•
•
•
The scanning time can be lengthened by switching in an unused scanning channel between the
two switched voltages. A time delay in the program will have no effect. Te k / C D S scanning
modules automatically release the previous relay a fixed time prior to the closing of a new
channel to automatically preclude two scanner sources being shorted.
The third consideration is the maximum contact breakdown of 250V DC or 1000V RMS. In DC
applications particularly, the maximum difference between any two DC sources is, as a result,
limited to 250V DC.
Example:
Input voltage on 2 successive 53A-334 channels = 1 6 5 V D C f i r s t channel; - 1 6 5 V D C
second channel
Backplane analog bus capacitance
=
2000 pF
Backplane analog bus impedance
=
1 megohm
Residual voltage after 3.0 ms
=
VR
V R = V x e-itiRCI
= 1 6 5 x e -I.003 / 1 x 108 x 2000 x 10-12)
= 37.0
For this example, the next channel to be closed is at -165V. Therefore, the voltage across the
relay contacts could be 37 - (-165), or 202V. Since 202V is more than 200V (the switching
voltage of the 53A-334 Card), an unused channel should be closed between the closure of these
two channels.
334 - 15