Re: differential probes

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Crow Leader by way of Terry Fritz <teslalist-at-qwest-dot-net>" <tesla-at-lists.symmetric-dot-net>

On Sun, 13 Jul 2003, Tesla list wrote:

 > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
 >
 > Hi Jimmy, Finn, and All,
 >
 > This is the present schematic and parts list I have in mind for the
 > differential probe.  I have NOT assembled or tested it yet!
 >
 > http://hot-streamer-dot-com/temp/DiffProbe-5.gif
 >
 > http://hot-streamer-dot-com/temp/DiffProbe-5p.gif
 >
 > It is still just using the basic 150MHz TI  THS4151ID differential
 > amplifier IC and other straight forward parts.
 >
 > One thing I am "trying" to add is a front end that can take a hard hit.  I
 > will have a system ground stud and MOVs at 1800 volts each.  These will be
 > fused with low current but high voltage fuses.  The idea is if you get a
 > say 20,000 volt cap on the input, the MOVs will clamp it and the fuses will

How about some neon lamps or one of those glass or ceramic tubes the size
of a pencil eraser with metal leads? Both should have a low capacitance,
and don't conduct until 70 volts or so for neon, and several thousand for
the larger ones. Maybe put them both in parallel. You probably want the
lowest possible impedance in a short mode, otherwise the probe or what's
left of it will dissipate lots of heat, which leads to it blowing up or
catching on fire. Large energy storage caps are designed this way- to
short out with the lowest impedance so their energy is dissipated outside
the cap tank in bussbar and fuses, not inside where the tank will build up
pressure and rupture. MOVs are basically firestarting kits.

KEN

 > break the circuit away without damaging anything like the scope.  Since we
 > play with big voltage stuff, I think super protection like this is a real
 > good idea to have.
 >
 > So that's were I am at the moment on this...
 >
 > Cheers,
 >
 > 	Terry
 >
 >
 >