Adjustable Constant Current Source

[graemec]

I've been trying to design an adjustable constant current source.

I need to drive a range of currents (150mA -300mA) into an experimental LED. I am limited to a supply voltage of +/- 5v and if I could would like to avoid using expensive components like voltage regulators.

G

 

[Nigel Goodwin]

You could use a simple wirewound variable resistor - constant current isn't a problem because you have a constant voltage.

 

[graemec]

Sorry I should have added I am not in control of the supply voltage. Want to design a circuit that delivers a constant current independent of the voltage(within reason obviously)

g

 

[k7elp60]

The easiest way would be to use a LM317T voltage regulator. A potentiometer between the the output terminal and the adjust terminal. The output would be from the adjust terminal. How simple can you get, two parts.

 

[Hero999]

5V is too low to use the LM317 if the load is >3V, the dropout is 1.5V at 200mA (plus a bit more at 300mA) + 1.2V.

What's the maximum voltage of the LED?

I take it 5V is the minimum supply voltage, right?

This might work.

Rb needs to be low enough so Tr1 will have enough gain.

Rs = 0.6 / I

 

[graemec]

No 5v is the maximum supply

 

[Hero999]

That's even worse then, what's the minimum supply voltage?

You didn't give a value of the maximum load voltage though, what's the maximum Vf for you LED?

 

[graemec]

Hero999

Your circuit is pretty close to something I came up with just about The problem is with me requring such a relativly high current from such a relatively low supply Im forced to use resistors in the 1-100ohm range which when you bring the issues of tolerance in I dont have much control over the output.

Someone had mentioned to me about using a buffer amplifier??

I dont know what the max load voltage is. its expermimental LED and its not even been deliverd yet!

g

 

[Hero999]

Use an LM317 constant current source and a much higher supply voltage, I strongly recommend building one with a 12V mains transfromer, a bridge rectifier and a large (>4700uF) filter capacitor.

 

[Styx]

you could use an OPAMP as a current-source

**broken link removed**

 

[graemec]

I cant Hero its gonna end up in the same package as an ASIC. 5v is all im gonna get!

Styx. Can u elaborate on how to use an OPAMP. Wont I have the same problems with having to use small resitors.

Thanks for the prompt replys guys.

g

 

[Hero999]

That would be fine but I thought he couldn't control the supply voltage and that depends on the supply being 5V.

 

[graemec]

Could I build a number of your circuits hero and feed each of there outputs into the LED? Im sure that its not as simple as that! But my gut feeling is that what im looking for is something pretty close to what you described.
g

 

[hjames]

Did you say ASIC? If you are trying to build an IC circuit, the rules are all different... current mirrors and external current references are the name of the game.

 

[graemec]

the analog portion will not be an asic but it will housed in the same package

g