Theory
of
Operation—MicroLab
|
Instruction
Power
Supply
The MicroLab
|
power supply
transforms
the
input
ac
line
voltage into dc
voltages
that
are
used
by
the other
boards
in
the
MicroLab
|.
The following
voltages
are
supplied:
+5 Vdc,
+12 Vdc,
and-12
Vdc.
Some
personality
cards
require
a-5
volt
supply. The
-5
Vdc
regulator
is
mounted
on
the
MicroLab| main interconnect
circuit
board. The voltages
supplied
by
the
power
supply
are
current-limited
to
prevent
damage
if
a circuit
board
fails.
The MicroLab
|
power supply may
require
calibration.
See
Section
6
for
the
calibration procedure. Section
6
also
tells
how
to
disassemble the
MicroLab|!
to
get at
the
power
supply.
Circuitry
Switches
And
Transformer
<>
cae
The
MicroLab
|
is
protected
from
overheating
by a
thermal
switch
(S100)
located
on
the
transformer
core.
The
thermal
switch
is
in
series
with
the
line
voltage
power
cord,
and
will
open
if
the
temperature
inside
the
MicroLab
|
exceeds the
switch’s rating.
The
line voltage
is
fed to
the
primary side
of
transformer
T80.
One switch (S96)
connected
to
the
primary
side
allows
the
input
line
voltage
to
be
either 120
Vac
or
240
Vac.
The
other
switch on
the
primary side (S94) allows
the
line
voltage to be slightly higher
than
normal. Switch
S94,
when
in
the
HI
position,
adds
a
few
more
windings
to
the
primary side
of
the transformer,
allowing
the secondary
side to
maintain
a
normal
voltage.
-12
Volt
Supply
»
Ae
The
-12
volt
supply
starts
from
the secondary tap
on
transformer
T80.
A
full-wave bridge rectifier
(CR50)
rectifies
the
ac from
the transformer and
converts
it
to
pulsating
dc.
The
filter capacitor
(C10)
smoothes the
pulsating
de
and
produces
about
~22
volts
dc. The-22
volts
is
applied to Q39,
which
acts
as
a
series
regulator.
In
the
bottom
center
of
schematic
7 notice
that the
base
of
Q39
is
tied to
the
collector
of
018.
Transistor
Q18
is
part
of
the current
foldback network
that
controls
the
output
of
Q39.
The
complete network includes Q18,
027,
and
28,
and
works
in
the
following way.
5-4
When
power
is
applied,
the
base
of
Q18
is
at
about 0.6
volts, which allows Q39 to come
on.
Transistors
027
and
Q28
create
a voltage comparator.
Notice
that
the
base
of
Q27
is
connected
to
a
3
volt
zener diode. This zener
regulates the
amount
of
current
flowing
through
Q27. Q28,
on
the other
hand,
is
biased
by a
resistor
network.
Let's
assume
that
the
power
supply
is
turned
on,
and that
no
load
is
present
on
the
output.
If
the base
of
028
is
adjusted
with
R13
to equal
the
voltage
at
the base
of
Q27, the
current
through
both
transistors
is
equal.
Now,
if
we
load
the
-12
volt
supply,
a
proportionally larger voltage
change
will
occur
at
the base
of
027
than
at
the
base
of
028,
because
of
the
3
volt
zener
diode. The diode
tries
to maintain the
voltage differential
between the base and
emitter
of
Q27.
For
this
reason, Q27
begins
to
draw
more
current than
Q28. The
current drain through
Q27
results
in
a
greater
voltage drop
across
R26,
thus
increasing the
collector
current
in Q18.
And
when
you
increase
Q18's
collector
current,
you
also increase
the collector
current
through
a39.
The
RC
combination
of
R7
and
C7
on
the
base
of
018
is
used to
prevent
the
regulator,
which
is
actually
a
feedback
amplifier, from oscillating. The
RC
combination
tends
to
dampen
any
rapid
voltage changes.
Transistor
Q6
is
the
current-limiting
device.
When the
power
supply
is
operating
with
a
normal
load,
the
base-to-
emitter
voltage
is
not sufficient to
turn
on
Q6.
However,
if
the
power supply
is
shorted,
Q6
comes
into
action
Remember
that
when
the
load
is
increased,
the
voltage
comparator
tries
to
increase the current
through
Q27.
And
the increased current through
Q27
increases
the
voltage
drop
across
R26.
If
the
voltage drop
across
R26
is
great
enough,
the
collector
of
Q6 will
be forced
in
the
negative
direction far
enough
to
turn
the
transistor
on.
When
Q6
turns
on,
it
keeps
the base
of
Q18 from
going
any more
negative,
thus
preventing Q39 from outputting any more
current.
+12 Volt
Supply
”»
fa
The
+12 volt
regulator
works
in
much
the same
way
as
the
-12
volt
regulator.
The
+12 volt comparator
is
referenced
to
the -3
volts
used
by
the
-12
volt
regulator,
and
to the
-12
volts
itself.
Transistor
Q35
acts as the current
controller
for
Q49.
Transistor
036
is
the
overcurrent
protection
device.
+5 Volt
Supply
»
FEE}
The
+5
volt
supply
is
also
referenced
to
the
-12
volt supply.
It
functions
in
exactly
the same
way
as the
+12
volt
regulator.
@
Summary of Contents for 067-0892-00
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