Heathkit IM-16 - Page 46

In
the
first six
positions
of
the Volts
range
switch
(.5
through
150)
the
AC
voltage
passes
through wa.fers 1
and 2
of
the
switch
and is
reetified
by
diode
Dl.
In the
500
and 1500
posi-
tions
of
the switch,
diodes
D2
and
D3
form
a
series
rectifier circuit
with
Dl. The three series
diodes
are
capable of
handling
the higher
volt-
ages
on
these two ranges.
Onthe
1500range,
the
input voltage
is divided
in a ratio of
approxi-
mately 3
to
1 by
resistors
R2
and
R3
before
reaching
the diodes.
These
resistors
also form
a
DC
return
path
for
the
diode
circuit
in
all
positions
of
the
Volts
range switch.
The
rectified output
voltage from
diode
Dl
charges capacitor
C2
to the
positive peak
value
of
the
AC
voltage.
This
DC
voltage
then
passes
through resistor
R15 to the voltage
divider
cir-
cuit
of
resistors
R4 through
R14.
The
voltage
divider function
is
the
same
as
for
the
DC
volts
input circuit,
with
one
exception.
Since the
AC
input
voltage
on
the 1500
volt
range was
divided
by
resistors
R2 and
R3 in
the ratio
of
approxi-
mately 3
to
1,
the
DC
(rectified
AC)
voltage
is
divided
in
the
same
ratio
as
the
500
volt
range.
The
voltage
from
the divider
thenpassesthrough
wafer
4
of
the
Function
switch,
and resistor
R16,
to
transistor
Ql
and
the
output
circuit.
Ohmmeter
Circuit
(Figure
9 fold-out
from
Pqge
39)
When the
Function switch
is in the Ohms position,
battery
E2, resistor R33,
and transistor
Q7
form
a constant current
power
supply. Ohms
Adjust control R17
sets the base
bias
for transistor
Q7
so
the voltage
across
R14
(approximately
150 mV), which
is
coupled to transistor
Ql,
will
cause a
fullscale
deflection of
the meter.
Assume
the Ohms range
switch
to
be
in the Rx1
position,
and the Ohms
Adjust
control
set
for
full-scale
meter
deflection.
If an external re-
sistance
(Rx)
of
10
ohms
is
connected
between
the
probe
tip
and
the common lead,
it would
parallel
the
internal
10
ohm
resistor
(R14)
and
result in
a resistance of 5 ohms.
Since
the
cur-
rent
in this
circuit
is constant,
the
voltage
would
be
reduced
proportionately,
causing
the meter
to
read half scale.
Note
that
the
center
mark
on
the Ohms scale
is
10,
but
the
other
divisions
are not in
proportion
(not
linear)"
For
other
positions
of
the Ohms
range switch,
the internal resistance
between
the divider tap
and
common
always adds up
to a multiple
of
ten,
and is equal to the
center
scale
reading
of
the
meter.
lnput
Tronsistor
0l
Transistor
Q1
is a field effecttransistor
(FET).
The
high
input impedance
of
the
FET
keeps it
from loading
the
input switching
and attenuating
circuits.
A
constant
current source,
transistor
Q4,
is
used in
place
of
a resistor in the source
(S)
circuit of
FET
Q1.
Since the
electrical
operating
characteristics of
FET transistors
varywidelyfrom one
transistor
to another,
a Bias
Adjust
control
and
a
Zero
Adjust
control
are
provided.
The dual Bias
Ad-
just
control,
R18
and
R20, determines the
DC
voltage range
available tobiasthe
gate
(G)
of
Q1.
Zero Adjust
control
R19 is
used
as a finer
gate
bias
adjustment
so
that
the meter
pointercanbe
set
to the left end
zero mark when
no
signal
voltage
is
passed
through
the
inputswitchingand
attenuating circuits.
Transistors
Q2
and
Q3,
with resistor
R16,
are
used
to
protect
FET
Ql
and
the output circuit
from severe
accidental overloads.
Q2
andQ3
are
connected
in
a
parallel
circuitthatperforms
like
a
9-vo1t
zener diode.
That
is,
this
circuit
pre-
sents
an infinite
impedance
to
input voltages
up
to about
9
volts, but
acts like
a short circuit
to
higher
voltages.
Excess
voltages
would
then
be
dropped across resistor
R16.