C
IRCUIT
A
NALYSIS
6-6
June 1997 Rosemount Analytical 748213-P
Model 755R Oxygen Analyzer
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 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.
6.4 DETECTOR HEATER CONTROL CIRCUIT
Figure 6-4 is a simplified heater control circuit drawing for the detector. Heaters 1 and
2 are actually connected in parallel and have a combined resistance of about 17
ohms.
The thermistor resistance (RT1) in the resistance bridge varies inversely with
temperature. The bridge is designed to maintain the temperature of the detector at
150
°
F (65.5
°
C).
The junction point between R55 and R56 is maintained at a specific voltage since
these resistances maintain a definite ratio. The thermistor resistance is 149 K at 150
°
F
(65.5
°
C) and increases rapidly as the temperature decreases. R59 in this bridge circuit
represents the setpoint value for temperature. Suppose that, at temperature,
resistance of the bridge (R55, R56, R59 and RT1) equals 149 K.
If the temperature goes down, RT1 increases in resistance and causes the junction of
RT1 and R59 to go positive in voltage value. Since R55 and R56 are of equal
resistance, their junction is at zero volts. Therefore, terminal 3 of AR6 is more positive
than terminal 2 and the base of Q2 is positive. Q2 conducts, allowing alternating
current to flow through heaters 1 and 2. The voltage drop across the heaters, when
completely cold, would be about 20 VAC and, when controlling, would be AC of very
low amplitude.
As the temperature increases, the resistance of RT1 decreases and the junction point
between RT1 and R59 becomes less positive. Terminal 3 of AR6 becomes less
positive with respect to terminal 2. The output of AR causes Q2 and Q3 to conduct
less. When terminal 3 equals terminal 2, or is less than terminal 2, the output of AR6
is zero or less. Q2 and Q3 do not conduct and the heater would not be supplying heat
energy to the detector.
