Thus, to ensure an accuracy of 1 part in 107 of full
of the first dial.
scale, the resistor accuracy need not be better than 104 (or 0.01%). During
manufacture, the ratio is adjusted to 0.001% and the construction is such
that a drift rate of not more than 0.01% per year may be expected. This is
equivalent to 0.01 V/step of the fourth dial per year.
The total range of the fifth, sixth, and seventh dials is produced by
similar current dividing resistors, but with accuracy requirements of 0.1%,
1%, and 10%, respectively. The errors of the resistive divisions apply only
to the dial totals, and not to the linearity of the dials which still
depends on the equality of the 10 turns and is the same for all dials. An
internal trimmer is provided to correct for any long term drift.
d. Measuring circuit thermal EMFs.
The only components in the
measuring circuit are the generating resistor, the low thermal pushbutton
selector switch, and the galvanometer circuit (with low thermal keys).
There are no Measuring Dial contacts in the measuring circuit. The design
and construction of this part of the circuit incorporates the best low
thermal techniques, with heavy thermal insulation and large heat sinks.
Thermal EMFs of a few nanovolts exist only when relatively large and rapid
variations in room temperature create temperature gradients as heat flows
into or out of the potentiometer.
e. Switch
contact resistances and contact EMFs. In the potentiometer,
all Measuring
Dial contacts are in circuits of at least 1000 megohms
resistance and
variations in contact resistance or generated thermal EMFs at
these contacts
are therefore negligible.
f. System noise.
The resolution of the potentiometer is limited by
system noise rather than the number of turns or the Measuring Dials. This
noise is about 1 AT and since the comparator is operated at an ampere-turn
level of about 30AT, the 1 AT of noise is therefore equivalent to 0.033 V
on the X1 range and 0.0033 V on the X0.1 range.
Learning Event 2:
DESCRIBE THE MODEL 9771 CONSTANT CURRENT SOURCE (CCS)
1.
Functional description.
The Guildline Instruments CCS Model 9771 is
designed to serve as a long-term working reference for precise electrical
With an output current of 30 mA 10% and regulation of
measurements.
better than 1 ppm, the CCS is an ideal replacement for the conventional
lead-acid battery usually used with a DC Potentiometer.
High current
stability is achieved by using a galvanometer light beam-photo transistor
resistor with the voltage of an external standard cell. The circuit gain is
so high that an output resistance greater than 109 ohms is achieved.
Regulation is better than 1 ppm for a 1000-ohm change in load resistance,
and better than 1 ppm for a 5% change in line voltage.
Four thumbwheel
switches and a fine control enable the operator to set the output current
accurately to any point within the operating range.
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