Switching And Heat Dissipation; Eads North America Test And Services - Racal Instruments 1256 Manual

1256 User Manual
Switching and
Heat Dissipation
Getting Started 2-8
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The 1256 accommodates all Adapt-a-Switch plug-ins, which gives
it exceptionally high-density switching capability. When switching a
large quantity of signals near its maximum rated loading current, a
plug-in generates heat due to the resistance of its conducting
paths and relay contacts. 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 plug-in requires de-rating, and to what degree,
depends on the type of plug-in, its switching density, and the
ambient temperature. The following guidelines provide practical
de-rating for plug-ins used in the 1256 chassis.
Signal Switch Plug-Ins
Signal switch plug-ins consist of low- to moderate-current modules
such as the 1260-117 and 1260-118. For these plug-ins, up to
25% of the channels on each plug-in may be simultaneously
closed under full-rated current if the ambient temperature is 50
or less. If the ambient temperature is greater than 50
the current by 20%.
Power Switch Plug-Ins
Power switch plug-ins are high-current modules such as the
1260-120 and 1260-121. For these, their full rated current is
allowed on all channels simultaneously at ambient temperatures
o
up to 50 C. For ambient temperatures above 50
current by 20%.
Digital I/O, RF/Microwave, Matrix, and Multiplexer Plug-Ins
Digital I/O (e.g., 1260-114), RF/microwave (e.g. 1260-150), matrix
(e.g., 1260-145), and multiplexer (e.g. 1260-138A) plug-ins require
no de-rating. As always, the current through any connector pin
must not exceed the rated channel current. For example, if current
passes simultaneously through two inputs of the same multiplexer,
the common side carries the current from both. The common
current must not exceed the rated channel current.
Publication No. 980855 Rev. A

EADS North America Test and Services

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