Rosemount 755, Instruction Manual

Page: 63 / 94

6
C
IRCUIT
A
NALYSIS
748183-H
6-1
Rosemount Analytical
December 1997
The electronic circuitry of the Model 755 Oxygen Analyzer consists of the following:
•
Case heater circuit
•
Detector heater circuit
• ±
15 VAC power supply
•
Voltage regulating circuit for a stable light source
•
Detector circuit with a first-stage amplifier to provide a feedback current for
mechanical feedback to the detector and a scaling amplifier circuit to give an
output change of 0 to +7.5 V for a 0 to 100% change of the operating span.
6.1 POWER SUPPLY
±±±±
15 VDC
The components of the
±
VDC power supply circuit are located in the lower the left-
hand corner of the case circuit board. 19 VAC should be measured with respect to
ground at CR5 (WO4). A +15 VDC should be measured at C27 (+) lead and -15 VDC
at the C28 (-) lead. If the specified voltage measurements are obtained, the power
supply is working correctly, see DWG 617186.
6.2 CASE HEATER CONTROL CIRCUIT
The case heater control circuit utilizes four voltage-comparators (LM339 quad
comparator). An understanding of how one of these comparators functions is
necessary before any circuit analysis can be attempted.
In Figure 6-1, comparators 1 and 2 are depicted having a comparator within an overall
comparator symbol. Also within this symbol, the base of an NPN transistor is
connected to the output of the comparator. A -15 VDC is supplied to the emitter. The
collector is illustrated as the overall output for the comparator package.
When the noninverting terminal of comparator 2 is more positive than the inverting
terminal, the transistor does not conduct and the collector of the transistor or
comparator output is at whatever potential is the present on the collector.
When the noninverting terminal of comparator 2 is less positive (more negative) than
the inverting terminal, the transistor conducts and the output of the comparator is -5V.
This value is the output of the OR circuit.