Scion Instruments 436-GC Service Manual page 79

436-GC/456-GC
Bake out mode.
If the jumper connecting J2, 15 & 16 is removed the temperature controller will run away and provide full
power to the heater Q4, limited only by R28 and R46. Under normal ambient conditions this will cause the
Aluminum block temperature to rise to approximately 80C which is a useful bake out temperature that can
be used for cleaning of the block and associated components. Under these conditions the current in Q4 will
be 567ma, with a power dissipation of approximately 13.5 Watts.
The junction temperature of Q4 under these conditions will be about 94C. (Tjmax = 150C)
Notes:
AR1 is an extremely low input bias current opamp that was chosen because it makes high resistor values
and low capacitor values easy to use. It is also a low drift amplifier.
RT1 is a low cost surface mount thermistor with a 10% tolerance of resistance at 25C. At 45C its nominal
value is close to 2.37K.
Reference Voltage.
The reference voltage is supplied by the main board, and connects to J1,8. R50 and C29 form a simple
filter intended to reduce noise relative to Ground 2 on the EIC PWA. The buffered 5.00V reference is
connected to U4,2&7, and then it is divided down to 2.5Volts for the transducer excitation reference. It is
divided down once more to 1.25 Volts for the two transducer instrumentation amplifier offsets voltages AR2
and AR4.
Any change in reference relative to Ground 2 is ratio-metrically canceled because it causes equal
percentage changes to the DAC output and the transducer excitation current as well as the instrumentation
amplifier offset voltage.
Precision resistor pack R3 is used to divide down the reference voltage; this ensures that the temperature
drift is kept low.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 78