Teledyne 402REU - Hydrocarbon analyzer - Manual (Page 48)

Chapter 5 Model 402 R-EU
Teledyne Analytical Instruments 5-14
manufacture) from the chart on schematic B-30974 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 resistor R1 and capacitor C1 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 13 is larger than that at pin 9. As the temperature of the controlled
chamber begins to rise, the resistance of the thermistor decreases, thus
reducing the voltage at pin 13. During this warm-up time the thyristor
gating circuit is continuously delivering gate current from pin 4, 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 tempera-
ture. Consequently, the balance point of the bridge formed by this combi-
nation of components can be altered by the selection of R3, causing the
circuit to seek a temperature at which the thermistor resistance balances the
bridge.
Because IC A1 triggers the thyristor at zero-voltage points in the
supply voltage cycle, transient load current surges and radio frequency
interference (RFI) are substantially reduced. In addition, use of the zero-
voltage-switch reduces the rate of change of on-state current (di/dt) in the
thyristor.