🚀 go-pugleaf

I built a HV probe for my scope as follows:

	10  	10 megohm 1/2 watt resistors 1% 
	1	1" diameter PVC pipe 18" long
	2	PVC end caps for above pipe
	1	small nail for the probe tip
	2	small machine screws about 1.5" long
	2	nuts for the screws
	2	blocks of paraffin wax to fill the tube

The idea is to solder the 10 resistors in series.  One end of the chain
should be soldered to the nail (probe end), the other to one of the machine
screws around the head (ground end).  Tap off a lead between the first and
second resistor (nearest the ground end) and solder it to the other machine
screw head (measurement point).  Drill a single hole in one end cap for the
nail, drill two holes side-by side in the other cap for the two machine
screws.  Put the nail through the end cap and epoxy into place from the 
inside.  String the resistors through the PVC pipe and secure the cap with
the nail on the pipe.  Melt the paraffin and fill the tube, providing extra
dielectric strength to prevent arc-around the resistors and through the pipe
to your hand.  Finally, install the 2 machine screws in the other end cap
and secure onto pipe.  Be sure to mark which screw is ground and which is
"hot".

This makes a simple 10:1 HV prope assuming your scope has 100 Megohm
input impedence.  Mine has 1 Megohm input impedence, making this a 100:1
(approximately) probe when used with the scope.  Current draw is under
a 120 micro amps when measuring 10KV.  Commercial probes from Beckman
have similar specs (and use the same passive divder circuit, as far as I
can tell), yet cost $100.  This one will cost less than $10.

Be very careful with the HV.  Always keep one hand behind your back and
wear rubber soled shoes.  Needless to say, one mistake can be your last.

Jim Christy
HP Labs
jchristy@hplren2.hpl.hp.com
Palo Alto, Ca.