Instruction Manual

Instruction Manual
748213-S
April 2002
Rosemount Analytical Inc. A Division of Emerson Process Management Circuit Analysis 5-5
Model 755R
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 con-
stants (R81, C38) are available for discharg-
ing 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 be-
tween 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 is to delay the tim-
ing 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 cross-
over point (0° and 180° of line phase).
Varistor, RV1 is a temperature sensitive re-
sistance 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 in-
creases 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 cir-
cuit.