SECTION VII THEORY OF OPERATION
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7-1. General
The information contained in this section describes the theory of operation behind the Valhalla Model 4300B
Digital Micro-Ohmmeter. The theory is divided into two sections. the first is a general description referring
to the block diagrams of Figures 7-1 and 7-2. The second section is a detailed description referring to the
schematics and diagrams at the back of this manual.
7-2.
General Description of Circuits
This section describes the general function of each section of the Model 4300B. The paragraph titles used in
this section refer to the block diagrams of Figures 7-1 and 7-2. "I" designates Figure 7-1 and "V" designates
Figure 7-2.
7-2-1. Reference (I)
The current source reference voltage is provided by an internally ovened zener reference and its associated
components. The reference circuit provides a 1.0000 volt output to the error amplifier circuit.
7-2-2. Error Amplifier (I)
The error amplifier compares the reference voltage with the signal from the range switching circuit to drive the
output stage. The ON/OFF signal from the control logic commands the error amplifier to turn on or off the
output stage.
7-2-3. Output Stage
The output stage provides current gain capable of d10 amps into the unknown resistance. The
ON/OFF signal from the control logic disables the current source output when the test current is turned off.
7-2-4. Range Switching (I)
The range switching circuits provide six ranges of precision current shunts to control the output current. The
current shunt values are selected so that 1 volt is dropped across the shunt for each current range (e.g. 10A in,
1V out; 1A in, 1V out; etc.). The operation of the current source is as follows: Current from the power supply
flows through the load, output stage and current shunt. The voltage dropped across the shunt is compared to
the reference voltage by the error amplifier. The error amplifier uses the voltage drop across the internal shunt
to control the amount of current passing through the load as necessary to maintain a precise constant current.
