Instruction Manual

748213-S April 2002

Model 755R

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 con- ducts. The output is -15 V. A full 30 V drop appears across R77.

The potential on the non-inverting terminal will now be about -2.3 V. 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 -1.85 V to -1.92 VDC.

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 (See Figure 5-4, page 5-6) does not allow pulses from the OR circuit (comparators 1 and 2) to operate Q6 or Triac Q7 in the case heater (See Figure 5-5, page 5-7).

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 tempera- ture, the error signal to comparator 4 may

vary from 0mV 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, page 5-2) is either -15 VDC 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 form multivibrator circuit, pulses 120 times a second. For about 100 microseconds the junction of R82 and R83 is some value between -1.85 V and -1.92 V, 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 -15 V.

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 turning it on.

At the beginning of the next 100 microsecond pulse, comparator 4 output is again -15V, with zero volts on the base of Q6. Q6 again con- ducts, discharging C38. At the end of the 100 microsecond pulse, Q6 ceases to conduct.

5-4 Circuit Analysis

Rosemount Analytical Inc. A Division of Emerson Process Management