Processor Technology Sol-PC, Manual

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PROCESSOR TECHNOLOGY CORPORATION
Sol THEORY OF OPERATION SECTION VIII
Fused primary power is applied through S5 to T1 (T2 in the
Sol-20). FWB1, a full-wave bridge rectifier, is connected across
the 8-volt secondary (green leads). The rectified output is filtered
by C8 and applied to the collector of Q1. Q1, a pass transistor, is
driven by Q2, with the two connected as a Darlington pair. The out-
put of Q1 is connected to R1 which serves as an overload current
sensor.
An overload current (approximately 4 amps) increases the
voltage drop across R1. The difference is amplified in one-half of
U2 (an operational amplifier) and the output on pin 7 turns Q3 on.
Q3 in turn "steals" current from Q1-Q2 and diverts current from the
output on pin 1 of U2. This in effect turns the supply off to reduce
the current and voltage. Note that the circuit is not a constant
current regulator since the current is "folded back" by R6 and R8.
The current is reduced to about 1 amp as the output voltage falls
to zero.
Divider network R11 and R12, which is returned to -12 volts,
senses changes in the output voltage. If the output voltage is 5
volts, the input on pin 2 of U2 is at zero volts. U2 provides a
positive output on pin 1 if pin 3 is more positive than pin 2 and a
negative output for the opposite condition.
When the output voltage falls below 5 volts, pin 2 of U2
goes more negative than pin 3. This means pin 1 of U2 goes posi-
tive to supply more current to the base of Q1. The resulting in-
crease in current to the load causes the output voltage to rise
until it stabilizes at 5 volts. Should the output voltage rise above
5 volts, the circuit operates in a reverse manner to lower the
voltage.
Protection against a serious over-voltage condition (more
than 6 volts) is provided by SCR1, D1, R2, R13, R14 and C8. Zener
diode, (D1), with a 5.1 zener voltage, is connected in series with
R13 and R2. When the output voltage exceeds about 6 volts, the re-
sulting voltage drop across R2 triggers SCR1 to short the foldback
current to ground. Since the overload current circuit is also work-
ing, the current through SCR1 is about 1 amp. Once the current is
removed, this circuit restores itself to its normal condition; that
is, SCR1 turns off. R13, R14 and C8 serve to slightly desensitize
the circuit so that it will not respond to small transient voltage
spikes.
Bridge rectifier FWB2, connected across the other T1 second-
ary, su12 and -12 V dc. The positive output of FWB2 is fil-
tered by C5 and regulated by IC regulator U1. The negative output
is filtered by C4 and regulated by U3. Shunt diodes D3 and D4 pro-
tect U1 and U3 against discharge of C6 and C7 when power is turned
off. (Note that should the -12 volt supply short to ground, the
+5 volt supply turns off by the action of U2.
VIII-7