TM8400B-L-00
1 November 2004
9. THEORY OF OPERATION
9.1. GENERAL
The measurement theory of the Autosal is explained in Section 3.1. The following sections
explain the operation of the electronic circuitry.
9.2. POWER SUPPLY
The power supply circuitry is contained on the Power Supply and Heater Control circuit board
schematic 18962.01.04. The power supply provides regulated 5 volts D.C. for the meter
assembly, and ±15 volts DC for the meter, and unregulated ±15 volts for the conductivity
circuits. Referring to the Power Supply and Heater Control Circuit Board schematic and
assembly , VR101 provides +15 volt and -15 volt regulation. VR102 is the 5 volt regulator. All
the other power supply components are also mounted on the board except the power transformer
T1 and T2 which are mounted on the rear of the instrument chassis.
9.3. TEMPERATURE CONTROL CIRCUIT
A schematic of the temperature control circuit is shown in 18962.01.04. The temperature control
sub-system is in the form of a bridge network where one half of the bridge is formed by the
parallel arrangement of two thermistors and their corresponding matching resistors. The
thermistor matching resistors are selected to provide a bridge balance within ±20 mK of the
selected temperature and are mounted on the front panel assembly along with the temperature
setpoint switch and sensor select switch. The thermistors are mounted on the top of the tank
assembly.
Resistors R224 and R225 form the remaining half of the temperature control bridge circuitry.
These resistors along with the bridge amplifier U204 (LTC1052) are mounted on the
conductivity card assembly (schematic 18961.01.04). The bridge out-of-balance voltage
(amplified by the low noise stable-zero-offset amplifier of U204 on the conductivity card
assembly) is filtered by the resistor capacitor network R115, R116, C110 and C111 mounted on
the Power Supply and Heater Control Card assembly (schematic 18962.01.04) and input to the
high gain amplifier U107 configured as a level detector. The outputs from the comparators U107
are routed to the opto-isolator triac circuits of U101 and U102 and also the opto-isolator logic
circuits of U103 and U104. The triac signals from U101 and U102 gate the silicon controlled
rectifiers (SCR). CR101 and CR102 respectively. Each SCR (connected through a small value
resistor R101, R102) applies AC half cycle power to a heater lamp on zero crossing of the AC
line frequency when triggered. The demand for heat is determined by the setting of the
TEMPERATURE SET switch and the temperature sensed by the thermistor pair. When more
heat is required, the logic output from U107 triggers the SCR's to turn on and apply power to the
heater lamps. The opto-isolator circuits of U103, U104, U105 and U106 monitor the presence of
current flowing in each heater lamp (through R101 and R105). Should the signal from U107
request more heat but no change (increase) in current through R101 and R105 is detected,
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