6.
Theory of operation. Figure 2-4 is a block diagram showing the major
components of the potentiometer. The unknown EMF is compared with a known
voltage generated by passing a known current through a stable-generating
resistor.
The current also passes through the secondary winding of a
current comparator toroidal transformer, where it is compared with a stable
30 mA current in the primary.
a. The comparator circuit automatically adjusts the secondary current
until the effects of the ampere-turns in the secondary and primary windings
exactly cancel.
In this condition, the ratio of secondary to primary
currents is equal to the ratio of secondary to primary turns. The primary
current is held constant by using a precision CCS while the number of turns
on the primary winding may be discretely selected using a set of switches.
The current in the fixed secondary winding, which depends on the turns ratio
between primary and secondary, may therefore be precisely controlled by
switching the appropriate number of turns into the primary winding.
Figure 2-4.
Block diagram of potentiometer
b. A resistor (known as the generating resistor) is placed in series
with the secondary current and the voltage which appears across it may be
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