Power Supply Calculation

smohamed

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Join Date
Mar 2008
Location
ON
Posts
108
Morning everyone,

I searched but couldn't find anything related to this question.

When looking in the manual for Allen-Bradley PLC's, they specify the 24VDC current and 5VDC current. I know for the SLC, they use these numbers ot calculate the power supply needed.

I really had two questions.

1. What is the relevance of both these currents? eg. Where or how are they related to powering the PLC and backplane?
2. I was wondering for other PLC's (ControlLogix, and other brands) what is the method to use to calculate the power supply?

I was attempting to figure out if I needed a extra power supply for Flex I/O other than the adaptor card.

Thanks!
 
Pretty much every AB PLC requires you to do a power supply check by identifying the modules that will be used in a particular system and then calculating the required 5V and 24V current consumptions to determine the size of the power supply required. There is typically a section in the user manual or ordering guide that gives you the requirements for every module.

Typically the digital input cards only require the 5V since the supply voltage is supplied externally. However, other modules like analog and specialty cards require both 5V and 24V.
 
i don't think you need to calculate by hand, just open RSlogix, configure your I/O and then select power supply. it will tell you if it's overloaded or not, what current is available and what is used. here is example for RSlogix500, don't have 5000 on this this machine but i think it was similar.

SLC-PSU.jpg
 
Last edited:
Never new about that feature.

Really I was wondering how the 24VDC and the 5VDC are actually used, like is one for the backplane and the other for something else?

I will definitely have to use this in the future though.

Thanks!
 
The 5VDC is used for general backplane communication and module logic power. The 24VDC is used for thinks like the supply for internally powered analog output modules and the relays on relay modules.

If you are really curious add modules to a test Logix500 program and see what it does to the backplane current numbers.

Keith
 

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