1257 User Manual
EADS North America Defense
Getting Started 2-7
Test and Services, Inc.
©
2001
Switching and
Heat Dissipation
The 1257 accommodates a large volume of optical, microwave,
and general purpose switches, which gives it exceptionally high-
density switching capability. When switching a large quantity of
signals near the maximum rated loading current, a drawer
generates heat due to the resistance of its conducting paths, relay
contacts, and actuators (typically motors or solenoids). To prevent
excessive heat, certain guidelines apply regarding the maximum
number of channels that may carry full rated current at one time. In
a high-density switching system, it is not reasonable to have all
relays simultaneously closed, all carrying full rated current,
operating at the maximum rated ambient temperature. Some form
of de-rating is necessary.
Whether or not a drawer requires de-rating, and to what degree,
depends on the type of drawer, its switching density, and the
ambient temperature. The following guidelines provide practical
de-rating for drawers used in the 1257 chassis.
Predominantly Optical Drawers
Drawers containing mostly optical content generally have limited
actuation mechanisms and have no resistive signal path heating
because of the optical nature of the product. Under most
circumstances, unless otherwise described by the specifications of
the optical switching drawer, no additional de-rating is necessary.
Refer to the drawer manual for additional restrictions or
specifications.
Predominantly Microwave Drawers
Microwave products like the 1260-CDS switch drawers that
typically utilize solenoid actuated switches that draw significant
amounts of power (typically 350 mA) per pole. In the case of the
1260-CDS 10x10 matrix, a total of 20 relays with 10 poles each
exist in a drawer. If it was possible to actuate all poles
simultaneously, the drawer would be dissipating 840 W! Not only
does this clearly exceed the power supply capabilities of the 1257,
but this would result in a large amount of heat dissipation inside
the 1257 enclosure. As a general rule of thumb, total current
drawn by all switches and actuators should not exceed 12.5 Amps
(for a 12 Volt power supply) up to 50
o
C ambient and should be de-
rated by 50% above this threshold. Refer to the drawer manual for
additional restrictions and specifications for typical device current
drawn.
Summary of Contents for 1257
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