Power Supply Sizing for Safety PLC

[shaunjones]

I'm looking for any guidelines and/or tools to use in sizing the 24Vdc power supply for a safety PLC which uses an energize to trip philosophy. In addition, is there any derating that could be done on such a system, or does one have to look at the worst case scenario (i.e. all inputs and outputs energized)in this situation?

Thanks

 

[itsmoked]

There are lots of considerations here.

For starters what are you calling "energize to trip" rings no bells with me with relation to powering PLCs.

You need to decide if you want a linear or switcher.
Panel mount, rack mount, din rail mount?
Price?
Current required?
Fusing?
Ambient temperature?
Life?
Parts count?

Better fill most these in if you want answers that are worthwhile.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ozmosis]

Typically, for safety related applications, the 24vdc power supply should be designed in according to IEC 742 for safety transformer or comparable DC/DC converter.
However, I'm not familiar with the term "energize to trip philosophy" either but for Safety related issues, the Category level will dictate the fundamentals of overall design. As an example, if relating to EN 954 with a SIL 3 Category 4 system, then the internal voltage of the failsafe modules must be used as the power supply.
So, it depends on the specific level of safety you are looking at but I would say reference IEC 742 first to get a better idea of where to start with your PS.