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Understanding Input Power Requirements and
Transformer Sizing
Use the graphs in Figure 10 to determine the input power requirements for the
supplies, given the power they are providing to the modules in the chassis. The
vertical axis of each graph shows the backplane power consumed by all of the
modules in the chassis; the horizontal axis shows input power requirements of the
power supply. To use the graphs:
1. Add all of the backplane power (i.e. watts) for all of the modules in the
chassis.
2. Find the number from step 1 on the vertical axis.
3. Follow that value to the right until it intersects the line on the graph.
4. Find the associated input power rating consumed by the power supply on
the horizontal axis.
For example, if the power consumption of all of the modules in the chassis is 30
watts, a 1756-PB75/B consumes approximately 40 watts of Real Power.
Figure 10
backplane power
load (Watts)
apparent power (Watts) = transformer load (VA) = real power (Watts)
backplane power
load (Watts)
1756-PA75/B (ac)
1756-PB75/B (dc)
real power (Watts)
Publication 1756-IN596A-EN-P - December 2004
ControlLogix Power Supplies
43896
13
43895
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