1260-21 User Manual
Racal Instruments
©
2005
Specifications 1-5
By substituting the actual values:
Total power dissipation =
[(10
A)
2
* (.020
Ω
) * 3] + [(12 V)
2
/ 80
Ω
* 3] + (9.0 W)
= 20.4 W at 55
°
C
This is acceptable power dissipation for an individual plug-in
module. The overall module power dissipation is approximately
20.4 W, which is well within the cooling available in any
commercial VXIbus chassis. In practice, rarely are more than 25%
of the module’s relays energized simultaneously, and rarely is full
rated current run through every path. In addition, the actual contact
resistance is typically one-half to one-fourth the specified
maximum, and temperatures are normally not at the rated
maximum. The typical power dissipation for each module should
be no more than 10.2 W.
Most users of a signal-type switch, such as the 1260-21, switch no
more than a few hundred milliamperes and are able to energize all
relays simultaneously, should they so desire.
Additionally, if fewer plug-in modules are used, more power may
be dissipated by the remaining cards. By using a chassis with high
cooling capacity, such as the Racal Instruments 1261B, almost any
configuration may be realized.
About MTBF
The 1260-21 MTBF is 1,500,000 hours, calculated in accordance
with MIL-HDBK-217-FN2, with the exception of the
electromechanical relays. Relays are excluded from this
calculation because relay life is strongly dependent upon operating
conditions. Factors affecting relay life expectancy are:
1. Switched
voltage
2. Switched
current
3. Switched
power
4. Maximum switching capacity
5. Maximum rated carrying current
6. Load type (resistive, inductive, capacitive)
7. Switching repetition rate
8. Ambient
temperature