How to build a constant current source?

[thomson.eric@gmail.com]

I have been frustrated for months working through an introductory
circuits book, as I want to build many of the circuits, but many of
them include constant current sources. Why is it so easy to find
voltage sources at your local store (i.e., batteries!), but searching
for current sources on the web leads to a complicated bunch of circuit
diagrams?

Here is my naive question: using thevenin-norton equivalent circuits,
couldn't I transform a battery (i.e., voltage source) into the desired
equivalent current source using Vth=InorReq. That is, can I put a
voltage source in series with a resistor (as opposed to its equivalent,
a current source in parallel with the same resistor)? What is the
problem with doing that?

Does anyone know where I could buy a cheap but reliable current source?
Why is this so hard?

 

[thomson.eric@gmail.com]

Wow, you have all been very helpful. I will print these suggestions out
and try to figure out which would be best. As far as specifics, I would
be happy with something that could generate a single DC current of
~100mA. In the best of possible worlds, I would love something that
would let me, through a turnpot, vary the DC current between 0 and 1
Amp. I don't want anything fancy, just something so that I can play
with my breadboard while working through an electronics book.

I guess we are lucky to have batteries. I wonder why no companies sell
a cheap current source that takes AC in or even uses batteries to power
it. They could probably make a KILLING with all the EE departments
around the country! Maybe I'll make one and provide the supply!

 

[thomson.eric@gmail.com]

Have you ever gone through basic electronics text book? They are full
of hypothetical circuits that contain voltage sources and current
sources, to demonstrate the analytical principles. If you are an
experimentalist, you need both sources to verify that they are not
lying to you.

Exactly, my main motivation. Secondarily, I am a neuroscientist. We
inject current into neurons all the time, and I want to stop treating
the stimulator as a black box!

 

[kell]

I am a neuroscientist. We
inject current into neurons all the time

Whose neurons?
Hey has anybody seen my foil hat!!?