DC power supplies and PLC's - newbie question

Here is a newbie question. Are DC power supplies used for any reason in the industries that use PLCs? This is not including any PLCs themselves. The power supplies that I am asking about are stand alone supplies that have been activated because of an external control. For example, if a temperature controller sends a constant voltage via one of its AC outputs or alarms, has anyone seen an application where a DC converter was used between this and some other device? The voltage is not a communication signal via DeviceNet or something similar. This is just a voltage that can run devices. I'm constructing a device and am curious if this option would be worth the trouble. I've seen 24 VDC inputs and 10/50 VDC outputs with a certain brand PLC, but the circuit included the + and - of those PLC devices and were not connected to anything else but a switch, etc. There were no external power sources involved. If you have seen something, what was the approximate voltage or the average voltage one might see in industry? Thanks for any input.

I almost always use them!!!

Yes, 24vdc power supplies are widely used in a variety of types. Other voltages are less common.

Today, the most popular types, are din rail mounted "switchers" (switching power supplies) since they have advanced to the point that the ripple can be tolerated by all but the most sensitive instruments.

PULS is my favorite brand of external power supply. In most cases it is ill advised to use the PLC power supply to power external I/O, and if you do, it needs to be separately fused. Some tiny little self contained systems may get away with this and be fine, but in most industrial settings, you don't want field wiring hanging off your CPU power source.

PULS (click on selection guide)