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?

Voltage sources produce a fairly constant output voltage for all loads

from infinite resistance (zero current) to some minimum resistance

(more than zero current), but they are only approximate voltage

sources, because their output voltage does sag and current passes

through them. This is because they have some internal resistance.

Practical current sources hold a very nearly constant current for all

loads from zero ohms (zero voltage drop) to some maximum value of

resistance (some maximum voltage drop called the compliance voltage).

Ideal current sources would produce a constant current, regardless of

the load resistance, including infinite resistance, by producing a

compliance voltage up to infinity volts.

It is a lucky accident that we have discovered chemical cells that

produce something close to a voltage source over useful ranges of load

current. There is no similar simple power sources that provide a

constant current output.

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?

The problem is that it is not a current source, but a current source

in parallel with a fairly low value of resistance. Mathematically

transforming a voltage source that has a little series resistance to

the equivalent current source with parallel resistance does not

actually alter the characteristics of the source, at all.

Does anyone know where I could buy a cheap but reliable current source?

Why is this so hard?

Almost all practical current sources are actually active variable

resistances that waste all the extra voltage from a voltage supply

that is not needed to force the specified current through the load.

This active circuit has to be designed to pass the required current,

while measuring it, in some way, and using some sort of feedback, vary

an active resistance as needed to hold the measured value of current,

constant, till the load uses up all the available voltage. These

active current regulators are specified with their effective parallel

resistance (equivalent to a perfect current source in parallel with a

resistor), their voltage compliance, their frequency response, and

sometimes how long it takes for them to recover current regulation

after being exposed to an excessively high load resistance and then

the load falls to within the normal operating range.

Name your specifications, and people, here will help you design

current sources (regulators) that meet your needs.