Instruction Manual

245364-V May 2002

Model 755A

 

T1

 

 

 

+15V

 

 

 

R78

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

19 VAC

 

R69

-

1

R74

-

249K

 

 

 

120 V

 

2 M

 

590K

 

 

R77

 

 

 

TO POWER

+

 

 

 

 

 

 

 

 

 

3

 

 

RMS

 

SUPPLY

 

 

 

 

 

 

10K

 

CR11

 

 

19 VAC

 

R71

 

R70

 

+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

C37

 

 

 

 

 

 

 

 

 

 

21.5K

 

20M

 

R75

R79

 

 

 

12

 

CR9

 

-

1.0uF

 

10K

Q6

T2

 

R67

 

 

 

 

210K

 

 

 

 

 

 

 

 

 

 

 

 

 

2

 

 

 

 

 

 

 

10K

 

R68

+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R76

 

C40

R80

 

 

 

 

 

 

3.3K

 

 

 

 

 

 

 

R72

C36

 

 

 

 

37.4K

2200uF

10K

 

 

 

CR10

 

 

R73

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4.75K

.18uF

 

 

20M

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R81

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

R85

 

R86

 

56.2

C38

 

 

 

-15V

 

 

 

 

 

 

 

 

 

 

 

 

 

R82

R83

 

 

 

 

 

 

 

 

 

 

 

 

11.0K

 

20M

 

 

.18uF

 

 

 

 

 

 

9.07K

 

 

 

 

 

 

 

 

 

 

63.4K

 

 

 

 

 

 

 

 

 

 

C39

RT1

R84

-

 

 

 

 

R87

 

 

 

 

 

.01uF

 

 

4

 

 

10K

 

 

 

 

 

169K

 

 

 

 

 

 

 

 

 

 

 

+

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

-15V

 

 

 

 

 

Figure 5-3. Case Heater Control Circuit

Theoretically, at 135°F (57°C) the potential at the junction of RTR1 and R84 is -1.85VDC. This is equivalent to a resistance of 21.2 K. By substituting a decade box for the thermistor and placing 20.2 K into the bridge, the heater should be off. With 22.7 K, the heater should be full on.

Since the potential at the junction of R82 and R83 can vary between 1.85V and 1.92V according to the 6 Hz ramp, and the potential at the junction of RT1 and R84 may vary around or within these limits, depending on temperature, the error signal to comparator 4 may vary from

0 mV to some absolute value. The polarity of the error signal will depend on the deviation from the desired temperature and the ramp value at the function of R82 and R83.

The input from the OR circuit comparator (See Figure 5-1 on page 5-2) is either -15VDC or the ramp effect on the bridge. When -15V, the junction of R82 and R83 is also this value. The error signal into comparator 4 is negatively large to the inverting terminal. Comparator 4 output transistor does not conduct. The base of Q6 is positive; therefore, Q6 does not conduct and a charge builds up on capacitor C38.

The input from the OR comparators 1 and 2, a form of multivibrator circuit, pulses 120 times a second. For about 100 microseconds the junction of R82 and R83 is some value between -1.85V and -1.92V, depending on the ramp generator. For this brief period of time (one pulse), comparator 4 compares the potential of junction R82, R83 with junction RT1, R84 of the bridge circuit. If the temperature at RT1 is low, the potential at the non-inverting terminal of comparator 4 is more negative and the output is -15V.

The base of Q6 is zero, because of the voltage drops across R79 and R80; therefore, Q6 conducts. Energy, stored in C38, flows through Q6 as current and capacitor C38 discharges to zero potential. No current flows through the primary winding of transformer

T2. At the end of the 100 microsecond pulse, the NPN transistor in the output of comparator 4 ceases to conduct, so the signal on the base of Q6 is +15V. Q6 ceases to conduct. C38 starts to charge, driving electrons (current) through the primary of T2. This induces a pulse into the secondary of T2 and to the gate of Triac Q7 (Figure 5-5 on page 5- 6) turning it on.

5-4

Circuit Analysis

Rosemount Analytical Inc. A Division of Emerson Process Management