5-1 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 am- plifier 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.
5-2 ±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 ob- tained, the power supply is working cor- rectly.
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 (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 compara- tor. 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.
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 -15 V. This value is the output of the OR cir- cuit.
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 20 M re- sistances, R70 and R73.
An approximate 8 V peak-to-peak AC signal is applied to comparators 1 and 2. As the signal starts going positive, comparator 2 transistor ceases conducting and comparator 1 transistor is off.
When the signal exceeds the +159 mV on the non-inverting terminal, it turns on comparator 1 and the output is -15 V. Comparator 1 stays on until the signal drops below +159 mV, at which time the output will be the value of the OR bus.
As the AC signal goes negative with respect to ground, the transistor of comparator 2 conducts and the output is again -15 V. The
