SFE has a OpAmp breakout board with gain 100:http://www.sparkfun.com/products/9816For anode ( high voltage side ) current measurements:http://www.sparkfun.com/products/8883With maximum gain 47 you can cover 0 - 100 mA range
Unfortunately the latter device has ground plane too close to the current-sense traces to be safe at 500V - elementary error in PCB layout! It might flash-over at that high a potential despite the actual chip being rated for 2.1kV...
The current path is through a sheet of copper, 25A isn't that hard to handle (it will be getting hot though, but the same copper conducts the heat to the PCB traces. These high current devices are just a sheet of copper next to a hall-sensor, simplicity itself... Of course it will act as a fuse at higher currents (an expensive fuse!)
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