Subject: X 100 HV probe Adaptor for DVM


When measuring or setting the HV output from a Geiger Counter, the problem
is that the source is a very high impedance. Trying to measure with most
DVMs or VOMs simply load down the supply and cause erroneous readings.
Using a X100 probe on a VTVM brings the load impedance up above 1000
MegOhms, where it needs to be, however there is the problem of reading the
analog meter accurately.
( See Archive for further details. Many discussions previously on this
subject).

One "perfect" answer is to use a Digital Voltmeter for accuracy of readout,
in conjunction with a
X 100 probe adaptor. All (!>?) the adaptor requires is a precision 990
MehOhm resistor in series with the 10 MegOhms presented by the DVM input.
Easier said than done. Fining such a resistor is difficult to say the
least,
but fortunately a member of the CDV 700 Club ( Thanks Norman!) had a some
precision (2%)
495 MegOhms for sale, and I acquired a pair.

This is the mechanical layout that makes using the resistors convenient..
Each resistor has a screw and washer connection at each end. Removing the
lockwasher and installing a solder-lug with built in lockwasher gave a
place
to solder your connections. At the HV end, a piece of stiff hookup wire
attaches the resistor string to an input BNC connector. In the middle,
where
the two resistors connect, simply solder the two lugs together. Then at
the
meter end, a short piece of flexible hookup wire connects to the output
"pin" jack. A Pin connector is similar to a Banana jack, but smaller in
size, and accepts the probe point on the lead from the DVM.

Input and output connectors are installed on 1/2" PVC plumbing pipe end
caps
as shown. A 15" length if 1/2" PVC plumbing pipe serves as the adaptor's
body. Before inserting the resistor string into the housing, slip a pair of
3/8" (hole) size rubber grommets onto each resistor body. What these do is
center the HV resistors into the middle of the housing, and also provide a
substantial shock mount for same, with a snug fit.

In use just connect a BNC jumper cable * to the PROBE jack on the Geiger
Counter, plug
the DVM probe into the pin jack and take a reading ( don't forget to
connect
the negative lead to the "ground" of the counter)> set the DVM to the 20
Volt scale, and multiply any reading seen by 100.
i.e. 10.01= 1001 V.




* An option is to have a regular probe on the Geiger counter end of the
jumper, for probing around on circuit boards etc.

Have Fun
but Be SAFE...HV can be lethal!!

George Dowell