Teledyne 4020 - Hydrocarbon analyzer - Manual (Page 26)

Operational Theory Model 4020
Teledyne Analytical Instruments 10
The thermistor acts as the sensor in the flame guard circuit. Its
ambient temperature resistance is in the 100 K ohms region. When the
flame is ignited, its resistance is reduced by a factor of 100. The
thermistor is coupled to a semiconductor control circuit on the anode-
flame guard power supply PC board, which will be described in a
following section.
The cell electrodes of both the anode-igniter and flame guard
thermistor are connected to the electronics chassis by means of a plug-in
cable.
The electrode section of the cell may be removed for inspection by
turning off the power, disconnecting the electrode lead plug, and
removing the screws which retain the electrode assembly in the sensor
body.
Figure 2-1: View Inside Cabinet
2.3.1 Electrometer-Amplifier
The collector cable is coupled directly to a coaxial fitting located on
the electrometer-amplifier PC board. The PC board is located on the side
panel next to but outside of the isothermal chamber. See Figure 2-1. and
consists of an electrometer amplifier and an operational amplifier. This
circuit is a very high-gain, current-to-voltage converter circuit, having