Differential voltage probes

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi All,

Finn and I have been thinking about differential voltage probes that would 
be useful for solid state Tesla coil work.  Rather than spend $$$$ on 
commercial ones, say from Tek, we were wondering about just making our own.

Since SS coils don't need high frequency response or high input impedance, 
I was thinking of something very simple and cheap to make like this:

http://hot-streamer-dot-com/temp/DiffProbe03.gif

I think it could do about

1MHz+ Bandwidth
2000 V across, 1000V off ground with plenty of safety margin (~10kV)
1000:1 ratio
1Meg ohm input Z
"stereo" for dual channel scopes.
Super easy to make (though hole and like $15 of parts)
One could simply use standard probes to hook to the output eliminating 
compensation worries.
Should be very safe.

It certainly would not be a 100MHz Tek P5202 or anything, but far better 
than nothing and should do very well for many SS coil uses.

I was thinking carbon film resistors would be reasonably trouble free.  The 
TL082ACP should have low enough offset that you will not notice it.  A 
metal case could be independently grounded from the electronics ground and 
the transorbs on the output could provide additional protection if 
something went wrong.  There are faster ICs too??  But some fast ICs have 
high input currents or get goofy in hand built stuff.  I thought the 
TL082ACP was easy to get and well behaved.  The standard TL082C could be 
used too.  The voltage are less than 2V so the gain is good for 1MHz+.  It 
would be very simple and limited in many ways, but far better than many 
alternatives for such low cost.

Any thoughts or suggestions are very welcome!  We just want to keep it very 
simple and easy to make but still real safe.  I do have a P5202 probe to 
test it against to be sure it does what it should and HV power supplies to 
"pop" it for hi-pot testing.

Cheers,

	Terry
	terrellf-at-qwest-dot-net