Sizing a Power Supply

Join Date
Apr 2020
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USA
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50
I have 120 VAC feeding a Micrologix PLC. I am feeding my I/O with 24 VDC using a power supply. If I add up the current draw of all I/O, I get 5 Amps (largest draw from a limit switch). Does my power supply need to match that value? Asking because not all I/O is on all of the time, yet, I know over sizing won't hurt. The first power supply we bought is rated at 0.42A (haven't hooked up), so want to get the proper size before we trial.
 
If I add up the current draw of all I/O, I get 5 Amps (largest draw from a limit switch).
Limit switches are generally wired to PLC inputs which don't draw much power. Are you looking at the rating for the limit switch? That would be how much current the switch can safely handle, not how much current actually flows in a particular application.
 
If you have a limit switch that draws 5 amps, there are issues with something possibly including my understanding of what you are telling us.

DC Inputs on a micrologix (or any PLC) are quite high impedance and will consume very little current. DC outputs could require more, but that depends on what the loads are. I frequently use a 60W power supply in small panels that can supply up to 2.3 amps of 24VDC for all the loads and it typically runs at less than 20% of that.

I can't suggest going forward with your very small supply without knowing more. Most modern switchers will protect themselves from an overload condition so even if you undersized it, no damage would be done, but again, I don't know enough about your hardware or machinery situation to safely recommend doing that.
 
Limit switches are generally wired to PLC inputs which don't draw much power. Are you looking at the rating for the limit switch? That would be how much current the switch can safely handle, not how much current actually flows in a particular application.

Perhaps that is what I misinterpreted. The switch spec sheet says 2.8 A @ 24 VDC.
 
If you have a limit switch that draws 5 amps, there are issues with something possibly including my understanding of what you are telling us.

DC Inputs on a micrologix (or any PLC) are quite high impedance and will consume very little current. DC outputs could require more, but that depends on what the loads are. I frequently use a 60W power supply in small panels that can supply up to 2.3 amps of 24VDC for all the loads and it typically runs at less than 20% of that.

I can't suggest going forward with your very small supply without knowing more. Most modern switchers will protect themselves from an overload condition so even if you undersized it, no damage would be done, but again, I don't know enough about your hardware or machinery situation to safely recommend doing that.

All of my I/O total is 5 amps, I was just saying a good chunk of that is from the limit switch (2.8 A). It appears I may have been misinterpreting the rating (see above).
 
It seems that you are confusing the maximum current that can go through the limit switch contact with the current that will flow through it connected to a PLC input at 24VDC, which will be a few milliamps, less than 15 mA.
 
Agreed, I did confuse that portion. So, let's backup. What rating do I use to fuse the above limit switch? Do I use the PLC input's max current rating (12 mA)?
 
Post a schematic and save us all a lot of time guessing at what you are looking at. If you don't have a schematic, make one, doing so will probably answer a lot of your questions.
 
Who fuses PLC inputs?

Good question.

Fusing is really only intended to protect the wire from overheating should a short circuit or ground fault occur. Therefore, fuses are typically sized according to the wire's ampacity.

Fusing a low voltage DC circuit can be redundant, since a fault will typically cause the PS to almost immediately reduce it's voltage output to zero.

I see no useful purpose in fusing low voltage DC input circuits. Although if the fuse is fast acting and has a low enough amperage rating, it might be able to isolate any fault that occurs in the circuit, rather than crowbarring the PS and zeroing the voltage for the entire IO system.
 
Good question.

Fusing is really only intended to protect the wire from overheating should a short circuit or ground fault occur. Therefore, fuses are typically sized according to the wire's ampacity.

Fusing a low voltage DC circuit can be redundant, since a fault will typically cause the PS to almost immediately reduce it's voltage output to zero.

I see no useful purpose in fusing low voltage DC input circuits. Although if the fuse is fast acting and has a low enough amperage rating, it might be able to isolate any fault that occurs in the circuit, rather than crowbarring the PS and zeroing the voltage for the entire IO system.


Bingo, that's why you do it. (see bold underline)
 
I started using solid-state DC distribution blocks after working on some automated assembly machines in a pharmaceutical application where we had to disinfect and wear bunny suits to get in the room. There was a worn cable that would short out, crowbarring the PSU and rebooting the PLC but only when the exact right set of machine moves occurred.

Since then all of my input circuits with flexible cables to the sensors get a protected feed, with the output also routed to an input of its own to show the status. Has saved my butt (and protected some delicate sensors) more than once.

Murr MICO and Puls PISA units are our usual choices.
 

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