Just for fun, I dug out my old Model 630NA
Triplett VOM with its cool anti-parallax mirrored scale
to see what it would read. I magnified the image in
Figure 4 and estimated the reading to be 2.488 volts.
The manual for the meter stated the accuracy to be
+1-1/2% of full scale. Bottom line: My Triplett was
accurate to within 1/2% on the three volt range. Not
too shabby for a meter as old as the hills!
EFFECT OF TEMPERATURE
I’m not going to discuss the effect of temperature
on any of the voltage references I’ll be covering later in
this article because that is a whole other
subject itself. Plus, it’s been extensively
covered in scientific studies. Temperature
is a critically important parameter for
many types of references, but the effects
are quite small for hobbyist ICs.
For example, the 5.000 volt output
of an AD584 only varies about +1.25 mV
over the full industrial range of 0°C ( 32°F)
to 70°C (158°F). If you are interested, I
conducted temperature tests on an AD584 and the results
are graphed in Figure A in the sidebar.
THE BLACK BOXES
Voltage references (or standards) in the past weren’t
quite as small or inexpensive as the current ICs. They
came in shiny black Bakelite
housings that cost $40 in 1963
dollars. On a whim, I bought a
brand new one (actually 39
years old) made by Eppley
Laboratory on eBay for $50.
It’s a real beauty. See Figure
5 for a size comparison to
the modern AD584 DIP.
For almost 80 years
(from 1911 to about 1990),
these shiny black housings
called Weston cells reigned
supreme as the world’s
primary and secondary voltage
standards. Inside the housing was a simple glass vial filled
with a bunch of high purity chemicals. Figure 6 shows one
of the H-shaped vials with the chemicals at the bottom of
each leg and filled with a liquid to just above the halfway
sulfates, cadmium-mercury amalgam,
and an electrode
of shiny metallic
mercury in the lower
right leg. Platinum
wires were used
to bring out the
voltage. The voltage
was produced by
chemicals and was
a little over one volt;
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cell made by
over 70 years.
FIGURE 4. My vintage Model 630 Triplett VOM is accurate
to within 1/2% after decades of hard use.
Figure A. Temperature plot of AD584 IC 5.000V output
shows just 2. 5 mV variation from 0°C to 70°C.
FIGURE 6. Inside the
standard cell was a glass vial filled with chemicals that generate an
accurate and stable voltage.
September/October 2018 55