Studio Technologies IFB Plus 2001 - Manual (Page 30)

Issue 7, November 2001 Model 2 User Guide
Page 30 Studio Technologies, Inc.
Model 2 Central Controller
Power Supplies
The Model 2 contains three independent
power supply circuits. It was felt that
reliability would be enhanced by splitting
the circuitry loads, and then optimizing
a power supply for each. Using this
scheme, the Model 2 will run reliably, even
with wide swings in ambient temperature,
varying operating duty cycles, and mains
voltage fluctuations. The result is a unit
that should prove quite hard to kill! The
power supplies use separate step down
transformers, the primary side of each
containing two 120V windings. This allows
them to be configured for nominal mains
voltages of 120 or 220/240V. For 120V
operation the primaries are connected
in parallel; for 220/240V operation they
are connected in series. The configuration
is performed using jumper straps on the
circuit board. For safety, a fuse is in series
with the incoming mains power.
Because the Model 2 is intended for
continuous operation, a power switch
is not included. This serves several pur-
poses: eliminating the chance of a power
switch being accidently turned off, main-
taining physical isolation between the
nasty 50/60Hz fields and the sensitive
analog circuitry, and eliminating the physi-
cal space required by a switch.
The first power supply generates filtered
and regulated ±12Vdc. Its transformer
has dual 17V secondaries which are
connected in series. The series connec-
tion point provides circuit common, as
well as being strapped to the metal chas-
sis and the ground pin of the power entry
connector. The transformer’s secondary is
fed to a full wave diode bridge. The output
of the bridge is filtered with electrolytic
capacitors, producing nominal ±24Vdc.
Two integrated circuit regulators produce
±12Vdc from the unregulated ±24Vdc.
Capacitors on the outputs of the regula-
tors provide stability. Interesting technical
note department: notice that a diode is
connected from the output pin of each
regulator to circuit common. These serve
to keep the ±12Vdc rails at, worst case,
one diode drop (0.7V) away from its
ground pin, i.e., the +12V regulator will,
worst case only, go to –0.7Vdc. This is
important when supplying bipolar loads,
such as operational amplifiers. Without
these diodes the regulators can “latch-up”
upon mains power being applied or re-
moved. When mains power is initially
applied one of the supplies can “come
up” (get to its operating voltage) sooner
than the other. This voltage is fed back
through the loads (e.g., the op-amps) to
the output pin of the other regulator that is
still coming up to full voltage. Upon seeing
this unexpected opposite polarity voltage
on its output pin, the regulator may get
very unhappy, possibly latching into a
nonoperating state, drawing lots of current
and burning up! The protection diodes
keep this condition from happening.
The Model 2 contains an LED indicator
light that displays the presence of mains
power being provided to the unit. The
LED is powered by the –12Vdc power
supply bus.
The second power supply generates
unregulated nominal +34Vdc. Its trans-
former has dual 10V secondaries which
are connected in series. The 20Vac, which
under low to moderate loads is quite a bit
higher, is fed to a full wave diode bridge
and a electrolytic filter capacitor which