5-1 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.
5-2 ±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.
5-3 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 5-1 on page 5-2, 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 5-1 on page 5-2) illustrate this logic principle.
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.
When the non-inverting terminal of comparator 2 is less positive (more negative) than the inverting terminal, the transistor conducts and the output of the comparator is -15V. This value is the output of the OR circuit.
Comparator 2 is biased at 0 volts on the inverting terminal. Comparator 1 is biased at about 159 mV on the non-inverting terminal. Positive feedback or hysteresis is built into each comparator circuit for stability or positive action. This is achieved by the 20M ohm resistances, R70 and R73.
