Tektronix 2213, Инструкция по эксплуатации

Страница: 50 / 225

Theory of Operation—2213 Service
Current from one side of the ac-power-source input will
go through L925 (a current-limiting impedance) and triac
Q925. Diodes CR931 and CR933 (on the Main board) and
CR932 and CR934 (on the Current Limit board) form a full-
wave bridge rectifier circuit. The rectifier converts the ac-
input voltage into dc pulses that charge C937. Surge
arrestor VR938, connected in parallel with C937, conducts
to protect the following circuitry should the Preregulator
output voltage become too high.
The two-transistor circuit composed of Q933, Q938, and
associated components provides over-current protection in
the event of triac misfiring or ac-power-source transients.
Transistor Q938 is an insulatd-gate FET used as a switch in
the charging path of C937. Transistor Q933 controls the
FET bias to limit the current under abnormal firing conditions
of Q925. In normal power-supply operation, the voltage
developed across R937 is not sufficient to bias Q933 into
conduction. The gate-to-source voltage of Q938 is set to
10 V by VR934 and R938, so the FET presents a low
resistance to the charging current to C937. If triac Q925
should misfire to cause excessive current, Q933 becomes
forward biased and Q938 is switched off to reduce the
current. When Q938 switches off, the current that was
flowing through Q938 flows through R939. The voltage drop
developed across R939 causes current to flow through
VR933 and R933, which holds Q933 on for most of the
remainder of the ac-power-source input cycle. Resistor
R939 limits the rate of collapse of the field around L925 to
prevent damage to Q938. Thermistor RT935 adjusts the
bias of Q933 over varying ambient temperatures.
PREREGULATOR CONTROL. The ac-source voltage is
full-wave rectified by CR903 through CR906 and applied to
a voltage divider composed of R911, R912, and R915.
Output from this divider serves as a reference voltage for a
ramp-and-pedestal comparator utilizing a programmable
unijunction transistor (PUT), Q921. Capacitor C912 filters
the line noise to prevent false triggering of the PUT.
Voltage-dropping resistor R914 provides current for zener
diodes VR914 and VR915 to produce constant voltages
during each half of the ac-power-source cycle.
When the instrument is first turned on, C917 is not
charged. Capacitor C915 charges through CR917 to the
voltage of VR915 minus the diode drop of CR917. When the
anode voltage of Q921 is greater than the gate voltage,
Q921 will fire and C915 will discharge through the primary of
T925. This event will happen after the peak of the voltage
waveform. Pulse transformer T925 is connected to the gate
of Q925, and the discharge of C915 through the T925
primary winding is coupled to the secondary to cause triac
Q925 to conduct. After firing, the triac will turn off again
when the sinusoidal source voltage crosses through zero.
As C917 charges through R917, Q918 current increases
proportionally to charge C915 more rapidly. When C915
charges at a faster rate, the anode voltage of Q921 rises
above the gate voltage earlier in the ac-source cycle and
thereby causes Q925 to conduct for a longer period of time.
The portion of the cycle preceding the zero-crossing point
over which the triac is conducting is called the conduction
angle. The conduction angle will increase from nearly zero
(at
turn
on) to an angle sufficient to supply the energy
needed by the inverter. Feedback from the inverter through
optical isolator U931 holds the correct conduction angle by
shunting current from R917. This shunting action controls
the voltage on C917, thereby controlling the increase in
base voltage on Q918. This action controls the charging
rate of C915 and therefore the conduction angle of Q925.
The Preregulator circuit can handle a wide range of input
voltages by changing the conduction angle of the triac as
the input voltage changes. As the input voltage increases,
the conduction angle will decrease to maintain the
Preregulator output voltage at a constant level. The voltage
divider composed of R911, R912, and R915 produces an
output voltage proportional to the input line voltage that is
applied to the gate of Q921. Since VR914 and VR915 hold
bias levels on Q918 constant regardless of input voltage,
the point on the cycle at which Q921 fires will vary with
changes in the ac-source votlage. This feed-forward,
together with the feedback from the Inverter through optical
isolator U931, ensures a constant Preregulator output to
the Inverter.
Prereg ulato r Board C onfiguration
The Power Input circuit converts the input ac-source volt­
age to filtered dc for use by the Preregulator.
The POWER switch (S901) connects the ac-supply
source through fuse F901 to bridge rectifier CR904. The
bridge full-wave rectifies the source voltage, and its output
is filtered by C909. Input surge current at the time of
instrument power-up is limited by thermistors RT901 and
RT902. Initially their resistances are high, but as they warm
up, their resistances decrease and they dissipate less
power. The instrument is protected from large voltage
transients by suppressor VR901. Conducted EMI is
attenuated by line filter FL9001, common-mode transformer
T901, differential-mode transformer T907, and capacitors
C901, C903, C904, and C905. Capacitors C907, C908, and
C910 form a high-frequency bypass network to prevent the
diodes in CR904 from generating EMI.
The Preregulator provides a regulated dc-output voltage
for use by the Inverter circuitry.
When the instrument is turned on, voltage developed
across C909 will charge C913 through R911. When the
voltage has risen to a level high enough that U920 can
reliably drive Q933, U920 will receive its Vcc voltage
through Q915. This level is set by zener diode VR917 in the
emitter circuit of Q917 and by the voltage divider consisting
of R912 and R913. The zener diode will keep Q917 off until
the voltage at its base reaches approximately 6.9V. Then
REV NOV 1982
3 -2 3