difference
and
applies
a
correction
voltage
to
the
on-chip
pass
transistor
to
raise
or
lower
the
output
voltage,
as
required,
until
the
desired
output
voltage
is
maintained.
To
provide
greater
current
capacity,
external
pass
transistors
SEeecemnieered
into
ithe
circuit.
A
TIP3055
transistor
is
used
to
bring
the
current
capacity
to
a
level
necessary
to
drive
the
pass
transistor
assembly.
The
pass
transistor
assembly
consists
of
four
transistors
connected
in
parallel
with
small
valued
resistors
in
the
emitter
leads
to
provide
correct
equalization.
Foldback
current
limiting
is
provided
to
limit
the
output
current
Preece
supply
in
the
event
of
an
accidental
short
circuit.
The
foldback
circuit,
in
essence,
monitors
the
impedance
of
the
load.
When
this
circuit
sees
too
low
an
impedance,
such
as
a
short
cir-
cuit,
the
output
current
is
reduced
to
a
low
continuous
value
of
about
1
amp.
Foldback
limiting
is
a
valuable
safety
feature
in
a
power
supply
since
it
can
prevent
malfunctioning
equipment
from
being
damaged
by
excessive
currents.
The
foldback
circuitry
can,
however,
be
fooled
under
some
circumstances
and
power
supply
oscillations
can
occur.
Consider
the
case
where
a
high
power
amplifier
is
being
powered
by
the
supply.
Assume
that
the
amplifier
draws
26
amps
during
startup
and
has
a
relay
in
its
supply
lead.
When
the
relay
pulls
in,
the
amplifier
will
attempt
to
draw
26
amps,
but
the
foldback
limiting
current
will
lower
the
supply
voltage
to
limit
the
current
to
1
amp.
The
relay
will
drop
out,
the
foldback
circuit
will
put
the
supply
back
to
normal,
the
relay
will
pull
in
again
causing
the
amplifier
to
attempt
to
draw
26
amps,
the
Pordpack
Circuit
will
reduce
the
voltage,
and
so
on.
The
output
voltage
of
the
power
supply-amplifier
combination
will
now
be
oscillating.
Oscillations
of
this
type
can
be
prevented
by
making
sure
that
startup
and
operating
loads
are
below
the
rated
current
for
the
SuppLy4
feGonsinuing.oscidtlations,of
this
type
may
cause
the
over-
voltage
protection
circuitry
to
short
the
output
of
the
supply
to
ground.
The
over-voltage
protection
device
(the
0.V.P.)
protects
the
equipment
being
powered
in
the
event
of
a
power
supply
malfunc-
tion.
If
the
power
supply
voltage
exceeds
15V,
the
output
of
the
supply
will
be
shorted
to
ground.
The
0.V.P.
will
also
operate
if
series
of
pulses
greater
than
15V
is
generated
by
the
load.
If
unwanted
O.V.P.
triggering
should
occur,
turn
the
supply
off
rewelemirnutle
to
reser
the,O0.Vr.2.
(3)
