MM0474
and Q712, it is necessary to provide a pre-bias from R729 and R730.
It is
important that this adjustment be made before any other adjustment in the
reference voltage generator unit.
h. 15 Volt Power Supply Circuit Description.
(1) Concerning the P100 boards, all of the power supplies required by
the bolometer bridge in the reference voltage generator units are contained in
the bolometer bridge unit.
These consist of two identical printed circuit
boards labeled P100 (refer to Figure 14). Terminals 14 and 15 are connected to
transformer T1 as indicated in Figure 15. The AC input power is rectified by
D105 through D108 and filtered by C105. The load current then flows from C105
through current sensing resistor R104 to pass-transistor Q111 mounted on the
rear panel of the bridge unit.
(2) An auxiliary power supply consisting of D101 through D104 provides
+30 volts for use within the P100 board and for the V400 board. With a 117
volt power line, approximately 60 volts appears on the collector of Q101.
Transistor Q102 compares a sample of the voltage which appears at the emitter
of Q101 with the drop across zener diode D109. As the emitter voltage of Q101
rises, the conduction of Q102 increases, increasing the drop across R103 and
decreasing the emitter voltage of Q101. This maintains the output of the +30
volt supply at nominally 30 volts. The trimmer, R108 is used to set the output
voltage.
(3) The voltage reference element for the +15 volt power supply is Q108.
It requires a constant current bias. This is supplied by a constant current
source composed of Q106 and Q107. Q106 and Q107 are complementary transistors,
cemented together for temperature tracking. The collector current of Q107 is
determined by the voltage drop across R114 and the voltage on the base of Q107.
Thus, the two base emitter drops cancel each other, and, to the extent that
they remain equal with temperature, the current through Q107 will be
independent of the temperature.
The reference voltage for the current
generator is provided by diode D110, which is itself a temperature compensated
zener element.
(4) Transistor Q108 is a hybrid device which combines an amplifying
transistor with a zener diode in the same case. The unit is designed so that
the positive temperature coefficient of the zener element is compensated by the
negative temperature coefficient of the base-emitter diode drop of the
transistor element. A sample of the output voltage appears at the slider of
R118 and is compared with the sum of the zener diode drop and the base-emitter
drop of the transistor. The collector load impedance for Q108 is the constant
current source discussed above, and consequently the voltage gain at this stage
is very high, typically about 1000. The collector of Q108 is connected to a
three stage Darlington amplifier composed of Q109, Q110 and the pass transistor
Q111 on the rear panel.
The output voltage is set by adjustment of R118.
Clockwise rotation causes an increasing output voltage. The output voltage is
also presented to Q105. Resistor R112 is selected so that Q105
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