120VAC or 24VDC power on new PLC?

I don't know if using 24 VDC started in Europe, but it became dominant there before it was common in the US. Remember, in Europe normal AC is 230 Volt. That makes the safety difference at 24 VDC more significant.

I have been using 24 VDC as much as possible for several years. A few engineering firms call for 120 VAC in their specs, but none of them can give a vaild reason. "24 VDC can't give accurate switching on long wire runs." Bull**** - just size the wire properly. "Our existing inventory is 120 VAC." Nonesense - most I/O cards are availale in stock from suppliers and I/O cards are cheap to inventory. "Our standard is 120 VAC" Dumbest reason of all - doing something because you always do it that way isn't engineering, it's superstition.


Advantages of 24 VDC:

1) Safety

2) For mciros and small PLCs I use 24 VDC for CPU power because it is one more layer keeping spikes, surges, and brownouts from my PLC

3) I can run DC and analog sgnals in the same conduit.

4) The hardware is usually less expensive.

5) Safety again
 
Mr Jenkins:
You mentioned a good point in that most people just go with 120VAC without weighing the pros and cons and just because the plant was set up on 120VAC.

Now what about redundancy of 120VAC vs 24VDC in the power supply end?

In 120VAC redundancy setup, one configuration I've seen is two 120VAC lines, one from hydro, one from ups. Down stream is a bumpless switch over unit, so if hydro fails, the bumpless unit flips over to ups, and during that switch transition, the unit will generate the power.

Now I am not familiar with the DC version of such back up.
But from a single 110VAC line, there are off the shelf AC-DC power supplies that come in pairs. So if one DC unit fails, the other takes over. But how would 2 110AC line be setup?
Thanks again for everyone's input,
George
 
Power the PLC from a battery. Now all you have to do is charge the battery. Depending on your battery size switching charge circuits in the event of a failure doesn't have to be high priority, a PLC can last hours on even a modest battery.
 
Now what about looking at the load end, for example, although some say DC sensors are cheaper, is it true some DC solenoids drain more power than AC solenoids which means the power requirements on the DC power supply is now bigger?
 
... and as a side benefit, it's quick and simple to check if you are getting noise on a DC signal; just measure the signal with your multimeter set to AC.

Could you elaborate on how this is performed? Is there a test setup I could toss together to simulate noise?
 
Now what about redundancy of 120VAC vs 24VDC in the power supply end?

In 120VAC redundancy setup, one configuration I've seen is two 120VAC lines, one from hydro, one from ups. Down stream is a bumpless switch over unit, so if hydro fails, the bumpless unit flips over to ups, and during that switch transition, the unit will generate the power.

Now I am not familiar with the DC version of such back up.

Looking at this comment (and considering in 2004 I was fresh out of University), weren't there any Quint Diode solutions for this back in 2004?

I find this particular subject interesting after working in a system that had its redundant processors powered by 2 redundant 24Vdc supplies, which were powered by two separate UPS's and each UPS itself was powered from 2 separate switchboards in the installation.
 
In general I think 24VDC has more advantages over 120VAC, but 120VAC persists simply because old habits die hard. They are getting old now but I still run into a lot of electricians who insist that “If I can’t test it with a Wiggy, it ain’t s#!t.”

For you youngster, a “Wiggy” was a popular brand of inductive voltage tester where it had an actual coil and a plunger that pulled a needle in to show the voltage level. I still like them for some things, but they were NEVER good for PLC circuits. Yet the attitude persist with a lot of older electricians used to working with 120VAC control circuits and 240-480V power circuits. In a few more years, they (we) will have all retired though. I say start off with 24VDC and use 120VAC only when absolutely necessary.
 
For DC power supplies with power => 500W much better with three-phase input, you save a large transformer, which needs space and gives heat.
 

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