[volt-nuts] Resistance standard
rob.klein at smalldesign.nl
Mon Dec 14 22:01:59 UTC 2009
My first post to the group, and it's Ohm-nut, rather than Volt-nut, but
I hope you'll forgive me for that :-).
The nutty idea is this: To build a Volt-nuts grade resistance standard.
Or, actually, two, perhaps three.
The first one is quite simple, but rather pricey: I have ordered four
Vishay VHP202Z's, at $ 96 each (ouch!). Expected to be delivered late
february/early march. They will be placed in a series/parallel
configuration to deliver a 10kOhm resistor that should easily rival a
probably be even better. When finished, I intend to have it calibrated
at regular intervals and use it as my house standard.
The other two are a bit more involved, but it will be interesting to
see the results.
For these two, I will use 9 each Z201's from Vishay. These use the same
chip as the VHP202, but are molded, rather than hermetically sealed.
Also, I will use 0.01% types, rather than the 0.001% VHP's. These are
much cheaper (the 18 I need to make two standards cost less than the
four VHP's!), but also less stable over time.
To overcome the stability problem, I am looking at two ways to *make*
them hermetically sealed. The first is to house the 9 (three in series,
sets in parallel) in an RF shielding can, fill this up with oil and
solder it shut.
The can I have in mind is a PCB mounted type, for which I shall have to
design a board. The PCB area inside the can will be solid copper,
some way beyond the outside, so I can make a proper seal. Connections
to the outside world will be through glass-sealed, solder mounted
feedthrough capacitors of low capacitance. After mounting the resistors
and a thorough cleaning, the whole thing will be baked at ~85°C
to get rid of any moisture, then filled with oil and soldered shut.
This assembly will be placed inside a sturdy metal box (Hammond model
1457K1201), which will hold four low EMF binding posts (Pomona
For the second solution, I want to use much the same method, but rather
than filling the can with oil, I want to fill it with epoxy or maybe
This is a much simpler solution, because there will be no need for the
feedthrough caps and no need to solder the can shut.
However, of course, I am aware that neither of these resins will
provide an actual hermetic seal, since they *will* absorb some
moisture. I am curious
though, as to how much of a positive effect can be gained from this
method, as the shear volume of the resin, as compared to the quantity
use to mold the
actual resistors, should at least greatly diminish any effects of
So, if you're still with me after this, I would very much like the
knowledgeables of this group to comment on these ideas. Are they
feasible? What potential
pitfalls might I have overlooked? What oil to use (as an avid cook, I
know my olive- from my sesame oil, but I haven't a clue what type of
mineral oil to
look for :-( ).
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