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General Circuit Descriptions
The ohmmeter may be easily thought of as consisting of two separate parts.
1) A constant-current source.
This half of the ohmmeter provides a stable test current that is passed
through the load to develop a voltage across it. The value of this current for each range is indicated
on the front panel of the instrument.
2) A digital voltmeter
(analog-to-digital converter). The voltmeter senses the voltage drop across the
load and translates this into the resistance reading on the display.
Use the following guidelines to determine whether the fault lies in the voltmeter or current source
circuitry of the ohmmeter:
1) If the fault occurs on one range only then the fault is probably in the current source section.
2) If the fault is display related (e.g., missing segments, non-numeric data, etc.) then the fault is
probably in the voltage measurement section.
3) If the fault occurs on all ranges, the voltmeter section may be verified as operational by applying a
precise -100mV (4100ATC) or -10mV (all others) to the VOLTAGE terminals. The display should
indicate 10000 counts no matter which resistance range is selected.
Detailed Circuit Descriptions
This series of paragraphs detail the actual operation of the above mentioned circuits, and are provide to
aid the technician in troubleshooting to component level. A basic knowledge of electronics is assumed.
The technician should refer to the schematics at the back of this manual.
Power Supplies
The ohmmeter uses several supplies to power the current source, the voltmeter (A-to-D converter) and
any optional interfaces. All of these supplies are similar in design. A secondary winding of the
transformer provides the basic AC voltage from which the DC supply will be produced. This AC
voltage is rectified using diodes, filtered using electrolytic and tantalum capacitors, and in some cases
regulated using a standard three-pin regulator.
The supplies are shown on the schematic 4100-074 sheet 1 (and sheet 2 for the optional unregulated
+5V supply). The 4150ATC and 4165 models use additional supplies for their preamplifier, shown on
sheet 2.
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