Fluke NetDAQ 2640A, Service Manual

Page: 52 / 260

NetDAQ
Service Manual
2-10
Open Thermocouple Check 2-17.
Under control of the Inguard Microprocessor, the open thermocouple check circuit
applies a small ac signal to a thermocouple input before each measurement. If an
excessive resistance is encountered, an open thermocouple input condition is reported.
A4 Analog Input PCA Block Description 2-18.
The following paragraphs briefly describe the major sections of the Input Connector
PCA, which is the “Universal Input Module” used for connecting the analog inputs to the
instrument.
20-Channel Terminals 2-19.
Twenty HI and LO terminal blocks are provided in two rows, one for channels 1 through
10 and one for channels 11 through 20. The terminals can accommodate a wide range of
wire sizes, starting with 12 gauge as the largest size. The two rows of terminal blocks are
maintained very close to the same temperature for accurate thermocouple measurements.
Reference Junction Temperature 2-20.
A semiconductor junction is used to sense the temperature of the thermocouple input
terminals. The resulting dc output voltage is proportional to the block temperature and is
sent to the A/D Converter PCA for measurement.
Detailed Circuit Description 2-21.
The following circuit descriptions describe the theory of operation for each Instrument
pca. For these descriptions, refer to the associated schematic diagram in Chapter 7.
A1 Main PCA Circuit Description 2-22.
The following paragraphs describe the operation of the circuits on the A1 Main PCA.
The schematic for this pca is located in Chapter 7.
Power Supply Circuit Description 2-23.
The power supply portion of the A1 Main pca consists of three major sections:
•
Raw DC Supply The raw dc supply converts line voltage (107V to 264V ac) into a
dc output of 8V to 35V.
•
5V Switcher Supply The 5V switcher supply regulates the 8V to 35V dc input into
the 4.9V +/-0.05V dc (Vcc) source.
•
Inverter Using the 5V switching supply output, the inverter generates the -30V dc
and 5.4V ac supply levels needed for the vacuum-fluorescent display and the -5V dc
supply for the RS-232 Interface. The inverter also provides is5.6V (Vddr),
+5.2V (Vdd), and -5.2V (Vss) outputs for the inguard circuitry.