I started with a crude prototype bodged together on a piece of copper clad FR4. It could probably have been done easily enough on a bread board but I've got used to jumping straight to solder.

Nothing fancy, a 100mm coil of 0.22mm diameter copper wound on 6mm silicone fuel hose, a TL431 and some passives.

It's a bit of a mess but it works! The dip package in the corner near the coil is a previous project.

 Calculations suggested a 1mV level for a few hundred amps. Add a bit of amplification afterwards and we'll have a useful probe.

First prototype didn't have any amplification so we'll need a decent current to get a useful signal.

Next problem... Finding a high current pulsed or repetitive source... 

Maplins to the rescue! I've got a terrible cheap digital power supply from Maplins that has slow short circuit protection and a decent output capacitance. If you short circuit the output and turn it on it gives a reliable, repeatable 5-15ms high current pulse depending on the output range.

A LEM hall effect sensor confirmed around 12A peak.

Result around 20mV for 12A! Roughly 450A/V. Strong signal, clearly something wrong with the maths but a good start and something to build on.