Overload protection is provided by fuse F900 ond F902.
Thermal cutout TK902 in the primary circuit of T900 opens
the transformer primary circuit if the temperature inside the
analyzer rises above a safe level. TK902 resets automatically
when the temperature returns to normal.
-10 Volt Supply. This is the reference voltage for the
other voltage supplies and the comparator circuits in the
Type 491. Reference voltage for the -10 volt supply is
set by zener diode D964, to approximately -0.9 V on one
side of a comparator Q960-Q961. The voltage to the other
half of the comparator is obtained from the voltage divider
R967, R968 (-10 volts), R969 and diodes D967-D968. When
R968 is properly adjusted the output voltage from the regula-
tor is exactly -10 volts.
Error voltage signal is sensed by the comparator Q960-
Q961 and applied as a correction signal through the com-
plementary amplifier Q952-Q951 to the base of the regula-
tor Q960. For example, an increased current demand by
the -10 volt supply load would tend to develop a negative-
going error signal on the error side of the comparator. This
produces a positive-going correction signal to the base of
Q950 and the extra current demand of the load is supplied.
The regulator circuit can never completely compensate
for changes in output voltage, because there must be an
error input for the circuit to operate. However, any error
in the output is reduced by a factor equal to the loop gain
of the regulator circuit.
+10 Volt Supply and Regulator. Error sensing far the
+10 volt supply is accomplished by the amplifier Q930.
A sample of the +10volt supply is applied from the voltage
divider, R934-R935 in series with the regulator Q920, to the
base of Q930. This voltage sample is compared with the
-10 volt emitter reference voltage. D932 provides thermal
compensation for Q930.
The amplified error signal from Q930 is emitter-coupled
through Q922 to the base of the regulator Q920. Q921 is
connected in cascade with Q922 and isolates the collector
of Q922 from the rectifier output. This reduces the amount
of the power supply ripple couple into the regulator circuit.
The +10 volt supply provides power for the POWER
indicating light B948. Current is also supplied from the
+10 V unregulated supply, through transistor Q940 to the
scale illumination lights.
The SCALE ILLUM control R940
sets the current through the illumination circuit and controls
lamp brightness.
Line signal for the LINE position of the SOURCE switch
is provided from pin 17 of the secondary winding of T900.
+150
Volt Regulated Supply. Error sensing for the +150
volt supply is provided by Q911. Error signal voltage in
the +150 volt supply is amplified by the cascode amplifier
Q911-Q910 and applied through the emitter follower Q900
to the base of the regulator Q901 as a corrective signal.
Diodes D904 and D905 protect Q900 from excessive volt-
age transients between the collector and emitter of Q900.
3-15
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