Heathkit IM-16 - Page 43

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CIRCUIT DESCRIPTION
For
a more
complete
understanding
of
the
cir-
cuits,
refer
to the Schematic Diagram
(fold-out
from
Page
43) while
you
read
this Circuit De,
scription.
Several
partial
schematics are
pro-
vided
to
help
explain individual
circuits.
The
Solid-State Voltmeter
consists
of
the
follow-
ing four major
sections,
which
willbedescribed
separately in
this Circuit
Description.
1.
Probe.
2.
Three
input
circuits.
3. Output
circuit.
4,
Power
supply.
PROBE
The
combination
AC-OHMS/DC
probe
contains a
switch and a
1
megohm
resistor.
When
the
probe
switch is in the DC
position,
the
1
megohm
resis-
tor
(Rl)
is
connected inseri€sbetweentheprobe
tip
and the
input
jack
of
the
Voltmeter.
This
re-
sistor reduces the
loading
of
circuits undertest.
In the
AC-OHMS
position,
the
probe
switch
by-
passes
resistor R1,
eliminating it from the
cir-
cuit.
A
voltage applied
to
the
probe
tip
is
con-
nected
through the
input
jack
to the
Function
switch.
INPUT CIRCUITS
The three
input
circuits
(AC
Volts,
DC
Vo1ts,
and
Ohms)
are
shown
on
the
left-hand side
of
the
Schematic. These
circuits
perform
the switch-
ing, attenuating, and
rectifying required
to
sup-
ply
the
correct voltages
to the
output
circuits
that are drawn
onthe right-handside of
the Sche-
matic. Approximately
.5
volt is required at the
gate
of
input
transistor
Ql
forfull-scale
deflec-
tion
of
the meter. Voltages
greater
than
.5
are
attenuated in the input
circuits.
DC
Volts
lnput
Circuit
(Figure
7 fold-out
from Poge
26)
When
the
probe
and
Function
switch are
set
to
measure
DC
volts, a
voltage appliedtotheprobe
tip
passes
through
resistor Rl
in
the probe
to the
voltage
divider
circuit
that
consists
ofresistors
R4
through
R14.
Wafer
3
of
the Volts
range
switch
selects
the
proper
tap
on
the
voltage dividerfor
the
voltage
being measured. Each
positionof
the
Volts
range
switch
is marked
on
the
front
panel
to indicate
the
full- scale voltage
for that
position.
The
total
resistance
of
the divider
network
is
fixed at
10 megohms. The
ratiobetweenthetotal
divider
resistance and the
resistance from a
selected
tap
to
common, is
the
ratio in which the
applied
voltage
is
divided
at that tap. For
ex-
ample,
jJ
the Volts
range switch is inthe
500
po-
sition,
there
is a
resistanceof 10,000ohmsfrom
the tap
to
common.
This
isone-thousandthof the
10 megohm total
divider
resistance andthe
input
voltage
will
be
divided
in
the
same
ratio.
Thus,
if
500 volts is
applied
to the
input,
.5
volt will
appear
at the
500
tap
of
the
voltage divider.
The
voltage
from
the
divider tap
passes
through
wafer
4
of
the Function
switch and resistor
R16
to input
transistor
Q1.
In the
.5
position
of
the
Volts
range switch
the
applied voltage is not
divided
since
.5
volts is required at
the
gate
of
transistor
Q1
for
full-scale
deflection
of
the
meter.
AC Volts
lnput Circuit
(Figure
8
fold-o.rt
from Poge
35)
With
the probe
and
Function
switch
set
to
mea-
sure
AC
volts,
resistor
R1
(in
the
probe)
is by-
passed.
An
AC
voltage applied
to the
probe
tip
passes
through
the
input
jackandcapacitorCl
to
waJer
1
of
the Volts
range
switch. Capacitor
Cl
prevents
passage
of
DC
voltage
so
AC
voltage
components
in
DC
circuits may
be measured.