Vanguard Instruments Auto-Ohm

General Description The Auto-Ohm micro-ohmmeter is made by Vanguard Instruments Company and features microprocessor-control for measuring very low resistances ranging from 1 micro-ohm to 300 milliohms with high accuracy. The Auto-Ohm is fieldportable, rugged, and is easily operated by first-time users having little or no training; it features a 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 what it is to do and how the test results are to be presented. The Auto-Ohm stores the last 3 resistance measurements, which can be displayed during test.

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 circuitbreaker’s contacts). Since the current (user selected) through the unknown resistance is known and the voltage across the unknown resistance is read by a precision voltmeter, the resistance read-out is a direct function of the voltage being measured. The voltage is read directly as resistance (translated and displayed on a digital readout). The dc test current is selectable in 2-amp steps, from 10 to 100 amperes. Test current is gradually ramped up and down (automatic function), which virtually eliminates magnetically induced transients through the circuitbreaker current transformers. The risk of inductively tripping a circuit breaker (bus differential relay) is virtually non-existent. The Auto-Ohm performs all of these functions in one simple process. Voltmeter test leads run separately from the currentbearing test leads to the resistive load; thus, voltages are measured at the terminals of the resistance being tested, eliminating any I•R voltage drop error in the current cables. The Auto-Ohm test voltage is supplied by a DC power supply. A precisely regulated constant-current source ensures measured voltage is a function of the resistance alone, and not affected by current-cable resistance losses. These Auto-Ohm features make very precise micro-ohm measurements possible without having to calculate compensations for current lead resistances errors.