Telex 38109-977 manual Temperature Range Consideration, System Current/System Capacitances/Loading

System Current/System Capacitances/Loading

The system currents are determined by several parameters:

1The current required to supply standby current for each user station.

2The current required to supply the dynamic current to generate line signal, headphone signals, speaker signals and call lamp signals.

3The current required to start up a system (inrush current) by charging up to (50) 4000 microfarad capacitors or 0.2 farad.

4The current limit imposed by the power supply to protect itself.

5The secondary current limit imposed by the power supply when a fault is close to the power supply (little or no circuit resistance). This limit, called the foldback current, further protects the power handling electronic devices in the supply and determines the system start-up time.

Currents 1 and 2 can be calculated by multiplying the number of user stations times the user station current data in the Complete User Station Specifications. Current 3 is usually limited by current 5. Currents 4 and 5 are listed in the Power Supply Specifications. Current 5 can be used to calculate the system start-up time: where:

T is the start-up time (approximated) in seconds. N is the number of stations.

C is the capacitance per station = 4 millifarads i is the power supply foldback current

dV is a change in voltage across the capacitors, say 10 volts.

For a 20-station system, a 1 ampere foldback current, and a 10 volt change on the capacitors:

The actual system start-up time will be longer since voltages in each user station have to stabilize before audio can be transmitted. This time is on the order of several seconds.

Temperature Range Consideration

All of the elements of the TW Intercom System have been designed to operate over the temperature range of 0 degrees Celsius (32 degrees Fahrenheit) to 50 degrees Celsius (122 degrees Fahrenheit). The high temperature range is extended another 15 degrees Celsius if the units are not operating at full capacity or some other worst-case condition. The low temperature range is extended another estimated 20 degrees Celsius if the full system gain range is not required. The major operating problem at lower temperatures will be the dew point and the resultant condensation. If this is the typical operating environment, then it is recommended that the equipment be opened, cleaned, dried and sprayed with several light coats of plastic spray. This will lessen the noises generated by leakage currents that occur when the moisture and any dirt or film combine. Cleaning can be accomplished a rinse of alcohol, a very mild detergent (saponifier type) wash and 2 or 3 thorough rinses with distilled water. This routine is to first wash off the nonpolar soluble substances, then the polar soluble substances.

Cooling Requirements

In general, only the power supplies require cooling consideration. Normally, leaving 2 inches clearance above and below the rack-mounted supplies is adequate. Portable supplies should not be left in the sun and these supplies should have clearance of 6 inches from five of the six surfaces. All other elements of the TW Intercom System require no special consideration. It is important to note that belt packs and other equipment left in the sun can cause burns to human flesh, due to the large amount of heat transfer possible. The

40 H a n d b o o k o f I n t e r c o m S y s t e m s E n g i n e e r i n g