Paralleling Multiple Power Supplies With Blocking Diodes for 24VDC PLC Power

Hi, it’s been a while since I posted here.
I looking to power a PLC that don’t have continuous power at a remote site. The PLC only needs power when one of the various power systems are running  on occasion. The power systems are various voltages from 12VDC to 120/240VAC. For simplicity and reliability, I don’t want to use relays and timers and zero power consumption till after a power system comes on. 
For each power system, I was thinking of possibly using an appropriate sized Class II power supply with a blocking diode between the power supply and 24VDC bus that will power the PLC. If two or more power supplies are on line, my presumption would be that the higher voltage one (power supply A) would power the 24VDC bus and the other power supply(s) (power supply B) would parallel or likely idle. As soon as power supply A drops out, power supply B will pick up the 24 VDC bus and keep the PLC powered. Any unforeseen issues with proposed concept?

Jerry, basically this should work..... But...- Use schottky diodes on each feed to minimise voltage loss- Use regulated supplies if possible- Have each supply as near to 24V as possible - a bit over rather than a bit under- obviously your 12V supply, if used, would have to be stepped up to 24V, you should be able to get a box that will do this- Put a reasonable sized capacitor (I am thinking 220/470 uf 35V electrolytic) on your main 24V bus after the diodes to smooth out any transients while switching from one supply to the otherBasically you'll have to trial it, make sure you don't have any issues with the PLC on switch on or on transfer from one supply to the other.  The more careful you are with regulation and matching, the less problems you are likely to have.There are hot-swap power control ICS available, they may be a solution but try something simple first.

There are both power supplies that can be paralleled and also modules that will act as very low voltage diodes (AKA active diode modules).In the case of the former they will actually share the load so each you can boost the current to above that each supply can deliverMost industrial component suppliers will have supplies or modules that will meet your needs..
Here is a Digikey app note that covers parallel power suppliesThis is a Phoenix module that ORs power supplies, probably active diodes.
The Weidmuller 9916250000 (our design) is an example of a power supply that has a load share feature that allows them to be paralleled. At the risk of self promotion, my employer makes some active diode modules: the 330283 and the 330345 (Class 1 Division 2 /
Zone 2)However there are many more variations. We have many competitors!

Thanx for the replies. They both have given me some food for thought

We use this module from Phoenix Contact to OR power supplies in power generation industries.Thanks. 

The Phoenix QUINT4 20A also allows for paralleling and N+1 redundancy operation. There is a description in the manual