Model 755A

Instruction Manual

245364-V May 2002

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 (Figure 5-2 below) and a temperature-sensing bridge are part of the case heater control circuit (Figure 5-3, page 5-4 and Figure 5-4, page 5-5).

OFF

OFF

OFF

 

+2.3V

 

 

 

 

 

 

 

 

 

 

-15V

 

 

 

-2.3V

 

 

 

 

 

 

 

 

 

R78

6 Hz

 

 

 

 

 

 

 

249K

+15V

 

 

 

 

 

 

 

+15V

 

 

 

 

 

 

 

R77

 

 

 

 

 

 

R74

-

 

 

INPUT FROM

 

 

 

 

10K

 

 

 

 

 

590K

+3

 

 

 

MULTIVIBRATOR

 

 

 

 

 

 

 

 

 

 

 

 

C37

-15V

R79

Q6

 

 

 

 

 

R75

10K

 

 

 

 

 

1.0uF

 

 

 

 

 

 

 

 

T2

 

 

 

 

 

210K

 

 

 

-15V to 1.88V ±0.3V

 

 

 

 

 

 

 

 

 

R80

 

 

R82

R83

 

 

R76

C40

 

 

 

 

10K

 

 

9.09K

63.4K

 

 

 

 

 

TO

37.4K

2200uF

 

 

 

 

 

 

 

 

 

 

 

 

 

-15V

COMPARATOR

 

 

R81

 

RT1

R84

 

 

 

 

56.2

C38

 

 

 

 

 

 

169K

 

 

 

 

 

 

.18uF

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R87

 

 

 

 

 

 

 

 

10K

 

 

 

 

 

 

 

 

-15V

 

Figure 5-2. Ramp Generator

On initial application of power to comparator 3 (Figure 5-3 on page 5-4), 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 ±2.3V because of the resistance divider, R75, R76. Capacitor C37 will now start to charge positively through R78.

When the positive potential across C37 and at the inverting terminal of comparator 3 exceeds the potential on the non-inverting terminals, the transistor conducts. The output is -15V. A full 30V drop appears across R77. The potential on the non-inverting terminal will now be about -2.3V. C37 will not discharge through R78 until its potential exceeds that on the non-inverting terminal. At that time, comparator 3 will switch polarity and start charging C37 again. The result is that the potential across C37 will vary almost linearly

with time and form a ramp signal of about 6 Hz.

As the potential across C37 increases and decreases linearly, it affects the potential at the top of the bridge circuit between R82 and R83 through R74. Because of the ramp action charging and discharging C37, the potential between R82 and R83 varies approximately from -1.85V to -1.92VDC.

The temperature sensing device, RT1, in the bridge circuit is a thermistor. The bridge is designed to control the temperature in the case at 135°F (57°C). When the temperature is 135°F (57°C), the resistance of the thermistor RT1 will be at its lowest and the potential at the junction of RT1 and R84 should be the same as the junction of R82 and R83. Comparator 4 (Figure 5-4 on page

5-5) does not allow pulses from the OR circuit (comparators 1 and 2) to operate Q6 or Triac Q7 in the case heater (Figure 5-5 on page 5- 6).

Rosemount Analytical Inc. A Division of Emerson Process Management

Circuit Analysis

5-3