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Chapter 7 Manual Operations
General
This Chapter contains operating instructions for the ohmmeter Models 4100ATC, 4150ATC, 4165, and
4165-1344. The information contained in this chapter should be used along with the descriptions in
Chapter 5 and 6 to become completely familiar with the various methods of operation using the
different ohmmeters.
Connections
Connections to the ohmmeter are made via 4 binding posts on the front panel of the instrument. When
using Valhalla test leads, the tabbed side of each banana jack is connected to the CURRENT terminals
(see below). This ensures that current is carried in the largest conductor of the cable, and that the
voltage input is shielded.
V
HI
☼ ☼ I
HI
←Tab
V
LO
☼ ☼ I
LO
←Tab
NOTE: If the V
HI
and I
HI
terminals are not shorted together the display will roll around and may or
may not indicate an overrange. This is a characteristic of the voltmeter and does not indicate a fault in
the instrument.
The 4-Wire configuration of all Valhalla ohmmeters eliminates errors normally caused by test lead and
contact resistances. In many applications the contact resistance can exceed the value of the load by
several orders of magnitude. The 4-Wire ohmmeter bypasses this potential error source by providing
two terminals of constant current and an additional two terminals for high impedance voltage
measurement. The result is a fast, accurate resistance measurement of the load, independent of the
resistance of the current carrying leads.
Figure 1 illustrates how the 4-wire principle is used to eliminate lead, wire and contact resistances as
potential error sources. The internal current source inherently overcomes all series resistance (within
compliance voltage limits) and delivers a precise constant current. The internal high-impedance DVM
senses the voltage drop across the load. There is negligible contact and lead resistance error created by
the voltage measurement because the high input impedance of the DVM limits current flow in the
voltage leads.
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