Studio Technologies IFB Plus 2005 - Manual (Page 25)

Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 25
Please refer to the block diagrams, located
at the end of this guide, and the schematic
diagrams, located in the separate schematic
packet, while reading this material.
Model 2 Central Controller
Power Supplies
The Model 2 contains three independent
power supply circuits. It was felt that reli-
ability 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 120 V
windings. This allows them to be config-
ured for nominal mains voltages of 120 or
220/240 V. For 120 V operation the prima-
ries are connected in parallel; for 220/240 V
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 purpos-
es: eliminating the chance of a power
switch being accidently turned off, maintain-
ing physical isolation between the nasty
50/60 Hz fields and the sensitive analog
circuitry, and eliminating the physical
space required by a switch.
The first power supply generates filtered and
regulated ±12 Vdc. Its transformer has dual
12 V secondaries which are connected in
series. The series connection point provides
circuit common, as well as being strapped
to the metal chassis and the ground pin of
the power entry connector. The transform-
er’s secondary is fed to a full wave diode
bridge. The output of the bridge is filtered
with electrolytic capacitors, producing nomi-
nal ±19 Vdc.
Two integrated circuit regulators produce
±12 Vdc from the unregulated ±19 Vdc.
Capacitors on the outputs of the regulators
provide stability. Interesting technical note
department: notice that a diode is con-
nected from the output pin of each regulator
to circuit common. These serve to keep the
±12 Vdc rails at, worst case, one diode drop
(0.7 V) away from its ground pin, i.e., the
+12 V regulator will, worst case only, go to
–0.7 Vdc. This is important when supplying
bipolar loads, such as operational amplifi-
ers. Without these diodes the regulators can
“latch-up” upon mains power being applied
or removed. 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, pos-
sibly 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 pow-
ered by the –12 Vdc power supply bus.
The second power supply generates un-
regulated nominal +40 Vdc. Its transformer
has dual 12 V secondaries which are con-
nected in series. The 24 Vac, which under
low to moderate loads is quite a bit higher, is