PV152
2-5
PRO1200 CIRCUIT DESCRIPTION
CIRCUIT DESCRIPTION
As soon this input exceeds the zener voltage, the out-
put switches high and saturates transistor Q9. The satu-
rated transistor Q9 pulls pin 2 of IC5 at ground level.
The incorporated Led has its max emission whereas pin
5 is pulled at ground level via R44. This ground level,
applied to IC100, drops the +HTHD voltage to a low
level.
The original overvoltage protection is now is causing
an undervoltage protection.
Undervoltage protection
(Page 6-3 Sect. C-5)
The stabilized zener voltage with Z8 is used as reference
voltage for the comparator 393, pin 6. Now, the other
pin 5 is the +HTHD voltage.
If the pin 5 drops below the reference voltage, the out-
put switches low, and the transistor Q8 saturates, pull-
ing again pin 2 of IC5 low.
Protection, too low +17 volts
(Page 6-3 Sect. B-9)
If, for some reasons, the +17 volts (and all the other
voltages as well) are, even temporarily, too low, it is
then advised to shut down the +HTHD voltage (coming
from the other SMPS).
The pin 6 of IC3 is factory preadjusted, at 14 volts with
P1 (refer to the adjustment procedure)
This happens with the comparator in IC3 and its output
pin 7 saturates again Q8.
Stand-by / ON-OFF switching
(Page 6-3 Sect. F-3)
An oscillator is built up around Q1/Q2 and the trans-
former T1.
Q1 gets its base current via R3. The collector current of
the latter flows in the winding 1-3 and induces a volt-
age in the winding 5-4 ‘ encouraging’ the base current.
As soon the emitter voltage of Q1 can drive the Q2 and
saturate it, this transistor clamps the base of Q1 at
ground level and cuts off Q1. The cycle starts all-over
again.
Two opposite polarity SB voltages (+/- 9 volts) are avail-
able at the secondary side.
a) Stand-by mode (OFF)
(Page 6-3 Sect. E-2)
The voltage at contact 4 of the J4 connector (‘OFF’) is
in this case ‘high’ and this means for the optocoupler
IC1 that the phototransistor is not conducting.
The transistor Q3 is thus saturated as R6 can provide
the required base-emitter current.
The collector ‘ON/OFF primary’ of Q3 is ‘low’. Further-
more, via connector J1 (pin 2), the pin 5 of IC102 is
below its “active level” via the diode D105, disabling
the drive output.
As a conclusion, only the stand-by vo/- 9 SB
voltages are available.
b) Operational mode (ON)
(Page 6-3 Sect. E-2)
The I/O block of the controller board ( collector of a
transistor) pulls now contact 4 of J4 at a low level ‘ON’
to light the LED in the opto-coupler IC1.
Now, the phototransistor of the latter is saturated and
brings the base of Q3 at nearly ground level. This means
now for this transistor an OFF state.
The zener Z100 on the motherboard can now i11
volts at the gate of the thyristor allowing the charge of
the capacitor C102.
DC Fan control of the fans
(Page 6-3 Sect. A-2&3)
The speed of the fans is regulated by means of a sensor
(NTC resistor) mounted close to the heatsink of the SMPS
board.
IC2 is an integrated circuit regulating the speed of the
fans by adapting the duty cycle of the output drive for
the power transistor Q4. L1 and C7 filters the output
voltage.
The feedback is applied to pin 5 which is protected
against arcing with D4/D5.
MC34063 is a switching regulator. An oscillator trimmed
with C6 is applied together with a dc voltage to an RS-
flipflop via an AND gate. That DC voltage now is the
result of a comparator output receiving an internal ref-
erence voltage of 1.25 volts and the feedback voltage at
pin 5 (comp). Consequently, the duty cycle depends on
the DC voltage that is built up as follows:
- it is determined by the output voltage via R13 / R14 /
R11 in order to stabilize the latter for a well-determined
value of the NTC resistor.
- it is equally influenced by any change of the NTC resis-
tor itself, sensing the heatsink of the SMPS board.
The minimun voltage is set by Z5 at approximately 7.5
volts and the maximum speed by Z4 + Z4. at 15 volts.
The maximum current output is limited by R10, and an
RC feedback straight from the output to pin 5 provides
a more regular speed at any time.
