Vanguard Auto-Ohm 100_200 s2 - Manual (Page 6)

AUTO-OHM 100/200 Series 2 Operating Procedures
6
Rev 3, 2009
1.0 INTRODUCTION
1.1 Applicability
This manual applies to the Model Auto-Ohm 100 Series 2 and Model Auto-Ohm 200 Series 2
(hereafter, Auto-Ohm), made by Vanguard Instruments Company.
1.2 General Description
The Auto-Ohm-100/200 Series 2 are the third generation micro-ohmmeters made by Vanguard
Instruments Company. The Auto-Ohm 100/200 features microprocessor-controlled measuring of
very low resistances ranging from 1 micro-ohm to 300 milli-ohms with high accuracy. The Auto-
Ohm is field-portable, rugged, and easily operated by first-time users having a minimum of
training. It features one-knob control and an LCD alpha/numeric display of the resistance
measured. The one-knob control operation is logical and simple: Turning the knob scrolls
through a menu of possible options (which display in sequence) and pressing the knob activates
the selected function. As its name implies, the Auto-Ohm operation is automatic, requiring little
more from the user than connecting it to an unknown resistance and selecting the desired
functions and its options. The Auto-Ohm automatically stores the last 3 resistance measurements,
which can be displayed after testing.
1.3 Functional Description
The Auto-Ohm’s operation is based on the electrical relationships described by Ohm’s law:
R=V/I, where I is a known current and V is the dc voltage measured across the unknown
resistance (typically, a circuit breaker’s contacts). Since the current (user selected) through the
unknown resistance is known and the voltage across the unknown resistance is measured by a
precision voltmeter, the resistance read-out can be calculated using Ohm’s law.
The Auto-Ohm test voltage is supplied by a true 5Vdc power supply. The true DC test current is
selectable in 2-amp steps, from 10 to 100 amperes for the Auto-Ohm-100 and from 10 to 200
amperes for Auto-Ohm-200. The test current is automatically ramped up and down slowly. This
current ramp rate is programmable from 5 seconds to 30 seconds.
The voltmeter test leads run separately from the current-bearing test leads to the resistive load
thus eliminating any I•R voltage drop error in the current cables. These Auto-Ohm features make
very precise micro-ohm measurements possible without having to calculate compensations for
current lead resistance errors.
1.4 Furnished Test Accessories
The Auto-Ohm is supplied with two 30-foot test cables with heavy-duty alligator clamps. Both
the current (#1 AWG) and sense leads are combined into one cable (Figure 1.0). A Ground cable,
and power cord are also included with each Auto-Ohm.
Users can select to have the current and sense leads separated as shown in Figure 2.0 and Figure
4.0. Optional hand spike sense leads are also available as shown in Figure 3.0.