HP 8133A Service Manual page 158

+
Control Power
Power-on Reset
Fan Control
7-26 Theory of Operation
No control cu.rrent
Control current producing a voltage drop below the gate threshold
voltage.
The low tapping path acts in a similar manner with the following
components:
Red 4 and Blue 4 wires of Tl,J7(1,3)
c102
R102
The unregulated low tapping path voltage UL+ 15
and X3(6), to the power control board. The function of this path is to
select which tapping path is activated (sometimes both). Line voltage
tolerance, and t.o some degree the load state, determine which tapping
path is activated.
Generally, the high tapping is activated at the line voltage that is
below the nominal value, but still within tolerance. The low tapping
is activated whlen the low voltage is below the nominal value, but
still within tolerance. Both are active
This keeps series regulator voltage drop and power losses as low as
possible.
UB
The high tapping AC voltage (from T1 wires Red 1 and Blue 1) is
used by diodes CR123 and CR124 to generate
voltage is applied to the power control board via J3( 19).
This circuit provides correct resetting of the MPU during power on,
power off and fail, by:
Monitoring thie +5.1V output
Supplying the HPSV signal to the MPU via Jl(3C).
The threshold voltage is approximately +4.85V at R119. If the +5.1V
supply equals tlne threshold at power on, the reset signal is delayed
for approximately 40ms by C120. HPSV then goes high (>4.35V). At
power down, the signal change to low is not delayed.
The fan control circuit supplies voltage for the two fans. The value of
this voltage is dependent on the temperature detected by the sensor
mounted on the
tapping voltages UL+ 15 and UL-15 and operates by current-flow to
ground. The temperature sensor, U50, on the
with a current
of
voltage drop directly proportional to the absolute temperature value
10 mVK. This voltage is compared with the threshold voltage drop
-
across R125, Which is supplied by
the control board.
Comparator U103A controls the output voltage
the threshold point is approximately 300°K (27°C) and its output
voltage, approximately 14V, is determined by the voltage drop of
at
around nominal line voltage.
with U l O l
board. It is supplied by the unregulated low
approximately
1
a
reference current of 1 mA from
fed, via J3(6)
is
+
UB. This unregulated
board is supplied
the power regulator
of
+
1). Thus,