected through a resistor to the positive supply) and the B to ground, but it doesn`t work at all!

Can anybody help me? Perhaps is there another solution, apart from using a transistor? (an IC maybe?)

 

[ante1]

Hi Nick,

Why not use the LM317 or LM350?

 

[nickagian1]

Hi Nick,

Why not use the LM317 or LM350?

Well... I haven`t understand what you mean Ante... :-[ I`m sorry...

How can the LM317 be used as a current sink?

Actually I mean something like the following circuit (except that instead of 1 I want to use more LEDs), where the ON or OFF state of the LED can be changed by the voltage applied at the base of the transistor.

But I obviously make a mistake because it simply doesn`t work at all!View attachment 39715

 

[ante1]

OK Nick, I did misunderstand your question!  :-[  I thought you needed regulated (limited) current for the LED’s! I can see now you are using a resistor for this.
So this circuit does not work? What is trigging the base of the transistor?

 

[nickagian1]

OK Nick, I did misunderstand your question!

It`s ok! Perhaps I didn`t express myself very clearly and I should have posted that picture too!

Well I simply triggered it by connecting it to the ground or to the positive supply (later at my real circuit this will be connected to the output port of a PIC, but I guess it is the same thing!)

I don`t know, I may have made a mistake... I will check it once more!

 

[nickagian1]

Thanks a lot Zeppelin! That resistor must be my mistake (I had thought of it, but couldn`t find the proper value). The guide is very helpful on that too!

With a 5V supply you can't drive that many LEDs, tops 3, I guess-you need a higher supply voltage.

I know that, I intend to use only two LEDs on series and organize all of them on doubles!

 

[ante1]

I believe it would work more reliable with only one LED in series with each transistor since the voltage is low.

 

[audioguru2]

Yeah, with only 5V for the supply, two 3.5V blue or white LEDs in series wouldn't be very bright. ;D

My red LEDs are only 1.8V and they would be fine with two in series.

 

[nickagian1]

Well I`ll use red LEDs, which -if I am not wrong- are 2.3V and I have tested two of them in series with a resistor of 47R. The current drawn was approximately 18mA and the brightness quite good! But I don`t doubt that alone it must have been much brighter!

 

[audioguru2]

If you connect a single 2.3V LED in series with a 47 ohm resistor and across a 5V supply then the current will be 57.4mA and the LED (rated max is 30mA) will burn out. It will be very bright for a few milli-seconds. The resistor's value must be recalculated for just one.

 

[nickagian1]

Yes of course! ;D  I didn`t mean that I have the intention to use a single LED with a 47 ohm resistor!! Thanks a lot anyway!