Eberline ESP-1 - Page 45

High-Voltage Power Supply
High
voltage
is
obtained
by
stepping
up
(T1)
the
voltage
of
the
oscillator,
(Q10),
rectifying
it
(CR10)
and
filtering
the
output
(C12,
C13,
R11).
High
voltage
is
regulated
by
feeding
back
the
output
to
control
the
oscillator.
At
tum
on,
Q103
is
on,
causing
Q10
to
turn
on.
Current
flow
through
T1
(pins
2,
3)
feeds
back
via
T1
(pins
4,
5),
turning
off
(blocking)
Q10.
With
Q10
off,
blocking
stops
and
Q10
turns
on.
This
is
a
blocking
oscillator,
the
frequency
of
which
is
limited
by
C16-R10,
maintaining
best
efficiency
of
ΤΙ.
:
The
high-voltage
output
is
fed
back
via
resistive
divider
R12
to
a
voltage
follower
(A102,
pin
3).
This
stage,
with
a
high
input
impedance,
allows
R12
to
be
large,
presenting
a
minimal
current
load
for
the
supply.
The
output
of
the
voltage
follower
is
then
proportional
to
the
high-voltage
output.
Because
the
input
impedance
to
this
stage
is
high,
CR104,
R113,
and
C114
provide
filtering
to
reduce
noise
interference.
The
filtered
high
voltage
sample
is
input
to
a
comparator
(A102,
pin
15)
that
is
referenced
to
the
high
voltage
adjusting
potentiometer,
R17.
When
the
sample
(pin
15)
exceeds
the
reference
(pin
14),
the
output
at
pin
16
goes
low,
turning
off
Q103,
which
tums
off
the
oscillator,
reducing
high
voltage.
As
the
sample
decreases
to
less
than
the
reference,
pin
16
goes
high,
tuming
Q103
on.
The
oscillator
then
runs
to
increase
the
high
voltage.
In
this
way,
the
high
voltage
is
` regulated to a value set by the HV adjusting potentiometer, R17.
The
filtered
output
of
the
high
voltage
sample
is
also
applied
to
pin
5
of
А102.
This
stage
is
connected
with
Q104
to
achieve
a
voltage-to-current
conversion,
with
current
through
R114
proportional
to
high
voltage.
Current-to-frequency
conversion
is
performed
by
the
voltage
comparator
A102,
pins
11,
12,
and
10.
The
output
at
pin
10
is
a
frequency
(Fuy)
that
is
proportional
to
high
voltage.
This
frequency
is
input
to
the.microcomputer,
which
can
then
convert
Ену
to
a
digital
value
and
display
it
as
high
voltage.
Amplifier/Discriminator
Transistor
Q106
and
the
amplifier
section
of
A103
form
a
dc-coupled
linear
amplifier.
The
gain
of
this
amplifier
is
set
by
R126
and
the
output
impedance
of
the
preamplifier,
Q106,
along
with
R127-R128
and
the
gain
of
the
preamplifier
stage.
Feedback
via
R141
provides
dc
stability.
The
dc
bias
is
set
by
R131-A132
to
half
of
+V
(2.5
volts)
for
a
linear
swing
of
signal
on
the
amplifier
outputs.
Input
protection
is
provided
by
CR103-R
122,
which
gives
а
charge
path
for
input
capacitor
C11
when
high
voltage
is
shorted.
The
amplifier
output
signal
is
coupled
to
the
discriminator
(A103,
pin
12)
with
capacitor
C110.
An output
(A103,
pin
10)
occurs
when the
amplitude
of
the
signal
(pulse)
exceeds
the
reference
(A103, pin
11)
set
by the
discriminator
potentiometer,
R16.
The
discriminator
output
is
divided
by
2
(A105),
yielding
a
binary
input
for
the
microcomputer.
The
microcomputer
counts
the binary
transitions
and
calculates
and
displays the
result
as
either
rate or
integrated
value
for the
operator.
Speaker
Pulses
that
cause
an
output
from
the
discriminator
are
input
to
the
speaker
control
either
directly
or
counted
down
for
slower audible
rate.
In
either
case, the
rate
from
the
speaker
is
proportional
to the radiation
level
at
the
detector
(probe).
The
speaker is
enabled.or
disabled
by
the
microcomputer
when the
operator
presses
the SPKR
keypad.
The d
is
enabled
when
"SPKR"
is
low
(0 volt).
ESP1.MAN/February
1992
4-3