Table of Contents
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o 4300C Main Board (4300C-700) – This portion of the ohmmeter contains
the power supply and the constant current source.
• Power Supply – This section converts the AC line power into the DC
levels necessary to power the ohmmeter's circuitry.
• Constant Current Source – This section of the main board 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.
Current Source Reference – The current source reference voltage
is provided by an internally opened zener reference and its
associated components. The reference circuit provides a 1.0000
volt output to the error amplifier circuit.
Error Amplifier – 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.
Output Stage – The output stage provides current gain capable of
driving +10 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.
Range Switching – The range switching circuits provide six ranges
of precision current shunts to control the output current. The
current shunt values are selected so that 1V 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.
Protection Circuitry – The protection circuitry protects the output
stage from inductive kickback voltages. The UNSAFE signal
indicates the presence of back EMF from the load.
Chapter 7 – Theory of Operations
7-3
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