Well, I'll assume you know how a transformer works in general.

There's the primary, the secondary, and the core, which is

magnetic material. If you use a toroid - there's an example

of a fairly large one here -

http://www.alltronics.com/transformers.htm
The toroid on top,

The windings are the secondary, the toroid core is the, well,

core, and the primary consists of one "turn" of wire, which

is the lead passed through the center hole of the toroid.

(not shown on that page above) For the formulas, it acts

exactly like one turn. So if you've got, say, 100 turns of

wire for the secondary, and the one "turn" (it actually

doesn't do any turning - if you did, and looped it through

the hole twice, that'd be two turns.) for the primary. And

the voltage ratio in a transformer is the turns ratio, and

the current ratio is the inverse of the turns ratio. So

with 30 A through a 100-turn-secondary current transformer

would give you a current of 30/100 A, or 30 mA. Stick a 100

ohm resistor across the secondary leads, and it will develop

100 * .03 = 3 volts. A 1K ohm would develop 30 volts and so on.

So in that case, you'd unbolt your alternator output lead,

thread it through the toroid, and bolt it back into place.

And you can probably find a suitable toroid at surplus

somewhere.

Good Luck!

Rich

Here's just one sampling of |"current transformer" schematic|

on google:

http://www.alltronics.com/transformers.htm