Rosemount 755A, Инструкция по эксплуатации

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6
E
LECTRONIC
C
IRCUIT
A
NALYSIS
245364-U Rosemount Analytical April 2000 6-1
Model 755A Oxygen Analyzer
6.1 OVERVIEW
The electronic circuitry of the Model 755A Oxygen Analyzer consists of the following:
•
A detector compartment heater circuit.
•
A detector heater circuit.
•
A
±
15VDC 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 +2.5V 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
±±±±
15VDC POWER SUPPLY
The components of the
±
VDC power supply circuit are located in the lower left-hand
corner of the Case Board. 19VAC should be measured with respect to ground at CR5
(WO4). +15VDC should be measured at the C27 (+) lead and -15VDC 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. -15VDC is supplied to the emitter. The
collector is illustrated as the overall output for the comparator package.
The use of a transistor, built into the output of the comparator, allows comparators to
be placed together in an OR circuit. Comparators 1 and 2 (in Figure 6-1) illustrate this
logic principle.