Teledyne 4020 – Hydrocarbon analyzer (special version for automatic background gas switching) - Manual (Page 94)

Maintenance Model 4020
Teledyne Analytical Instruments 76
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.
5.5.4 Electrometer-Amplifier PCB
Schematic No. B79159
Assembly Dwg. No. B79153
The ions formed in the process of burning hydrogen in the presence
of hydrocarbon components of the sample gas cause an electrical
conduction between two electrodes in the combustion chamber (or
detector cell) that is amplified by a high sensitivity and high input
impedance electrometer-amplifier circuit. The electrical output of the
electrometer-amplifier is directly proportional to the quantity of flame
ionizable hydrocarbons present.
The electrometer amplifier PC board is located on the side of sample
module, interconnected to the electronics circuitry by means of a single
8 pins cable, so that the ease of replacement of a board is maintained.
The high input impedance requires a shield, or cover, which is
removable for access, as well as a shielded input conductor.
Interconnection with the collector is made by a coaxial cable.
Although the cable and fittings are intended for coaxial service,
the cable is actually being used as a shielded single-conductor
connection. The collector cable plugs into a coaxial connector on the
electrometer amplifier PC board, which is located at the side of the
sample module.
The circuit consists of an electrometer amplifier and an operational
amplifier. It is a very high-gain, current-to-voltage converter circuit,
having an input impedance measuring in the billions of ohms. It is static
sensitive and highly susceptible to contamination, and special handling
precautions must be taken.
Because of its high impedance, the input circuitry to the electrometer
has had careful design consideration. The resistors (R2 and R3) in the
input gain circuit (see schematic) are installed on Teflon-insulated