[volt-nuts] Traveling Standards

[WB6BNQ]

Hi Bob,

I have been monitoring your thread since it got started.  Personally, you would
be better off spending your money on a used Fluke 731B off of eBay.  You would be
way ahead of the game compared to where you are at with your current concept.

First, the biggest issue is not having your circuit set to a "cardinal" voltage
point; i.e., 10 volts.  By having some other level the measuring party would have
to ratio against his standard to report a value.  That adds an unnecessary level
of complexity and increases the error limits in reporting the uncertainty.

Second, you need to provide separate power supply regulators for any heating
system and the reference voltage circuitry.

Third, your choice of a power supply regulator is very poor.  You should consider
something more exotic like the Linear Technology LT-3080 or the LT-3083.  In
particular, pay attention to the noise reduction considerations in the
specification sheet.  Preferably a two stage affair using a LT-3083 (handles 3
amps) as a pre regulator set to 18 volts and then the separate LT-3080 (handles 1
amp) devices to their respective circuitry.

Fourth, you need to pay particular attention to ground loops and your ground
plane layout.  Think along the lines of a star pattern, i.e., single point of
return.

Fifth, to do it right you should provide four (4) separate 10 volt references
with their own outputs in the package.  This would provide for independent
analysis of the "bank mean" and allows for determining if any individual
reference shifts outside of a nominal "bank mean."

Sixth, in designing the 10 volt reference, the output loading should provide the
highest possible isolation as to affecting the actual reference cell.  The output
circuit should provide a minimum of 10 milliamps without a any change in the
output level.

Seventh, if any adjustable elements are used in the design, they should be of the
smallest possible size as to resistance value.  This will help to minimize
temperature affects.  A "cermet" potentiometer type should be used with values
under 50 Ohms.  For example a 89pr10 (or 20 etc.).

Eighth, if it is going to be powered from the mains voltage, then a faraday
shielded transformer should be used.  Proper EMI & RFI shielding is also needed.

Just food for thought and, by the way, having done all of the above, you just
constructed a 731 if you just do one reference.


Bill....WB6BNQ


Bob Smither wrote:

> Fellow Voltage Fans,
>
> To review - I would like to have a lab standard here that I can trust to
> 10 ppm or better to use to calibrate my expanding collection of
> voltmeters.  I have proposed to built a portable standard that I will
> ship to participating volt nuts on this list who will record their
> readings of the standard and share those readings with the list.
>
> I have designed a Traveling Standard that incorporates two references
> (an LM199AH and a MAX6350E) along with an LM34 to measure the
> temperature of the MAX6350 (the LM34 is glued to the top of the MAX6350.
>
> The schematic is attached to this page:
>
>    http://www.c-c-i.com/TS
>
> Following suggestions on this list (thanks!) I will use an external
> "wall wart" supply for the input 24 Volts.  This reduces any shift
> caused by the line regulation of the three terminal regulator.
>
> My design notes are included on the above page.
>
> I have constructed a bread board of the circuit on a plugboard.  I will
> move the circuit to a printed circuit version after some testing.
>
> Some preliminary readings from the breadboard of the LM199AH part are
> noted on this page:
>
>    http://c-c-i.com/node/121
>
> Note that these readings are taken with an HP3455A which has not been
> calibrated since I bought it new.  I would expect the accuracy to be no
> better than .01% and may be much worse.
>
> My plan is to mount the printed circuit board version in a short length
> of 2" PVC pipe with connectors for the three outputs.
>
> After the printed circuit board version is aged some it will be ready to
> ship.
>
> Are there any more on the list that would be willing to make a
> measurement of the Traveling Standard?
>
> I will collect the readings that we collectively get on the TS and make
> them available to the list.  I anticipate that with some experience we
> can all benefit from having this TS well aged and with a well known
> output voltage.
>
> I will handle the shipping - all I ask is that you take careful
> measurements and provide them to me, then return the unit.
>
> Thanks!
>
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