Model 755A

Instruction Manual

245364-V May 2002

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 conducts, discharging C38. At the end of the 100 microsecond pulse, Q6 ceases to conduct. C38 charges and a pulse appears at the gate of Triac Q7, turning it on again. The charging time for C38 is about one-half a time constant (C38, R87) and ten time constants (R81, C38) are available for discharging C38.

The above action is repeated as long as the temperature is low, causing an error between R82, R83 junction and RT1, R84 junction. As the temperature approaches the desired case temperature of 135°F (57°C), differences between these two junctions will exist for only part of each ramp and the number of pulses operating Q7 will be proportional to the amount of error sensed by the 6 Hz ramp.

The pulses arrive at Q7 just as the supply AC line voltage is passing the zero-volt crossover point. The purpose of C36 (Figure 5-2 on

page 5-3 and Figure 5-4 below) is to delay the timing pulse, relative to line frequency, so that a pulse arrives at the gate of Triac Q7 as the line potential just passes the zero-volt crossover point (0° and 180° of line phase).

Varistor, RV1 is a temperature sensitive resistance device. When case temperature is low, such as ambient, the value of RV1 is low. Applying power at that temperature might cause a current surge to damage Triac Q7. RV1 with its low initial value of resistance acts as a bypass and most of the current is shunted through it.

As the temperature increases and approaches the desired case temperature, the resistance of RV1 increases to a large value. This limits the current through it and gives fine control of the heater to Triac Q7 and the temperature-sensing circuit.

HEATER

HR3

F2

FAN

MOTOR

MT2

Q7

T1

MT1

RV1

T1

NEUTRAL

120V RMS

HOT

Figure 5-4. Case Heater Circuit

Rosemount Analytical Inc. A Division of Emerson Process Management

Circuit Analysis

5-5