Choosing/ connecting power supply

[omrontempo 0]

Hey guys I have an control panel which has few missing pieces. The control panel controls 2 electrical motors, 1 of them through VFD. The system also has 6 electrical valves, which control the water flow in the pipes. The solenoids of the valves are switched thorugh a power optocoupler. The most important part missing is PLC. I can manage choosing the suitable one, but the problem is about 24V power supply. Now I assume I add up max current which is drawn by the system. Valves are 11 Watts, that gives about 0,5 amps, 6 of them equals 3 Amps. Flow meter takes 0,2 A, motor contactors take 0,32 A, PLC takes something and sensors. So I figured Ill choose 5A power supply. The part, which is bothering me, is the switching of the valves. I switch them through optocoupler, but they still all are connected to the same supply source. Will it cause any harm too plc? Should I install two different supplies? Which power supply should I choose? Thanks

[Bering C Sparky 7]

Hello, First you will need to select a PLC and see what power requirements it has. If you are planning to have all components you describe working at the same time you may need larger than 5 amp power supply. Also this would give you some room for future expansion. (you will be able to determine this once you have chosen all of your components) As far as damaging the PLC the best advice i can give is FUSES, FUSES, FUSES. As long as you have proper overcurrent protection in all the right places your PLC will be fine. You of course want to fuse the primary and secondary of the power supply, the PLC itself and make sure the outputs of the PLC are fused at or below what they are rated for. I usually over do it a bit when fusing for I/O, it adds a layer of protection and helps me sleep well at night knowing that if something happens it will be easy to see what it is and fix it fast. (If something goes wrong when I am asleep my crew does not hesitate to wake me up) The ship I work on has several PLC systems, some where installed long ago and the I/O is fused in such a way that if one sensor or load device shorts or a cable is damaged we end up with several pieces of machinery down, and you have to do extensive trouble shooting to find the one that is the problem. (Time = Money during production) If you fuse each of the outputs to the coils on your valves and one goes bad it will just pop the one fuse and everything else will keep right on working, you will see the fuse blown and know exactly what the problem is right away. If you group several together on one fuse and it goes pop then you have multiple functions down and have to figure out which is the problem. I am speaking of small applications like the one you describe of course where it is easy and inexpensive to do this) With inductrive coils there is a certain amout of voltage kickback when the coil is turned off and collapses. This can cause an arc acrross the contacts of what ever is switching the load and will shorten the life of the contacts or damage solid state devices. For DC coils you can add a diode to direct this kickback from the contacts of your relay or solid state device. For AC coils I usually install a RC snubber accross the coil or you could also use an MOV to limit the stray volatage. (you say you are using octocouplers and I have no experience with these so maybe this will not apply to your application) Good luck with your project and post again to let us know how it turned out. Best Regards. BCS Edited 7 Mar 2014 by Bering C Sparky