Co2 Sensor Pcb - Thermo Scientific 3403 Manual

Section 8
Appendices

CO2 Sensor PCB

8-24 Midi 40 CO2 Incubator
Pin 1 – "+5V'" If not present, check for "+12V" signal at J1 pin 1. If
"+12V" is present while "+5V" is missing, U2 circuitry would be suspect
and the 192039 assembly should be replaced. If both signals are absent,
troubleshoot "+12V" back to the CPU board.
Pin 2 – No Connection
Pin 3 - Ground
This circuit is simply comprised of a regulator supplying constant voltage
to a resistive bridge. One leg of the bridge (R1 & R4) consists of precision
resistors; the other leg is the TC sensor itself, also made up of two resistive
elements. Each of these is a basic voltage divider circuit. The voltage
produced at the center of each divider is provided to the CPU board A/D
converter via JP1 and JP2. These values are then used to calculate CO2.
Test Points
JP4 Pin 1 – "RAW"
Pin 1 is closest to the "JP4" label. "RAW" is the main supply voltage to
the board and the regulator. A specific voltage level here is not impor-
tant: it will vary with line voltage. Just determining that there is a volt-
age here is the important item (15-20 VDC, typically ~18 VDC). If this
is not present, yet the system is running in all other respects, the harness
to JP4 is probably the problem. In most cases, if "RAW" is not present,
there will be more significant system problems.
R2 Pin 1 – +12V
Regulator output, measure at leg of R2 closest to JP2. If +12VDC is not
measured, unplug JP1, JP2, and JP3 then recheck. If still not present,
assuming "RAW" has been confirmed, replace the board. Else, reconnect
harnesses one at a time and troubleshoot the problematic connection.
JP3 Pin 3 – TC Supply
This is the voltage supplied out to the TC sensor. This should typically
measure ~4.8 VDC with the sensor connected; ~11.7 VDC without. Pin
1 is closest to the "JP3" label.
JP3 Pin 4 or JP1 Pin 3 – TC Sensor (V
This is the voltage produced by the TC sensor and supplied to the CPU
board. Under proper operation, this will measure in the ~2.4 VDC
range. If the sensor is open circuit, this point will toggle 0 & ~5.4VDC.
Pin 1 is closest to the "JP3" & "JP1" label.
JP2 Pin 3 – V +
m
This is the voltage produced by the R1/R4 voltage divider. Under proper
operation, this will measure in the ~2.4VDC range. If the sensor is open
circuit, this point will measure ~5.4VDC. Pin 1 is closest to the "JP2"
label.
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