Studio Technologies 5121 - Manual (Page 13)

Model 5121 User Guide Issue 3, October 2014
Studio Technologies, Inc. Page 13
Model 5121
Line/IFB Output Module
damage. Exceeding 120 milliamperes for
more than one second will cause the auto
shut-down mode to become active. A con-
tinuous overload condition will cause the
output voltage to cycle through a 1-second-
on, 5-seconds-off sequence. Restoring the
output load to be within the rated 120 mil-
liamperes will allow the IFB output to again
operate normally. In extreme cases, such
as where the Model 5121 is located in an
environment with elevated temperatures, a
few minutes may be required from the time
an overload condition is removed to when
normal operation will again take place.
Please don’t test the Model 5121’s ability
to sustain frequent overload or short-circuit
conditions! The long-term reliability of the
unit can be impacted by the stresses caused
by these fault conditions.
The DC Pin 2 status LED makes it simple to
know if an excessive load, or a short circuit,
is being placed on the IFB circuit. Techni-
cally the LED, under software control, pro-
vides a direct indication of the IFB circuit’s
DC output voltage. And the output voltage
is directly related to the amount of current
being drawn.
The LED lights steadily when the IFB’s
DC output is within its normal range. Dur-
ing normal operation the DC level on pin 2
of the IFB output will be approximately 28
volts. The LED will begin to flash on and off
if the level falls below approximately 24 volts
DC. This will occur when the current draw is
greater than nominally 120 milliamperes. If
the fault lasts for more than one second the
LED will flash at a faster rate. In addition,
the DC output will shut down for a 5-second
period.
There’s really only one piece of advice when
it comes to understanding how to use the
status LED: if it flashes there’s a problem
that must be corrected! The most likely
cause will be too many user devices being
connected to the IFB output connector. It’s
also possible that a wiring problem could
cause a partial or full short circuit between
pin 1 (common) and pin 2 (power with chan-
nel one audio). Troubleshooting should
prove quick and easy. Begin by disconnect-
ing the IFB user devices. Observe the status
LED and see if the problem has gone away.
If not, review the interconnecting cables and
find the fault condition. Within five seconds
of the problem being “cleared” the status
LED will stop flashing.
Cable Length
There are no “hard and fast” rules defining
the maximum cable length possible when
connecting user devices to the Model 5121’s
IFB output. The maximum cable length is
directly related to the amount of resistance
in the connecting cable; the lower the re-
sistance per foot (or meter), the longer the
cable can be (although cable capacitance
affects high-frequency performance, resis-
tance is the limiting factor in most cases).
For example, a contemporary microphone
cable is Belden 1172A which has 18 ohms
resistance per conductor per 1000 feet.
Since we’re using two conductors to carry
the signal (pins 1 and 2) you’d get 36 ohms
per 1000 feet of cable. By knowing the cable
resistance value, along with the minimum
voltage and maximum load current required
by an IFB user device, a simple “ohms law”
calculation will tell you the maximum cable
length.
Let’s use the example of a Studio Technolo-
gies Model 32A Talent Amplifier being con-
nected to a Model 5121 IFB circuit. We’ll
select Belden 1172A as the interconnecting
cable. For correct operation, the Model 32A
needs at least 24 volts DC between pins