Powering a PLC with 24VDC vs 120VAC

Anybody know any advantages of powering a PLC with 24VDC vs 120VAC?

You can run 24VDC off of a battery???

What do you actually mean. Electronics inside the PLC do anyway work with under 24V DC...

You should look at the inrush current and total wattage of the PLC power supply. The PLCs that I use allow you to choose a 120VAC or a 24VDC backplane mounted power supply. If you choose to use the 24VDC power supply, you will need to factor in the total wattage of the PLC power supply along with all of your other 24VDC requirements. I also look at the inrush current to make sure that the main 24VDC power supply can handle it. Basically, if you decide to use the PLC 24VDC power supply, then your main 24VDC power supply will have to be larger.

Regards

I prefer 24 VDC, mainly because the instruments also use that voltage. I usually use 24V for solenoids and switches as well for safety reasons.
Roy

I generally choose 24V as you can then keep the control panel classified as low voltage and outside of the NFPA 70E arc flash standard. No more suiting up to troubleshoot the PLC.

I choose to use a 120VAC PLC when I accidentally order one instead of a 24VDC PLC. That way, I don't have to return it and pay a restocking fee!

They are usually the same, except that you possibly could get away with a smaller 24VDC power supply. I don't think you can get away from a 24VDC power supply completely unless the application is really limited.

ya i have to agree with Chris

it really comes down to what you feel more comfortable working with - or what the customer prefers - alot of the companys ive done projects for require their control circits to be 24VDC for safty reasions.

it also depends on cost sometimes - depending on how large of a DC power supply you'll need.

there's more to the story ...

going from AC to DC – or vice versa? ... how deep do you want to go with this discussion? ...

basic idea: actually there MIGHT BE some significant operational differences that many people don't usually take into consideration ... some of these differences hinge upon what happens during the inevitable "power failures" that can shut things down unexpectedly – and upon what SHOULD and/or should NOT happen once the power is subsequently restored ...

here's a quote from one of my Email Quizzes (#121 Surprises at Go-To-Run) ...

Note: Be especially careful in this last set of questions because we're going to change the way the system is electrically powered as we go. We'll keep the same program – and use an MLX-1000 platform each time – but we'll shift from AC to DC power for the last two questions. This isn't an attempt to trick you – this stuff is tricky enough as it is.

Click to expand...

you can find this quiz (and the answers) by going through the "Sample Lessons" page of my website (just scroll down past the YouTube Video to find the link) ... the questions that apply directly to what you're asking in this thread are numbers 13 through 17 – but I'd STRONGLY recommend that you start at the beginning of the quiz ...

going further ... following through with this discussion requires that you tell us what type of PLC you're using ...

key point: just considering the most popular Allen-Bradley models, there can be SIGNIFICANT differences in how a PLC-5 responds – when compared to the MicroLogix, SLC-500, or ControlLogix platforms ...

and also ... are you talking about powering the processor/chassis with AC or DC – and the field I/O with AC or DC – and will you be using a UPS (Uninterruptible Power Supply) for either piece of the puzzle - or for both? ...

and finally (for now) ... in some cases it becomes necessary to consider how the system will react to a power "flicker" which MIGHT indeed go all the way OFF (to zero volts) – but which lasts less than a second or two ... OR ... are we dealing with a power "failure" which stays OFF (again to zero volts) for more than a few seconds? ...

these concepts aren't necessarily "complicated" – but most programmers don't consider their implications – until they've crashed a machine or made some other similar "mess" of things ...

Ron how difficult would it be to put your lesson on here -- please?


Based on experience in some equipment (submarine, aircraft) where loss of control is life critical I would tend to go with 24 VDC control. IF I could see may way into it I would do it on nearly all equipment to keep things standardized and to minimize spare parts inventory.

Assuming your battery has amp hour capacity to meet current load over the time frame and voltage will not droop too far when AC is lost AND you maintain battery properly (daily inspect volt meter etc and daily or weekly gravity on pilot cells) this should give utmost reliability.

This kind or reliability would be very critical in the short term ie 1 to 3 second utility power interruption / surge (lightning strike tree or bird on transformer etc). Some equipment where PLC MUST stay energized and keep track of where "things are" both in the program and on the machine are CNC machine tools, punch presses etc. VFDs may not be an issue here they often can ride thru this but will need to bring motor back to demanded speed.

For brief outage 3 second to one minute I think it is still critical to keep PLC energized in some cases. VFDs may be at the upper end of their ride thru due to capacitor aging and this may be a concern. Combustion equipment may be a concern ie boilers etc. Life critical systems in hospitals is another.

After one minute I think for much of industry it does not matter too much. The exception here may be for life critical systems ie elevator return to ground floor, circ water systems, boilers.

The thing I see in industry is too much equipment is allowed to restart automatically on return of power. Not only is this hard on power distribution but it is a big safety issue in my mind. I favor drop out relays that will require operator to press the start button for most indusrial equipment.
Yes I realize maintenance will have to go around and restart circ pumps air compressors etc.
Dan Bentler

Greetings Dan ...

regarding your request ...

Ron how difficult would it be to put your lesson on here -- please?

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the "issue" is that the Question part of that particular quiz takes up 23 pages in a PDF file ... and then the Answers take up 20 pages in a separate PDF ... that would take up a LOT of posts on the forum – and would require a lot of cutting and pasting ... honestly I just don't have time to convert it over right now ...

BUT ...

it's really easy to download ... just click the link below my signature and go to the Sample Lessons page from the menu ... scroll down and follow the Email Quizzes link ... then download the PDFs ... there's no registration necessary ...

now to the subject at hand ...

at the level I'm talking about, it's not so much the "safety" or the "reliability" of using one type of power over another ... instead it's the "surprises" that can occur when a programmer/technician/etc. believes (incorrectly) that there are NO operational difference between the various power types ...

in a nutshell, I'm not saying that one type is "better" or "safer" or "more reliable" than the other ... all I'm saying is that their CAN be DIFFERENCES in their operation ... many (most?) people don't seem to know that ...

... PLC MUST ... keep track of where "things are" ...

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along those lines, here's a brief overview of ONE PART of the lesson I mentioned ...

suppose that we have TWO normally-closed pushbuttons connected to the same SLC-5/04 processor ... ONE button uses a 24VDC power source – and is properly connected to a DC input module ... the OTHER (identical) button uses a 120VAC power source – and is properly connected to an AC input module ...

everything is working perfectly today – and HAS BEEN working perfectly for years ... current is flowing through BOTH of the input circuits – so BOTH input bits have a status of ONE ...

but now the power to the plant suddenly fails ...

here's the tricky part ... the SLC processor KEEPS ON SCANNING its program for a second or two ... (its built-in power supply has enough power to keep it chugging right along for a "brief" period) ... but ... what about those two inputs? ...

first - for AC ...

when the plant's power dies, the AC power USUALLY drops instantly to ZERO volts ... quick question: do we still have current flowing in that input circuit? ... quick answer: NO, we do not ... and so – that particular input bit changes to a ZERO status ... the SLC processor continues to scan – and IT SEES THAT INPUT GO TO A ZERO STATUS ... in simple terms, the SLC processor thinks that the button has just been pressed ... does that affect the program? ... maybe it does – maybe it doesn't ... it all depends on how the program has been written ...

now then – for DC ...

when the plant's power dies, the DC power USUALLY does NOT drop instantly to ZERO volts ... (in most cases the DC power supply's filter capacitor keeps the current flowing for several seconds) ... quick question: do we still have current flowing in that input circuit? ... quick answer: YES, we (probably) still do ... and so – that particular input bit does NOT change to a zero status the same way that the AC input did ... (note the DIFFERENCE in operation) ... the SLC processor continues to scan – and IT SEES THAT INPUT STAY AS A ONE STATUS ... in this case, the SLC processor does NOT think that the button got pressed ... does that affect the program? ... maybe it does – probably it doesn't ... once again, it all depends on how the program has been written ...

in simplest terms, the POINT is this ... there CAN definitely be a DIFFERENCE in the operation of a DC input when compared to an AC input ... debugging something like this can be a nightmare – unless you know enough to consider potential "issues" like this ... many people never think along these lines – and significant "messes" can result ...

now one step further ...

suppose that we use a PLC-5 instead of an SLC-5/04 ... same set up – same wiring – same program ... just a different Allen-Bradley platform this time around ...

in this case the PLC will usually NOT see either of the inputs change state to a ZERO condition ... that's because the PLC-5 usually STOPS SCANNING INSTANTLY as soon as the plant's power dies ... so ... you could have a DIFFERENCE in operation between a PLC-5 system and an SLC-5/04 system – even though the wiring – and the program – are both completely identical ...

hang on ... now suppose that someone decides to put a UPS on the PLC-5 system to "clean up the power" ... NOW the PLC-5 processor can "keep on scanning" just like the SLC processor did ...

illustration: the plant's power dies ... this time around the PLC-5 processor "keeps on chugging along" – and it DOES get a chance to see the AC input drop to a ZERO status – but the DC input stays at a ONE status ... once again, we could have a significant DIFFERENCE in operation - just by installing a new UPS for the processor - but not for the I/O power ...

now to compare a brief power "flicker" to a power "failure" ... suppose that we take what we've already discussed above, and decide to "fix" any potential "power-up surprises" by using the processor's "First Scan" bit ... (usually S:1/15) ...

the programmer's line of reasoning usually goes something like this:

I'll write the program to use the First Scan bit to "home" or "reset" or "initialize" the machine to a known safe condition anytime the power fails and then comes back on again ...

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here's a potential problem with an SLC-5/04 system ... suppose that the power only "flickers" OFF for a very brief period (say one second just to have a number) ... now then, a normally-closed AC input will usually go to a ZERO status (as explained above) ...

quick question: can we absolutely count on the First Scan bit to fire – and do our programmed "home" or "reset" or "initialize" function – and prevent a potential mess? ... quick answer: no, we can't ... the point is that if the processor keeps right on scanning (through that brief power "flicker"), the First Scan bit will NOT fire ... oops! ...

like I said, that's just an overview – there's more in the actual lesson ... and I have at least three more quizzes planned along those same lines which go into even more detail ... they're all laid out - I just need to find time to write them down and post them ...

The thing I see in industry is too much equipment is allowed to restart automatically on return of power. Not only is this hard on power distribution but it is a big safety issue in my mind. I favor drop out relays that will require operator to press the start button for most industrial equipment.

Yes I realize maintenance will have to go around and restart circ pumps air compressors etc.

Click to expand...

you and I are on the same track – and one size does NOT fit all ... (unmanned pump stations, etc. come to mind) ... I tend to not enter into too many debates about what's RIGHT and what's WRONG ... I just do everything that I can to make sure my students understand the underlying principles of how the PLCs operate ... once THAT happens, then much of the fog and confusion goes away ...

final thought (for now at least) ... most people think that there are NO differences in the operation of AC inputs when compared to DC inputs – as long as you use the proper input modules to match the input signals ...

that is an INCORRECT assumption ... there CAN be differences ...

Ron

Thanks for reply. I thought putting your course work here would be fairly long which is why I asked "if difficult".

I was a bit too simplistic when I chose a battery as a source for 24VDC to supply PLC. Was mostly thinking of CPU and did not include enough about inputs and outputs - guess I blanket covered them with "where things are" statement.

Brother makes good money fixing CNC machines after they crash sometimes with great damage. Sometimes this is caused by programming and sometimes something else. Sometimes they never find the real cause, but you can bet the programmer guy is always called to task anytime it happens. What they do now is run the first part on something soft like wood to check the programming.

What bugged me being in Safety was the operaotors who would trust the PLC to keep the drive motors turned off while doing tool changes. I wanted some way to disconnect the motor but Equipment Eng said it could not be done. I did not know enough of PLC at the time 1993 to 5. Looking back maybe I should have talked them into sending me to PLC school so I would have been better able to deal with this. Ended up going to school on my own in 2001 and still do not have the answers just a better comprehension of the questions.

Dan Bentler

a simple one liner 110v hurts more than 24!

Parky,

Excellent answer, my thoughts exactly.

My theory is, if 24V is an option, use it. It is a lot harder to die from 24V, than it is 110V. I'm sure alot of us here have taken a 110V hit, at some time or another. We were just lucky.

I had a GFCI save me from a 110V hand to hand, while checking christmas lights. I have respect for it.

The original question was is it better to power PLC from 24 VDC than 120 VAC.

From the safety standpoint lower voltage is safer based on that criteria alone. Does it make a cabinet with 480 safer? What if there are fairly large capacitors in the circuit?
Automotive ignition starts with only 12 V but grab a spark plug wire - it will not electrocute you but I guarantee you will regret nearly bashing your brains out on the hood.
Dan Benter