Emerson Process Management 755R manual 46

Instruction Manual

748213-S April 2002

Model 755R

output remains at -15 VDC 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 sine wave,

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 ±0.03 V.

The on-off effect of the comparators to the OR circuit results in application of a positive -going pulse (from -15 V to -1.89 V) to

the temperature bridge at the rate of 120 pulses per second.

Capacitor C36 is added to the input circuit to delay the incoming AC signal so that the pulses will occur at or just after the line frequency crossover point.

Circuits for a ramp generator and a temperature -sensing bridge are part of the case heater control circuit (See Figure 5-2, page 5-3 and Figure 5-3, page 5-3).

On initial application of power to comparator of Figure 5-2 (page 5-3), no potential exists on the inverting terminal because no charge exists on capacitor, C37. If the transistor of comparator 3 does not conduct, +15V is at the output terminal. With +15V at the output, the potential on the non-inverting terminals will be about ±2.3 V because of the resistance divider, R75, R76.

100µ

-1.7V
-15V
159mV
	0°	180°	360°	180°
			0°
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

5-2 Circuit Analysis Rosemount Analytical Inc. A Division of Emerson Process Management