Re: How to obtain a constant current source?

A constant current source is usually created by 
monitoring the voltage drop across a resistor 
and keeping that voltage constant.  You can do 
this with OP amps.  You can also obtain 3 
terminal parts called current mirrors.  The 
current through one input of the current 
mirror is a multiple of the current through 
the other side.

Quite a bit of the decision to design a particular 
constant current source is based on the amount 
of current you want to deliver.  I can answer 
more fully if you give me more details.

Gus