TM
11-5820-765-34
Functional Description of PC101(A1) Power Supply Circuits
Voltage of all secondary windings in transformer T1 becomes zero at same instant. At
this instant:
Anode-to-cathode voltage of controlled rectifier SC R1(Q1) also becomes zero and shuts
OFF.
As anode-to-cathode voltage rises, SC R1(Q1) stays OFF until PC101(A1) supplies signal to
gate.
Clipped pulsating voltage to unijunction transistor Q101(Q1) also becomes zero at this ins-
tant.
Voltage to voltage divider R108, R109(R8, R9) builds to maximum value in approximately
0.3 millisecond.
Voltage to base 2-base 1 circuit of Q101(Q1) builds to maximum value in 0.3 millisecond.
Voltage to emitter Q101(Q1) builds slowly, due to charging time of capacitor C105(C5).
Emitter-base
1
decreases,
which
connects
C105(C5)
to:
resistor R109(R9),
gate of controlled rectifier SCR101(Q6).
SC R101(Q6) turns on.
Connects output of rectifier bridge Z102(CR18 through CR21) to gate of SCR1(Q1).
SC R1(Q1) turns on.
Results of Voltage Changes
Action described above occurs twice per cycle (120 times/second with 60 Hz input).
SC R1(Q1) is fired at same instant of each cycle.
IF: Output voltage changes (due to change in ac input voltage or dc output
voltage).
THEN: Base current of transistor Q103(Q3) changes.
WHICH:
Changes
current
through
R117(R17).
WHICH: Changes base voltage of transistor Q102(Q2).
IF: Output voltage increases.
THEN: Base voltage of Q102(Q2) increases.
WHICH: Increases current through R111(R11).
WHICH:
Reduces
charging
current
into
capacitor
C105(C5).
WHICH: Delays firing time of unijunction transistor Q101(Q1).
WHICH: Delays firing of SCR1(Q1).
I F : SCR1(Q1) fires later in each half-cycle.
THEN: Output voltage reduced.
WHICH: Restores output voltage to proper value.
OVERVOLTAGE
CONTROL
OVERVOLTAGE
CIRCUIT:
Components:
Silicon
controlled
rectifier
SCR2
(Q2).
Connected
in
series
across
dc
output.
CONNECTIONS TO CONTROL CIRCUIT:
Gate of SCR2 (Q2) connected to control circuit in PC101 (A1).
Control
circuit
connected
to
positive
(+),
negative
()
dc
output
terminals.
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