PLC controlling different devices

[Shohadawy]

I know that the operation of the PLC is based on passing the driving electrical signal or not passing it according to the sequence of its program. If I'm connecting a 24vdc solenoid valve to a PLC and a 220 VAC motor to the same PLC what driving signal shall I apply to my PLC, 24vdc or 220vac?

 

[SPDCHK]

PLC's are computers with normal IC's (CPU) inside like you will find in your desktop PC. These IC's run off (normally 3.6VDC to 5VDC). The total power supply range to a PC is 12V that is regulated to lower voltages for the motherboard and the 12V is used by the electrical motors spinning the Hard drive and CD ROM motors etc.

The AC power feeding the computer is stepped down by either switch mode or transformer power supply to get the desired DC voltage.

Industrial control systems operate on 24VDC as a standard. Therefore all electronic devices (i.e. transmitters, recorders, local controllers, etc.) run either on 24VDC or via AC stepped down by a power supply.

A PLC used to control equipment is usually fed by a UPS backed up power supply. In our case we use 220VACto power external 24VDC/5A power supplies, connected to the SIEMENS PLC. Other applications use only redundant 24VDC supply directly to the PLC racks powering both the PLC and the I/O. Some PLCs may allow for an AC voltage input.

When actuating 220VAC devices, the I/O is usually rated for the for the desired voltage, i.e. you will buy either a 24VDC Digital input/Output card or an 110VAC or 220VAC Digital input or output card. The operating voltage of the I/O cards are usually only 24VDC. (Reduce the risk of being shocked to death while trying do fault finding)

To answer your question, you will most likely find that the PLC being used only allows for a 24VDC power input in any case. How you get the 24VDC is up to you.