Tektronix 7834

and C1217 ; C1216 and C1217 discharge slowly through
R
1221 to allow for thermistor thermal-recovery time . This
ensures sufficient thermistor resistance to limit the turn-on
surge current to a safe level . Since C1216 and C1217
dis-
charge slowly, dangerous potentials exist within the power
supply for several minutes after the POWER switch is turned
off . The presence of voltage in the circuit is indicated by
the
relaxation oscillator R1219, C1219, and DS1219 . Neon
bulb DS1219
will blink until the potential across C1216and
C1217 drops
to about 80 volts .
Spark gap electrodes E1208 and E1213
are surge-voltage
protectors . When the Line Selector switch is in the
115 volt
position, only E1208 is connected across the
line
input
. If a
peak voltage greater than 230 volts is
present on the line,
E1208 will conduct and quickly open line fuse F1200
to
interrupt the input power before the instrument can be
damaged . In the 230 volt position, E1208 and E1213 are
connected in series across the line input to provide protec-
tion for peak voltages greater than 460 volts .
Transformer T1208 provides a sample of the line voltage
to
the plug-in connectors for triggering at line frequencies .
This line-frequency signal is also connected to the Inverter
control stage to sense when line voltage is present .
Start Network
Network R1210, R1242 and C1242 is connected between
the input line (ac) and the negative side of C1217 (through
T1225) . C1242 charges on each cycle of the input line
volt-
age . When the charge on C1242 reaches about 32
volts,
trigger diode CR 1238 conducts providing base drive to turn
on 01241 through C1239 . When 01241 turns on, it shock-
excites series-resonant network L1237 and C1237 to
generate a damped oscillation . This damped oscillation
pro-
vides the drive
necessary to start the Inverter switching
action . After the Inverter is operating, the recurrent
wave-
form at the collector of 01241 keeps C1242 discharged
through CR1242, thus disabling the Start Network
while
the instrument is on .
Inverter
The
Inverter stage converts the do voltage across C1216 and
C1217 to
a
sine-wave
current to drive Power Transformer
T1310 . Once the Inverter
has been started by the Start Net-
work, transformer
T1230 provides feedback to the bases of
01234 and
01241 to sustain oscillation . These transistors
operate
at a forced beta of 4 due to the turns ratio of
T1230 . Also, T1230
provides a 60-turn, center-tapped
winding for pre-regulation
and fault protection shut-down .
The Inverter Control
stage short circuits one-half of this
winding to either delay
the turn-on of 01234 and 01241 or
to completely
stop
their
switching action .
Over-Voltage Stop
Theory
of
Operation---7834
The switching action of 01234 and 01241 generates a
square-wave voltage with an amplitude approximately equal
to the do voltage at the input to this stage . The square-wave
voltage at the emitter of 01234 supplies the drive necessary
to maintain a sine-wave current in the series-resonant net-
work
of L1237 and C1237 . Diodes CR1234 and CR1241
provide
paths for series-resonant current when 01234 and
01241 are held off for pre-regulation .
To aid in understanding circuit operation, Figure 3-41 (A)
shows a representation of the Inverter stage as a switch . The
three possible states of the Inverter are depicted by the three
possible
switch positions : 01234 is on in position (a) ;
01241 is on in
position (c) ; or both transistors are held off
for pre-regulation
in position (b) . In the composite current
waveform
of
Figure
3-41(B), the relative phase and
ampli-
tude of each component
of
It
is
shown
for periods
T a , Tb,
and T c corresponding
to the
three
switch positions . Figure
3-41 (C) and Figure 3-41 (D)
show the relationship of
the
Inverter voltage and primary winding
voltages
with
respect
to the current waveform .
The normal sequence
of operation is as follows : Assume
that It is passing through
zero and is increasing in the direc-
tion which forward biases CR 1241
to
conduct 11
as
shown
in Figure 3-41(B) . When
the Inverter current crosses
through
zero the Inverter
Control stage holds off 01234 and 01241
.
At a time
determined by the Inverter Control stage, 01234
is allowed to conduct 12 which
reverse biases CR1241 .
01234 conducts
as
12
goes through its peak and back to
zero . At zero crossing,
the Inverter Control stage again
holds off 01234 and 01241 . During
this hold-off
time,
CR1234 conducts 13 . Next,
01241 is turned on to conduct
14 which reverse biases
CR1234
. 01241
conducts as
14
goes
through its peak and back
to
zero
.
The cycle
then repeats
itself .
During
conduction of 01234, power is delivered to the
series resonant
circuit L1237-C1237, and to T1310 . Part of
this power, stored in the
resonant circuit, is returned to the
supply when diode CR 1234
conducts . Pre-regulation is
achieved by varying the holdoff
of the inverter transistors,
Tb in Figure 3-41(B), thereby determining
the net power
delivered to T1310 .
Whenever the voltage across the primary of T1310 exceeds
a safe level, the Over-Voltage Stop stage stops the Inverter
to protect Inverter components from damage . For example,
this stage activates whenever the normal voltage regulating
path through 01252 and T1230 is inoperative .