6
C
IRCUIT
A
NALYSIS
748213-P Rosemount Analytical June 1997
6-1
Model 755R Oxygen Analyzer
6.1 CIRCUIT OPERATION
The electronic circuitry of the Model 755R Oxygen Analyzer consists of the following:
•
A detector compartment heater circuit.
•
A detector heater circuit.
•
A
±
15 VDC power supply.
•
A voltage regulating circuit for a stable light source.
•
A 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 +2.5 V for a 0 to 100% change of the operating span.
•
A digital output circuit for the digital read-out.
•
An analog output circuit for recorder, optional alarms and current output.
6.2
±±±±
15 VDC POWER SUPPLY
Refer to Drawing 617186. The components of the
±
VDC power supply circuit are
located in the lower left-hand corner of the Power Supply Board. 19 VAC should be
measured with respect to ground at CR5 (WO4). +15 VDC should be measured at the
C27 (+) lead and -15 VDC at the C28 (-) lead. If the specified voltage measurements
are obtained, the power supply is working correctly.
6.3 CASE HEATER CONTROL CIRCUIT
The case heater control circuit utilizes four voltage-comparators (LM339 quad com-
parator). 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 the 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 non-inverting 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 then present on the collector.
