IET Labs RS-925D, Руководство пользователя и обслуживания

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6
RS-925D
6 Operation
4.3 Dial Setting
The resistance setting may be read directly from the
dial settings. For additional flexibility and range, each
decade provides a “10” position setting. This “10”
position on any one decade equals the “1” position
on the next higher decade if any. It adds about 11
% to the nominal total decade resistance. The most
significant decade also has an “11” position to extend
the resistance range to over 1.2 M
Ω
. The l0 m
Ω
de-
cade, however, does not go below the “ 1 “ position
in order to maintain a precise and constant minimum
resistance of 10 m Ω , so that no subtraction of zero
resistance is required.
To determine the resistance obtained when any one
or more “10” or “11” settings are used, simply add 1
to the next higher decade. For example, a setting of
“10-11-10-10-10-10-1-10” becomes:
11 1 1 0 0 0 0. 0 0 0
10 1 0 0 0 0. 0 0 0
10 1 0 0 0. 0 0 0
10 1 0 0. 0 0 0
10 1 0. 0 0 0
10 1. 0 0 0
1 . 0 1 0
10 . 0 1 0
Total 1 2 1 1 1 1. 0 2 0
In order to obtain a zero in the l0 m
Ω
position, set
the l0 m Ω decade to the “10” position, i.e. l00 m Ω ,
and take the l00 m
Ω
setting, in the next decade, into
consideration. To get 1.000 Ω , for example, the
switches should be set to show “0.-9-10-0”.
Since the highest decade has an additional “11” po-
sition, resistance values of over 1.2 times nomianl
maximum value can be obtained.
4.4 Rheostat Operation
For high-resolution applications, an optional rheostat
may be used. It provides up to 10 mΩ of resistance in
0.1 mΩ steps. To get the rheostat reading in milliohms,
multiply the dial setting by 0.1. See Figure 4-1.
Figure 4-1: Rheostat Dial
In order to eliminate contact resistance and thermal
emf, the RS-925D integrates the rheostat as shown in
Figure 4-2. This way, the wiper is in the low potential
circuit, which is the high impedance lead. As a result,
voltage and contact resistance effects are removed by
being effectively added to the input impedance of the
measuring instrument.
Figure 4-2: Rheostat Schematic
4.5 power Considerations
To maintain the maximum possible accuracy and pre-
cision, power applied to the RS-925D should be kept
as low as possible, preferably below 0.1 W. For best
protection of the instrument, it is advisable to limit
the input power to 1 W. This may be implemented
with a series resistor or fuse.
HI SEnSE
HI CURREnT
LO SEnSE
LO CURREnT
DISCRETE
DECaDES
RHEOSTaT