5800 Refrigerated Sampler Service Guide - Isco

5800 Refrigerated Sampler Service Guide
Section 4 CPU and Power Supply
4.1.14 General Outputs – U37,
U39, P13
4.1.15 Analog Input – U7, 25,
34, 35, P13
There are four programmable outputs that come out on P13.
These outputs provide logic signals at 0 or 5 volts. U37 and U39
provide one of these two levels, depending on the signal input to
them. The signals go through current limiting PTCs R118, 119,
121, and 122, and protection circuits.
P13 also has the analog input (4-20 mA) which can come from a
flow meter and provide flow data for flow proportional sampling.
This input is electrically isolated from the other circuitry on the
CPU. The current coming in flows through zener diode CR28 and
the 10 ohm resistor R98. The zener drops 4.7 volts to provide a
local power source for the isolated circuitry. The rest of the
voltage drops across R98, which is the sensing element. U35 is a
voltage-to-frequency converter that runs on the isolated 4.7 volts
and outputs an AC signal based on the voltage across R98. The
pulses from U35 go through opto-isolator U34, which delivers
them to Schmidt trigger U25-B and the U7. The pulses are then
counted by the CPU.
4.1.16 Vref – U15 U15 is a 2.5 volt reference. It provides Vref to the CPU for the
a-to-d converter. It also provides power to the two refrigerator
temperature sensors.
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