Circuit Description—434
Start Circuit
The rectified DC power is applied in itia lly to the Power
Supply Primary Circuit by the S tart Circuit made up of
D S 1020, DS1024, Q1025, Q 1030, Q1032 and their
associated circu itry. DS1020, DS1024, C1024, R1020, and
R1024 form a relaxation oscillator w ith a very low repe
titio n rate (waveform period of approxim ately 5 seconds).
When DS1024 is on and DS1020 is o ff, the base-emitter
junction of Q1025 is forw ard biased, causing Q1025 to
conduct. The collector circu it o f Q1025 moves negative
(very near the floating comm on potential) causing the base-
em itter junctions of Q1030 and Q1032 and the diode
CR1032 to be back-biased, preventing them from con
ducting.
When DS1020 is on and DS1024 is o ff, conditions are
reversed. Q1025 is o ff and its collector rises to an approxi
mately +51 vo lt level (with respect to the floating common)
determined by zener diode V R1025. Q1030, Q 1032, and
CR1032 conduct, causing the junction of CR1032 and
R1034 to rise to an approximately +50 volt level. This
establishes a tem porary Vcc voltage supply for the T urn On
and Turn O ff circuits in lieu of the normal V cc voltage
supplied by one winding of T1085 and half-wave rectifier
CR1085. Normal c ircu it operation now takes place and a
sufficiently positive V cc voltage is generated to back-bias
and prevent CR1032 from conducting.
Current Driver
The C urrent Driver stage, Q1080, controls the current
conducted through the prim ary w inding of Power Trans
form er T1085. Q1080 is in turn controlled by signal pulses
from the Turn On and Turn O ff circuits. Quiescently
Q1080 is biased o ff, and must receive a positive-going pulse
at its base from the T urn On circuit in order to conduct
current through T 1085. The Turn O ff circ uit m onitors the
current conducted through T1085 and generates a tu rn -off
pulse when a preset maximum level of current is reached.
Zener diodes VR 1084 and V R1085 prevent the collector of
Q1080 fro m going more than 250 volts more positive that
the reference level at the anode o f V R 1085. This reference
level w ill be dependent upon the value of line voltage
applied to the instrum ent. This clamping action prevents
exceeding the em itter to collector breakdown voltage rating
of Q1080 during transform er flyback tim e when Q1080 is
turned o ff, and also aids in regulation of the negative high
voltage power supply.
Turn On Circuit
The Turn On circu it generates the pulse required to
cause Q1080 to turn on and conduct current through the
prim ary of T1085. W ith an adequate V cc voltage supply at
the em itter of Q1045, Q1045 w ill be biased in to saturation.
The collector of Q1045 w ill be w ith in about 0.2 vo lt o f the
em itter. C l046 starts to charge toward this positive level
through R1046. When the charge on C l046 reaches a level
about 0.6 vo lt more positive than the level at the gate o f
Q1050, Q1050 w ill turn on and start to conduct current to
discharge C l046 and, at the same time, forward bias the
base-emitter junction o f Q1064. When the current flow
through Q1050 falls below the level necessary to keep
Q1050 on, Q1050 w ill turn o ff and C l 046 w ill start to
charge again.
When Q1050 turns on, Q1064, which is quiescently
biased o ff, starts to conduct current through the parallel
com bination of R1064 and the prim ary winding of trans
form er T1064. T1064 couples a positive-going movement
to the em itter o f Q1065. This positive movement is coupled
back to the base of Q1064 through CR1060, R1060, and
R1062, which reinforces the turn-on bias of Q1064. The
positive movement at the em itter of Q1065 turns it on,
which causes a positive pulse to be coupled to the base of
Q1080 through CR1068 and R1068. Q1080 now turns on
and starts to conduct current through the prim ary winding
of T 1085.
Turn O ff C ircuit
To prevent Q1080 from conducting excessive amounts
o f current, the T urn O ff circuit m onitors the am ount of
current flow ing in the prim ary winding o f T1085, and
generates a turn -o ff pulse when a preset current level is
reached. The secondary winding of T1080 develops a
voltage across R1075 and C urrent Sense Adjustm ent
R1074. When this voltage level becomes about 0.6 volt
more positive than the voltage level at the gate of Q1075,
Q1075 turns on and conducts current through R1076,
CR1075, and the secondary winding of T1080. The
resulting positive movem ent at the cathode o f Q1075 is
coupled through C l076 to the base o f Q1078.
Q1078 now turns on and conducts current through the
parallel com bination of R1078 and the prim ary w inding of
T 1078. The negative movement at the collector o f Q1078 is
coupled through R1071, C l071, and CR1070 to the base
of Q 1064 in the Turn On circuit to interrup t the turn-on
process while the Turn O ff circuit is try ing to stop conduc
tio n throu gh Q1080. Transformer T1078 couples a
negative-going pulse through CR1078 and CR1079 to the
base of Q1080. The negative-going pulse at the base of
Q1080 is of sufficient am plitude to cause reverse current to
flo w through the base-emitter junction of Q1080, and
insure rapid tu rn -o ff. CR1081 and CR1082 clamp the base
of Q1080 at an approximate —4 v o lt level during trans
form er flyback.
Damping C ircuit
When the C urrent Driver stage Q1080 is turned o ff and
stops conducting current through T1085, the magnetic field
in T1085 collapses very rapidly. A fte r this flyback tim e,
high am plitude ringing is generated in T1085. Q1090,
©
3-20
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