High Voltage; Battery Charger; Line Input; Battery Charger Converter - Tektronix 213 DMM Instruction Manual

junction voltage. This provides a faster recovery time at
the multivibrator frequency that results in an increase in
the overall efficiency of the power supply. C544 prevents
a 700 to 800 hertz self-oscillation by the power supplies
which could otherwise be caused by the feedback circuitry.
HIGH VOLTAGE
~
High voltage is generated by an 8 stage multiplier that takes
about 190 volts peak to peak from the secondary of T 490
and produces approximately 1300 to 1400 volts across the
multiplier. The crt accelerating voltage, which is obtained
after the seventh stage of multiplication, is adjusted to ex-
actly -1000 volts by R525 and regulated to within 1 per-
cent by 0525, 0527, and 0528.
In a stable condition with the intensity turned all the way
down, 90 microamperes of focus current flows in the high-
voltage section through VR535, R565, R530, R532, R537,
0527, 0525, and the multiplier. Transistor 0528 is a con-
stant current source which is adjusted to a nominal 10 mi-
croamperes by R525 and held stable by a zener-diode regu-
lated +9 volt supply. When 10 microamperes of current
flows through R528, the supply is set at -1000 volts. Also,
a negative gate voltage is applied to 0527 where its drain-to-
source transfer characteristics allow the 90 microamperes of
focus current to flow.
When intensity is turned up, load current increases, which
causes the -1000 volt supply to decrease. Th is decrease
reduces the negative gate on 0527, which causes an increas-
ed current flow through it and the multiplier, and returns
the -1000 volt supply to its set level. A decrease in load
current would cause the opposite effect, thus a constant
-1000 volts is maintained.
Since the regulator is the feedback type, C525 prevents
self-oscillation. The first 8 capacitors in the multiplier have
a mylar dielectric which increases supply efficiency at low
temperatures.
BATTERY CHARGER
~
The battery charger consists of a line input section, battery
charger converter, and a battery charger regulator.
Line Input
The line input section contains a line filter consisting of all
components from the power-line plug through C412, and a
Power-Line Rectifier.
The Power-Line Rectifier consists of CR412, CR413, CR-
414, CR417, C418, L417, and L418. This section acts as
REV. A, JAN. 1977
Theory of Qperation-213 Service
a voltage doubler and rectifier when operating on 90 to 136
volts ac. In Option 1, the section acts as a bridge rectifier
when operating on 180 to 250 volts ac, and as a de filter
when operating on 180 to 250 volts de. Its output is ap-
proximately 330 volts de which operates the Battery Char-
ger Converter.
Battery Charger Converter
The battery charger converter is frequency controlled with
the battery charge current regulated by the control trans-
former T423.
Transistors 0422 and 0424 form a power converter. Feed-
back through R427 and pins 4 and 5 of T423 maintains
oscillation at a frequency near the LC resonant frequency
established by C427 and the primary of T430. When opera-
ting near the resonant frequency, the output voltage of the
secondary of T430 is about 3 to 4 volts peak. When this
voltage is rectified and filtered by CR434, CR435, C436,
and L435, it causes the batteries to charge.
The converter transistors 0422 and 0424 are not regenera-
tive when first turned on; therefore, CR426, a bidirectional
negative-resistance breakdown diode, is added to provide a
start pulse. As C425 begins to charge through CR425, a
point is reached where breakdown of CR426 occurs and
pulses 0424 into operation.
Battery Charger Regulator
As the battery charge increases to about 350 milliamperes,
the voltage developed across R44 7, which is sensed by 0444,
is sufficient to cause an increase in current through 0442
and pins 6, 7, 8, and 9 of T423. This increased current flow
changes the permeability of T423, which effectively reduces
its inductance and raises the operating frequency of the con-
verter. As the converter frequency shifts away from reso-
nance, the output voltage from T430 decreases and thus
reduces the charge rate.
CR427 and CR428 are fly back diodes which catch stored
energy in the LC resonant circuit to improve efficiency.
CR421, CR422, CR423, and CR424 are protection devices
for 0422 and 0424. Rectifiers CR421 and CR423 prevent
a short circuit of the supply by insuring that one transistor
is off before the other turns on. CR422 and CR424 prevent
forward biasing of the collector-base junction during the fly-
back period. R423 and C423 prevent feedback oscillation
in the drive transformer. V R440 prevents charger runaway
should the batteries be disconnected while the charger is in
operation. When the charger reaches the breakdown voltage
of VR440, current flows through the T423 control windings
and drives the converter off resonance which reduces the out-
put voltage. CR430 and CR432 provide a negative supply
voltage to operate 0442 and 0444.
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