Instruction Manual

245364-V May 2002

Model 755A

100µ

-1.7V

 

 

 

 

-15V

 

 

 

 

159mV

 

 

 

 

 

0°

180°

360°

 

 

 

 

0°

180°

COMP 1

ON

 

OFF

ON

 

 

 

 

COMP 2

OFF

 

ON

OFF

 

 

 

 

+15V

 

 

 

 

 

 

+15V

 

 

R69

-

 

 

 

2M

 

 

 

 

+1

 

 

OUTPUT

R71

 

 

 

 

21.5K

 

-15V

 

+15V

 

 

R70

 

 

 

20M

-

 

 

 

 

 

 

 

 

+2

 

R68

 

 

 

-15V

3.3K

 

 

 

INPUT

 

 

 

R73

R72

C38

 

 

20M

4.75K

0.18uF

 

 

-1.88 VDC

 

 

 

 

Source

Figure 5-1. Two-Comparator OR Circuit

An approximate 8V 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 -15V.

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 -15V. The output remains at -15VDC until the incoming signal crosses zero value and the positive signal causes the comparator 2 transistor to cease to conduct.

Summing the effects of the two comparators in the OR circuit results in no output from the comparators for about 4° of the sinewave, 2° after the signal goes positive (0 to 2°) and 2° before the positive signal reaches 180° (178° to 180°).

During the period that neither comparator is conducting, the value on the OR bus is the potential from the temperature-sensing bridge plus the effect of the ramp generator, probably -1.88 ±0.03V.

The on-off effect of the comparators to the OR circuit results in application of a positive-going pulse (from -15V to -1.89V) to the temperature bridge at the rate of 120 pulses per second.

5-2

Circuit Analysis

Rosemount Analytical Inc. A Division of Emerson Process Management