Product data

Publication 1785-SG001B-EN-P - June 2006
40 Select Power Supplies
Power Requirements and
Transformer Sizing
Each ac input power supply generates a shutdown signal on the backplane
whenever the ac line voltage drops below its lower voltage limit. It removes the
shutdown signal when the line voltage comes back up to the lower voltage
limit. This shutdown is necessary so that only valid data is stored in memory.
The external transformer rating (V A) of each power supply is greater than its
real input power (W) because a capacitor-input ac/dc supply draws power only
from the peak of the ac voltage wave form. If the transformer is too small, it
clips the peak of the sine wave, when the voltage is still above the lower voltage
limit. The power supply will sense this clipped wave form as low voltage and
could prematurely shut down modules in the chassis.
The following Power Load and Transformer Sizing graphs display the
backplane power load on the vertical axis.
Since these supplies have multiple outputs, the backplane power load is given
in watts.
• Use the real power value (W) for determining the amount of heat
dissipation you will have inside the enclosure.
• Use the apparent power value (V A) for estimating power distribution
sizing.
• Use the transformer load value (V A) of each power supply plus all
other loads on a transformer to determine the required transformer size.
Power Load and Transformer Sizing
0
2
4
6
8
0 255075100
Apparent Power (V A) Real Power (W )
Backplane
Current Load
(A)
T
ransformer Load (V A) =
Real Power (W ) x 2.5
0
2
4
6
8
0 15304560
0
2
4
6
8
0 40 80 120 160
0
2
4
6
8
0 255075100
Apparent Power (V A) Real Power (W )
Backplane
Current Load
(A)
T
ransformer Load (V A) =
Real Power (W ) x 2.5
0
2
4
6
8
0 15304560
0
2
4
6
8
0 40 80 120 16
0
0
2
4
6
8
0 255075100
Apparent Power (V A) Real Power (W )
Backplane
Current Load
(A)
T
ransformer Load (V A) =
Real Power (W ) x 2.5
0
2
4
6
8
0 15304560
0
2
4
6
8
0 40 80 120 160
1771-P4S, -P6S,
-P4S1, -P6S1
ac/dc
1771-P4R, -P6R
ac/dc
1771-P4R, -P6R
ac/dc
2 Units
1 Unit