Eberline E-520 - SECTION III THEORY OF OPERATION; A. GENERAL; B. FUNCTIONAL THEORY

MODEL E-520
SECTION Ill
THEORY
OF
OPERATION
A. GENERAL
The
high
voltage
supply
develops
*900
volts,
which
is
applied
to
the
geiger
tube,
giving
it
the
proper
operating
voltage.
When
radiation
reacts
in
the
geiger
tube,
negative
pulses
are
generated.
These
pulses
are
coupled
into
the
amplifier
where
they
are
amplified.
They
are
then
coupled
to
the
trigger
circuit
where
they
are
converted
to
standard
size
pulses
of
power.
These
standard
pulses
are
applied
to
the
meter
driver
which
converts
them
to
standard
pulses
of
current,
averages
this
current
and
drives
the
meter.
Thus
the
meter
deflection
is
proportional
to
the
average
rate
of
radiation
at
the
geiger
tube.
B.
FUNCTIONAL
THEORY
(See
figures
3-1
and
6-1)
1.
HIGH
VOLTAGE
SUPPLY
The
oscillator
transistor
(Q1)
drives
T1
primary
and
gets
its
feedback
from
Tl's
red-orange
winding.
The
voltage
is
stepped
up
by
T1's
secondary,
rectified,
filtered
and
app-
lied
to
V1.
V1
regulates
at
900
volts.
The
current
through
Vi
is
sensed
by
Q2,
amplified,
and
used
to
control
the
current
through
Q3.
The
current
through
Q3
controls
the
bias
level
of
the
oscillator
Q1.
This
tends
to
hold
the
current
through
V1
to
a
constant
value
regardless
of
battery
voltage.
The
result
of
this
is
that
power
is
not
wasted
with
new
batteries,
just
so
it
will
function
with
lower
voltage
batteries.
This
greatly
extends
battery
life.
2. AMPLIFIER
Q4
and
QS
form
a
feedback
controlled
preamplifier
which
amplifies
the
negative
pulses
from
the
detector.
The
feedback
enhances
stability
and
the
biasing
on
Q4
protects
from
overdrive.
Q6
is
biased
just
into
cut-off
so
its
output
is
near
O
volts.
A
pulse
turns
it
on
and
the
resulting
positive
output
pulse
starts
the
trigger
circuit.
ORIGINAL
3. TRIGGER
Integrated
circuit
Al
is
connected
to
operate
as
a
mono-stable
multivibrater
whose
pulse
width
is
controlled
by
the
RC
time
constant
between
its
pins
7
and
5.
This
time
constant
is
established
by
the
setting
of
SID
(scale
selection)
which
selects
a
particular
R
and
C.
The
cali-
bration
controls
form
the
R
for
each
scale,
making
the
pulse
width
continuously
adjustable
for
calibration.
When
the
trigger
is
initiated
by
the
pulse
from
Q6
the
output
at
pin
6
goes
positive
and
holds
until
the
predetermined
time
(RC)
elapses.
4. METER DRIVER
The
driver
Q8
is
normally
off,
so
no
current
flows
through
M1.
When
the
trigger
is
on,
Q8
is
turned
on
and
current
flows.
The
amount
of
current
is
determined
by
the
voltage
on
the
base
of
Q8
and
R25.
The
length
of
time
that
current
flows
is
determined
by
the
pulse
width
of
the
trigger.
This
(current
times
time)
forms
a
certain
charge
which
is
transferred
to
C10
for
each
event
counted,
C10
discharges
through
MI,
yielding
a
certain
average
current
dependent
on
the
rate
of
input
pulses.
Changing
the
pulse
width
of
the
trigger
(i.e.,
changing
scales
or
calibration
pot
setting)
changes
the
average
current
fora
given
input
pulse
rate.
This
allows
the
meter
to
be
cali-
brated
to
read
in
CPM
or
mR/hr
at
the
detector.
The
response
time
of
M1
is
controlled
by
the
RC
time
constant
of
C10
and
R27,
the
response
control.
With
R27
set
to
low
resistance
the
time
constant
is
fast,
and
at
high
resistance
it
is
slow.
5. PHONE DRIVER
Q7
amplifies
and
inverts
the
output
pulse
from
the
trigger,
yielding
a
large
amplitude
negative
going
pulse
which
is
capacitively
coupled
to
the
PHONE
connector.