Heathkit IM-16 - Page 47

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OUTPUT
CIRCUIT
(Figure
t0
fold-cut
from
this
pqge)
The
heart
of
theSolid-StateVoltmeter
isthe out-
put
circuit.
The
switching,
attenuating, andrecti-
fying
that
is
done
in
the
circuits shown
on
the
left
side
of
the Schematic
provide
the
correct
signal voltage
to
the
output circuit.
Whether AC
or
DC
voltage,
or resistance,
is being measured,
the meter
pointer
is driven
by
the
voltage ap-
plied
to
the
output circuit from transistor
Q1.
The
source
(S)
of
FET
Ql
is direct
coupled to the
base
(B)
of
transistor
Q5.
Transistors
Q5
and
Q6
are
used as
emitter followers
thatprovide
the
voltage
and
current required to
drive
the
meter,
which is
connected
between the
emitters
of these
two transistors.
Transistor
Q6
operates with
its
base
bias fixed
by resistors R2B
and R2g"
When
the Bias Adjust
and Zero Adjust
controls
are
properly
set,
and no
voltage applied
to the
gate
of
Q1
from
the
switching
circuits, the
emitter
voltage
of
Q5
is
equal to the
emitter
voltage
of
Q6.
Since there
is no
voltage drop
across
the
meter,
no
current flovrs through
it, and
the
pointer
remains
at zetoo
Since the
source
current
of
FET
Q1
is
constant,
and transistor
Qb
is a
direct
coupled
emitter
follower,
voltage
variations
at the
gateof
el
are
transferred
to
the
meter
circuit. Apositive
volt-
age
from the
input
cireuits to
the
gate
of
e1
will
cause
the
emitter
of
Qb
tobecomemorepositive
than
the
emitter
of
Q6,
resulting
in aforward
(up-
scale) indication
on
the
meter. A
negative
inp-ut
voltage would
lower the
voltage
at the
emitter
of
Qb
and
cause the
meter
pointer
to move
back-
ward. However,
in the DC-
position
of
theFunc-
tion
switch, the
meter
polarity
is
reversed
so
that
negative
input
voltages
cause
forward
(up-
scale) meter
readings.
The AC
Cal andDC
Cal
controlsprovide
calibra-
tion
of the
AC
and DC
voltage measurement
functions
of
the
Voltmeter"
In
the DC
position
of
the Function
switch,
for example,
with
the
Volts
range
switch
set
to
,5
and
one-half volt
applied
to
the
probe
tip, the voltage difference
between
the emitters of
Qb
and
Q6
would be
too
great
for the meter.
With
DC Cal
controlR26
in
series
with the meter,
the actual
voltage
applied
across
the meter
can be
adjusted
so
the
pointer
will
indicate the
voltage
that is applied
to the
probe
tip.
The
ohmmeter
circuit
requires
no
calibration
control. When
the Function
switch
is turned
to
Ohms, the
Ohms
Zero
control
adjusts
the
base
bias
of transistor
Q7
to
supply the
correct
amount
of voltage
(from
battery
E2
to the
gate
of
FET
Ql)
for fullscaledeflectionofthe
meter.
POWER
SUPPLY
(Figure
1t)
Either
of
two
separate power
sources
can
be
selected
with
the Power
switch. In
the
Line
posi-
tion,
the
power
supply
uses
a dual-primary
transformer
for
operation
from
either
120volts
or
240
volts,
E\/AO
Hz,
depending
on how
the
Voltmeter
was
wired
during
assembly.
The
two
primary
windings
are
connected
in
parallel
for
120
volt
operation,
or
in
series
for
240
volt
o-peration,
Aneonpilot
lamp
(NE-2H),
glows
when
the
power
cord
is
connected to
anAC
source
and
the
Power
switch
turned
to
Line. A
B-wire
line
cord
is
used, with
the
ground
lead
connected
to
the
cabinet
of
the
Voltmeter.
Silicon
diode D4
and
capacitor C4
form a half-
wave
rectifier
circuit in the
transformer
second-
ary,
Zener
diode Db
regulates
the
Br
voltage to
approximately
6,8 volts, R31
is
a
dropping
resis-
tor which
limits the
current
through
zener
diode
D5.
In the
Batt position
of
the Power
switch,
the
g-
volt
battery
is
connected, through
resistor
RBZ,
to the
B+
circuit. Zener
diode
Db
regulates
the
Br
voltage to
approximately
6.8
volts
to
compen-
sate for
an
aging
or weakening
battery
and
pro-
vides
constant B+
when
switching
from
line to
battery,