Dennis' suggestion of using a 100x probe got me to thinking of an
alternative as the 100x scope probes are sorta' pricey, even on the used
market. As Dennis points out, the 100x scope probes are useful when
sampling high voltage, typically greater than 1KV. Since my focus has been
with the grids of tube receivers, these measurements (even into the hundreds
of volts) are adequate with the right 10x probe.
The 100x probe uses a 10 meg sampling resistor with a 100K shunt at the
scope input. Most scopes have a ~ 1 meg input Z. The input Z of my Fluke
8060A computes to 10.5 meg and is a value that is probably close to most
portable DMMs.
Taking Dennis' suggestion, I used a 1 meg Simpson VTVM probe and shunted it
with a value close to 100K to create the necessary 10:1 ratio. A 200K pot
was used to find a value that produced an exact 10:1 ratio. For the Fluke
8060A and my Simpson VTVM probe, that value is 112.5K. I then created shunt
consisting of two resistors in series whose total value is 112.5K and
inserted this combination into a Pomona dual banana jack. Different shunt
jacks can be created if one wants best accuracy over a choice of DMMs in the
shack.
In the links below, notice that the Simpson probe is connected to a
BNC-to-dual banana adapter with standard 3/4" pin spacing. The adapter has
holes to allow shunting from a dual banana connector; the latter of which
has the 112.5K resistor combination. The black plug simply plugs into the
red plug.
http://tinyurl.com/bc6ggo8
http://tinyurl.com/b56tfm5
Testing this idea in the real world shows that Dennis' idea works in a 10x
configuration. For example, when a +12V source is measured, the DMM reads
1.2V. It's then just a simple matter of multiplying displayed results by a
factor of 10 which is a whole lot easier than using the DMM's RELative
button and trying to find a 1.07V source as the calibrating reference.
It appears good accuracy can be achieved when using a DMM with a traditional
VTVM probe. The VTVM probe has the benefit of a 1 meg isolation resistor
which is highly useful for grid readings. The trade-off is the requirement
of multiplying displayed readings by a factor of 10 -- the same as mentally
moving the display's decimal place to the right by one digit.
Paul, W9AC
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