Tektronix 485, Инструкция по эксплуатации

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Circuit Description—4 8 5 /R 4 8 5 Service
2. At a controlled time after the resonant circuit current
passes through 0 ampere, regulator transistor Q1900 is
turned o ff by the Inverter Control circuit. When Q1900 is
turned off, the direction of current flow in the feedback
winding T1831 is such that it induces a voltage in the base
windings of T1831, which turns on Q1844 and holds o ff
Q1834. Transistor Q1844 conducts while the resonant
circuit current builds up to a maximum and falls o ff toward
0 ampere.
3. When the resonant circuit current reaches 0 ampere
and begins to increase in the opposite direction, regulator
transistor Q1900 is again turned on by the Inverter Control
circuit. This holds both Q1834 and Q1844 o ff for a
controlled amount of time. While Q1900 is on, resonant
circuit current flows through CR1844.
4. When Q1900 is turned off by the Inverter Control
circuit, the direction of current flow in the feedback
winding of T1831 is such that the induced voltage in the
base windings of T1831 turns on Q1834 and holds off
Q1844. Transistor Q1834 conducts as the resonant circuit
current increases to maximum and falls o ff toward 0
ampere.
5. When the resonant circuit current reaches 0 ampere
and begins to increase in the opposite direction the cycle
begins to repeat.
Inverter Regulator
The Inverter Regulator circuit schematic is located in
diagram 14 at the rear of this manual. The purpose of the
Inverter Regulator is to maintain constant voltages at the
semi-regulated supply outputs. This is accomplished by
varying the inverter frequency. The nominal resonant
frequency of L1835 and C1835 is 28 kHz. Regulation is
achieved by operating on the low side of resonance, in the
range of 20 to 28 kHz. At the lowest line voltage and
highest load, the Inverter w ill operate at a frequency close
to resonance. If either the line voltage is increased or the
load is reduced, the inverter frequency will decrease.
Power and phase information to the regulator circuit is
provided by current transformer T1848. CR1931, CR1932,
CR1933, and CR1934, are connected as a bridge rectifier to
deliver both positive and negative voltages. The +7.5 V at
pin 6 of U1910 is internally shunt regulated. The —2 V at
pin 7 is unregulated. VR1945 provides a stable reference
for the sensing divider string consisting of R1940, R1941,
R1942, and R1944.
U1910 contains the regulator circuit consisting of a
voltage amplifier and a variable pulse width monostable
multivibrator. Pin 15 (normally near ground potential) is
the input to the voltage amplifier. The charging ramp of the
monostable is available at pin 12. Inputs that trigger the
start of the monostable appear at pins 10 and 11. The
output at pin 9 drives the regulator transistor Q1900.
Circuit operation is as follows: In the stable state of the
multivibrator, pin 9 will be near ground, holding Q1900
off. As the inverter current goes thru zero, either pin 10 or
pin 11 will go positive depending on polarity. This positive
pulse sets the multivibrator into its unstable state. During
the unstable state, pin 9 will be positive holding Q1900 on.
The duration of the unstable state is determined by the
voltage sensed at pin 15. If the voltage is low, the duration
will be short. As the voltage increases, the duration
becomes longer.
The monostable pulse width controls the inverter fre­
quency by the fact that when Q1900 is on, both Q1834
and Q1844 are off. Power delivered to T1960 varies with
inverter frequency because the impedance of the series
resonant circuit LI 835 and Cl 835 varies with frequency.
Inverter Current Limiting Circuit
U1910 also contains the Inverter Current Limiting
circuit. Circuit operation is similar to voltage regulation
except that the pulse width of the multivibrator is varied so
that the inverter current never exceeds a safe level. The
current lim it circuit takes over control of the multivibrator
during the turn on surge or whenever an overload on a
semi-regulated supply causes the inverter current to reach
the lim it value. R1926 is the current sensing resistor.
Voltage at TP1926 w ill be the negative rectified inverter
current to a scale of approximately 0.7 V/ampere. The
current sense input at pin 13 will normally be held positive
by R1922. During current lim it, the negative voltage at
TP1926 pulls pin 13 toward ground. Multivibrator pulse
width then increases until the current limits at a value
which holds pin 13 near ground. If the circuit remains in
current lim it for more than approximately 30 ms, pin 8 will
go positive, tripping the stop monostable, which stops the
Inverter. (See protection circuit description.)
Overvoltage Stop Circuit
Q1840 and Q1846 provide a circuit to stop the Inverter
whenever the voltage across the primary of T1960 exceeds
a safe level. This circuit w ill be active whenever the
connector between the Inverter and power board is
removed or the normal regulating path thru Q1900 and
T1831 is inoperative. CR1848 charges C l848 to the peak
of the voltages across T1960. If this voltage exceeds a safe
level, VR1846 will conduct turning on Q1846. C l848 will
then discharge thru R1846 into the base of Q1840. When
Q1840 is on, Q1844 will be held o ff stopping the Inverter.
The Inverter cannot restart until CR1843 has charged
Cl 848 to the breakdown voltage of VR1831.
Line Stop Circuit and Surge Limiting
The Line Stop Circuit stops the Inverter when the
POWER switch is turned off or the AC line voltage falls
below a minimum value. This circuit function is necessary
to lim it the turn-on surge current and thereby protect the
POWER switch, Line Fuse, and Line Rectifier Bridge.
When the instrument is first turned on, thermistors
RT1821 and RT1822 have a value of approximately
3-8
REV. B, AUG. 1975