Teledyne 4020 – Hydrocarbon analyzer (special version for automatic background gas switching) - Manual (Page 93)

Total Hydrocarbon Analyzer Maintenance
Teledyne Analytical Instruments 75
The control temperature is determined by the value of resistor R3
and C3 on the temperature controller printed circuit board, selected (at
the time of manufacture) from the chart on schematic B-31077 to
provide the desired control point.
The thermistor used in the circuit is a negative temperature
coefficient (NTC) device; as the chamber temperature increases, the
resistance of the thermistor decreases, and vice versa.
The resistance of the thermistor in the circuit is compared with the
value of resistor R3; when their resistance is equal, or when the
resistance of R3 is less than that of the thermistor, the heating circuit is
activated.
When a temperature deficiency is sensed by the thermistor,
integrated circuit A1, acting as a zero-crossing switch, applies a pulsed
signal to triac Q1, which in turn applies full wave power to the heating
element.
IC A1 employs a diode limiter, a zero-crossing (threshold) detector,
an on-off sensing amplifier (differential comparator), and a Darlington
output driver (thyristor gating circuit) to provide the basic switching
action. The DC operating voltages for these stages are provided by an
internal power supply, with only capacitor C4 added externally.
The on-off sensing amplifier in this circuit is configured as a free-
running multivibrator. This scheme adds proportional control, which
takes over when the comparator inputs are at the design differential
voltage.
Initially, when cold, the thermistor resistance is large, and the
voltage at pin 7 is larger than that at pin 8. As the temperature of the
controlled chamber begins to rise, the resistance of the thermistor
decreases, thus reducing the voltage at pin 8. During this warm-up time
the thyristor gating circuit is continuously delivering gate current from
pin 4 of A1, thus maintaining constant fullwave AC power to the heater.
When the temperature reaches the selected control point, pin 13
voltage is about the same as pin 9 voltage, and proportional control takes
over. The rate at which thyristor (triac) Q1 conducts and allows power to
be delivered to the heater is determined by the combination of
components R2 & R3, R4, C3, R5, and the thermistor resistance at the
control temperature. Consequently, the balance point of the bridge
formed by this combination of components can be altered by the