Studio Technologies 42 - Manual (Page 12)

Model 42 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 13
action will depend on the specific over-
load condition that is present. In general,
the more extreme the overload condition,
the sooner normal operation will cease.
Restoring the output load to be within the
rated 200 milliamperes will allow the
output to again operate normally. A few
seconds may be required from the time
an overload condition is removed and
when normal operation again takes place.
Please don’t test the Model 42’s ability to
sustain frequent overload or short-circuit
conditions! The long-term reliability of the
unit can be impacted by the stress caused
by these fault conditions.
If there is concern that excessive loads
are being placed on one or more of the
IFB circuits, performing a simple test is
recommended. This can be performed
using any good-quality digital multimeter.
Begin by setting the meter to measure
DC current. Then place the meter leads in
series with the pin 2 lead of the XLR-type
connector associated with the IFB circuit
to be tested. The easiest way to measure
the pin 2 current is to create a simple
adapter cable using one female and one
male 3-pin XLR-type connector. Connect
pin 1 on both connectors together. Con-
nect pin 3 on both connectors together.
Connect separate wires to the pin 2 leads
on both connectors. Then connect the
meter leads to these two wires. The meter
will indicate the DC current being drawn
while normal operation of the connected
device(s) takes place. Be certain to con-
nect the maximum number of devices
that might be powered by the IFB circuit.
That is, measure the worst-case condition
and ensure that the load is within the rated
200 milliamperes output. If possible,
leaving a 10 or 20% reserve margin is
a good practice.
Technical Notes
Cable Length
There are no hard and fast rules defining
the maximum cable length possible when
connecting user devices to Model 42 IFB
circuits. The maximum cable length is
directly related to the amount of resistance
in the connecting cable; the lower the
resistance per foot (or meter), the longer
the cable can be. (Although cable capaci-
tance affects high-frequency performance,
resistance is the limiting factor in this
case.) For example, a standard 20 AWG
microphone-type cable is Belden 8412,
which has 10.9 ohms resistance per con-
ductor per 1000 feet. Since we’re using
two conductors to carry the signal (pins 1
and 2) you’d get 21.8 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 Tech-
nologies Model 200 Announcer’s Console
being connected to a Model 42 IFB circuit.
We’ll select Belden 8412 as the intercon-
necting cable. For correct operation, the
Model 200 needs at least 24 volts DC
between pins 1 and 2 of its IFB input
connector. It has a current draw of 95
milliamperes. The Model 42’s IFB circuit
presents an output voltage of 30 volts
across pins 1 and 2 and can supply a
maximum current of 200 milliamperes.
(As the Model 200’s current draw is well
within the Model 42’s capability, this is not
a limiting factor.) The difference between
the voltage supplied by the Model 42
(30 volts) and the voltage required by