IET Labs HARS-LX, User And Service Manual, Page: 15 / 22

9
HARS-LX Series
9 Operation
4.10 Rheostat Operation (-RH Option)
For high-resolution applications, an optional rheostat
may be used. The rheostat provides up to 10 mΩ of
resistance in 0.1 mΩ steps. To get the rheostat read -
ing 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 HARS-LX 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
HI SENSE
HI CURRENT
LO SENSE
LO CURRENT DISCRETE
DECADES
RHEOSTAT