Tektronix 422, Instruction Manual

Page: 61 / 213

Circuit Description—Type 422 AC-DC
current through voltage divider R1043-R1044 produces a
more positive voltage at the base of Q1055 to bias it into
conduction. The current through Q1055 flows through
R1054, D1054, R1047 and R1046 to pull the base of
Q1045 even more negative. This produces negative
feedback to Q1055, which makes it conduct even harder.
When Q1055 conducts, its collector drops negative to pull
the voltage at the junction of D1057-R1057 negative
enough so the POWER neon does not have sufficient
voltage applied to ignite it. However, under this condition
C1057-R1059 and the POWER neon form a neon-bulb
relaxation oscillator to provide a blinking-light indication of
low battery potential. Current flows through R1059 from
the +95-volt supply to charge C l057. As the charge on
C1057 builds up to the firing potential of the POWER
neon, the neon ignites and discharges C l057. Then the
POWER neon extinguishes until C l057 recharges to the
firing potential o f the bulb.
DC-DC Regulator
General. The DC-DC Regulator circuit produces
regulated DC output voltages fo r the indicator from the
rectified (unregulated) DC power output o f the AC-DC
Power Selector circuit. Fig. 3-13 shows a detailed block
diagram of the DC-DC Regulator circuit. A schematic of
this circuit is shown on diagram 12 at the back of this
manual.
Principle of Operation. Fig. 3-14A gives a simplified
schematic to show the operating principle o f this circuit.
Switch SW1 is closed to allow the current flo w through the
primary o f T1 (inductor) from the voltage source (battery)
to build up to a given level and produce a flux-field in the
primary o f T1. When SW1 is opened, the field around the
primary o f T1 collapses to induce a voltage into the second­
ary. The secondary voltage is rectified by D1 to provide
current to load R[_.
Notice that output current is produced by the collapsing
field o f inductor T1 (source voltage removed). Therefore, if
this field is always the same, a constant voltage will be
induced into the secondary o f T1 on each cycle. The graph
shown in Fig. 3-14B represents the linear rising current o f a
purely inductive load in response to tw o different source
voltages. With high source voltage, current through the
inductor reaches arbitrary level Y — Y' at time T i and with
low source voltage the current reaches the same level at
time T2- However, if the applied voltage is removed at the
time the current reaches level Y — Y' in each case, the
collapsing fields are the same fo r both source voltages as
shown by the shaded areas. Applying this principle to the
circuit shown in Fig. 3-14A, the same current would be
induced into the secondary o f T I in both cases since the
collapsing fields are the same.
With a constant current level being induced into the
secondary o f T I w ith changes in source voltages, to
produce a regulated output voltage across the load R|_ it is
only necessary to establish a fixed cycle in which this
action occurs. By use of idealized waveforms. Fig. 3-14C
shows how this can be done. Notice that the pulse period is
the same fo r both high source voltage and low source vo lt­
age. However, the pulse duration is changed so the current
induced into the primary of T I is constant w ith each source
voltage, thereby creating the same field in the inductor
(primary of T I). Since the pulse period is the same regard­
less of the source voltage, the voltage induced into the
secondary o f T I remains constant to produce a constant
voltage across load R|_. Filter capacitor Cl filters out any
variations during the pulse period to maintain a constant
output voltage.
This discussion gives the basic concept of the DC-DC
Regulator circuit. The method in which the pulse period
and pulse duration are changed to maintain a constant out­
put voltage is given in the following circuit discussions. Fig.
3-15 shows idealized waveforms from the DC-DC Regulator
circuit. These waveforms w ill be referred to throughout the
following discussions to show the inter-relationship be­
tween the various stages in this circuit.
Start Circuit. When the AC-DC Power Supply is first
turned on, there is no output from the Isolated Supply
since there is no voltage being induced into transformer
T1201. Therefore, a starting voltage must be supplied so
this circuit can start operation. This voltage is provided by
the Start Circuit D1192-Q1193-Q1194. The input voltage
from the AC-DC Power Selector circuit is applied across
R 1 191-D 1191-D1192. Zener diode D1192 and diode
D 1191 set the base voltage o f Q1193 at about +9.7 volts.
Transistors Q 1193 and Q 1194 operate as a voltage regulator
stage to provide about +9.1 volts starting voltage at the
positive output o f the Isolated Supply (only at turn-on
time). This voltage is sufficient fo r the Blocking Oscillator,
M ultivibrator, Steering Switch, Power Control and Isolated
Supply stages to begin operation. A fte r a few cycles the
Isolated Supply output raises to its normal +12-volt level.
Then the emitter o f Q1193 also goes positive to about +12
volts and, since its base is held at about +9.7 volts by
D1191-D1192, it is reverse biased. With Q1193 reverse
biased, its collector rises positive toward the level o f the
input voltage and pulls the base of Q1194 toward this level
also. This reverse biases Q1194 to turn o ff the Start Circuit
and isolate the input voltage from the Isolated Supply.
Blocking Oscillator. Transistor Q 1120 along w ith its
associated circuitry comprises a blocking oscillator9 . When
power is applied to Q1120 through decoupling network
C1120-R1120, Q1120 conducts through the collector
winding o f transformer T1120. The current flo w through
the collector winding o f T 1 120 induces a current into the
base winding o f T1120 which aids the forward bias of
Q1120 and quickly drives it into saturation. When Q1120
reaches saturation, the field around T1120 begins to col
lapse and a negative-going pulse is coupled to the base of
Q1120 through the base winding o f T1120 to reverse bias
Q1120. Diode D1120 by-passes the collector winding of
T1120 so the collapsing field does n o t‘change the level at
9Millman and Taub, pp. 597-601.
3-23