K, I'll leave it in there for now, with the two stages, one can basically run a number of combinations or just put a simple solder bridge in there. Same for the auto-zero, I'll leave it in there as an option
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/xsmiley.gif.pagespeed.ic.R8GFI-pF6f.png)
Since no other complaints came up, I would call the input conditioning done for now
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
except for:
Let's talk CAT-Protection and over-voltage protection (not overrange, thats already done
![Tongue :P](https://www.eevblog.com/forum/Smileys/default/xtongue.gif.pagespeed.ic.J5mTe0A2NA.png)
) to finish the voltage input section
![Cool 8)](https://www.eevblog.com/forum/Smileys/default/xcool.gif.pagespeed.ic.cOxV6sPNRd.png)
I'd love to have CAT-II (for measuring mains stuff), which would be rated 1kV (withstanding 6kV input impulses if I remember it correctly ^^ ). I don't have access to a 61010-1...but I could certainly use the DO-160 Class 5 as an equivalent
![Cheesy :D](https://www.eevblog.com/forum/Smileys/default/xcheesy.gif.pagespeed.ic.KPzuH4J-TG.png)
(just kidding). Maybe someone could post a quick wrap-up what the unit needs to survive in terms of energy and pulse duration....
Littlefuse offers some nice gas-discarge tubes wich might do the job, with low capacitance and certainly no other leakages during normal operation:
http://www.littelfuse.com/products/gas-discharge-tubes/high-voltage-gdt/cg3.aspx.
My fist try would be to put a high wattage resistor after the GDT to absorb the energy once it arcs over, together with a fuse in series with the resistor after the tube, to stop the "magic-smoke-and-fire generator" if it fails (obviously sacrificing the unit, but hey, you just managed to blow the protection
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/xsmiley.gif.pagespeed.ic.R8GFI-pF6f.png)
). I really wouldn't like to see a PTC in a "precision" instrument (all that temperature compensation stuff, and a PTC in the input path?!). But maybe one could use one in series with the high-wattage R instead of the fuse to cool down the current in a constant overvoltage condition, which might be a better idea after all instead of the fuse. I've actually never used a GDT before...so, what do you think? Any chance this arrangement could do the trick? Do we need some additional "constant high energy overvoltage, eg. plug the damn thing in your 5kV generator" protection?
So long,
Lukas
Edit:
![Face Palm :palm:](https://www.eevblog.com/forum/Smileys/default/xfacepalm.gif.pagespeed.ic.EBDwh1hCfo.png)
I guess I wrote some BS up there regarding the idea for the protection....basically, It wouldn't do anything to protect the unit...I'll leave it in the thread as a bad example:D