Greetings Dan ...
regarding your request ...
Ron how difficult would it be to put your lesson on here -- please?
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" ...
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 ...
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.
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 ...