"Looking for direction"

[arkyny]

No, Little Rock, Ar home of the losing hogs !!

 

[Ron Beaufort]

from your post #9 ...

Maybe if I ask specific questions I could grasp this ?

let's try that approach ...

Does the PLC run the same voltage from inputs to outputs on every device?

I'm not sure that I understand your question ... let me GUESS about what you meant to say – and you correct me if I'm wrong ...

is this what you meant? ...

Does the voltage from the input (ex: a switch in the field) run to outputs (ex: a lamp in the field) ?

well, here is what does NOT happen – and honestly, over the years, I've had MANY students believe this idea ...

the following is WRONG! ...

suppose that you have a switch wired up to 120VAC ... you cannot use that switch in your PLC program to control a 24VDC lamp ... the reason being - that when the 120 volt "juice" hits that 24 volt lamp bulb it will "blow it up" ...

if that's what you have stuck in your mind, then you need to get that OUT of your mind IMMEDIATELY ... but as I said, some students (even some with several years of experience) actually BELIEVE ideas like that ...

the reason that some people believe this notion is that they have been TRAINED to believe it by hearing statements such as:

(1) "a PLC program works just like switches and coils" ...

(2) "this XIC (contact) represents the switch in the field" ...

(3) "this OTE (coil) represents the lamp in the field" ...

(4) "when you write the PLC program, you're CONNECTING the switch in the field with the lamp in the field" ...

OOPS! ... the student gets the (incorrect) idea that somehow or other he's actually WIRING/CONNECTING the (120VAC) switch to the (24VDC) lamp through the RUNG (the "wire") inside the PLC processor ...

this is absolutely NOT what's happening – but well-meaning instructors (and coworkers – and so on) have been trying to explain PLCs to "beginners" for years using exactly this type of INCORRECT "switches and coils" analogy ...

the fact is – a PLC is NOT "just like" a bunch of switches and coils ... (in fact, it's not even "a lot like" a bunch of switches and coils - once you get past the beginner level) ...

as I said earlier, I'm just GUESSING as to what you meant by the question you posted ... I am absolutely POSITIVE that I have a correct answer for you – but I'm just not sure about your question ... if the answer that I've given doesn't "fit" with your question, just post again and rephrase your question ...

tip: instead of ASKING us – TELL us instead ... in other words, explain what you THINK happens (as though you're the instructor – and the forum members are the students) ... that way any mistakes that you have in your head will be a lot more obvious to us – and we can help you understand ...

there comes a point at which the instructor needs to LISTEN to the student – rather than TALKING to the student ... help us out here – tell us what you THINK – and be as DETAILED as possible ...

your next question:

Is the PLC capable of energizing the output device directly or through a control relay?

either way – depending on the "load" drawn by the output device – and on how much current the PLC output (its internal "switching" device) is capable of carrying ...

here are some examples - based on the "grinder" project shown near #49 on the Boot Camp Experience page of my website ... (click the picture for a larger view) ...

consider the lamps, and the exhaust fan, and the little grinder wheel motor ... all of these are very light/small loads – of less than 0.25 Amps each ... so THOSE types of devices CAN be wired directly to the PLC output screws ...

but ...

consider the "feed motor" that raises and lowers the head ... this is actually a pistol drill motor – which draws at least 4 or 5 amps when it's running ... that heavy/large load can NOT be wired directly to the PLC output screws ... that's why you see the "interposing relays" (contactors) located along the right side of the cabinet ... it varies with the model – but most PLC output circuits are only rated for about 0.25 to 0.50 amps MAXIMUM load ... any load heavier than that requires something like an "interposing" relay ...

so ... back to the "feed motor" on the grinder ...

the PLC controls the COIL of the relay (another very light load) ... when the coil is energized, the relay's contacts close (or "pick up") and the heavy/large current required by the feed motor flows through ONLY the relay contacts – and NOT through the PLC output module ...

hopefully this will help ...

 

[leitmotif]

Ice cold

I tried learn at home also. Concluded I had an idiot for an instuctor. Went to a Junior College and found myself a qualified instructor. Worked much better.

Not only do you get a bit of coaching but you get to use their equipment. I can recommend a good college in Seattle but Indiana is a bit of a reach.

Reread Bernie Carlton post - I think that is the best overview of the whole deal with PLC.

Mr Melores manuals are also good starting material. You can get em right here. Good info at a very good price. Read and read again 3X the portion on binary - good understanding of this will be needed when you get into the analog control issues.

Dan Bentler

 

[Icekold]

I do understand about only specific voltage can only be used on a device or equipment requiring that voltage.

I am understanding this is logic rather than physical, pertaining to the PLC.

I'll rephrase my question: I want to turn on a two pole contactor with a 110 volt coil. 110 volts is the control voltage to do this.
How is the PLC wired to make this happen?
The input voltage is what?
The output voltage will be the same?

So, I would wire 110 volts to a terminal on the PLC, when all of the bit boxes are true the output terminal for that circuit will energize the lighting two pole contactor's coil.

Another device may have 24 vac control circuit. I would use the same procedure.
And so...

OR...

Is there just one control voltage used for the entire PLC ? Eg. 24vdc. And all the outputs are 24vdc coil relays and the 110 volt lighting circuit runs through a set of dry contacts to the lighting contactor?

 

[leitmotif]

OK for the simple answer -- it depends

Now to get complicated and real. Comparing to something you know and understand - it is quite similar to relays you can get the coil in differnt voltage and the contacts can come in different voltages and current capacity.

SO for PLCs depending on what you need you can power the PLC "brain" from almost any voltage you want.

Inputs can be many voltages either supplied by PLC itself or external source.

Same is basically true for outputs.

Yes you can drive a 120 VAC relay with the output of the PLC - just pick the right PLC whose "brain power supply", input power supply and output power supply match what you want.

Dan Bentler

 

[sthompson]

I'll rephrase my question: I want to turn on a two pole contactor with a 110 volt coil. 110 volts is the control voltage to do this.
How is the PLC wired to make this happen?
The input voltage is what?

Whatever you want it to be. It could be 24VDC, 110VAC, or any other Voltage compatible with the Device and Input Card, even an Analog Voltage.
This is usually decided by the Device, and the Input Cards you have available.
The PLC Logic could not care less. It only looks at the zeros and ones.

The output voltage will be the same?

No, again the Output Voltage will be decided by your load. If the Contactor requires 110VAC, you would use a 110VAC Output Card.

Is there just one control voltage used for the entire PLC ? Eg. 24vdc. And all the outputs are 24vdc coil relays and the 110 volt lighting circuit runs through a set of dry contacts to the lighting contactor?

Some OEMs do this, especially for International destinations. The entire PLC will have all 24VDC Inputs and Outputs, and then Interposing Relays are used for the Local Voltage. However, generally the previous method is more common. The Input and Output Cards are picked based on the Input Devices and Output Devices in the system.

Stu....

 

[Icekold]

Thanks for all your help. I'm beginning to grasp what I have been missing. Input and output cards and such .. Making much more sense. So, you have an input card for each specific voltage you need to control eg. 110 volts, 24vac, 208 volts etc... then through PLC logic, the program, when all the parameters, bit boxes, are true... PLC sends a signal to an output card that energizes a device. This is why the PLC doesn't care what the voltage is that its controlling...Just needs to choose the correct Input and Output cards. The PLC interfaces the input/output cards. Is my thinking correct ?

 

[Icekold]

In addition, this where (sinking) NPN and (sourcing)PNP is determined ..yes?

 

[Mickey]

In addition, this where (sinking) NPN and (sourcing)PNP is determined ..yes?

Yes,

http://www.patchn.com/index.php?opt...vices-and-connections&catid=20:plc&Itemid=118

 

[Ron Beaufort]

So, you have an input card for each specific voltage you need to control eg. 110 volts, 24vac, 208 volts etc...

basically, yes ...

then through PLC logic, the program, when all the parameters, bit boxes, are true... PLC sends a signal to an output card that energizes a device.

basically, yes ...

This is why the PLC doesn't care what the voltage is that its controlling...Just needs to choose the correct Input and Output cards. The PLC interfaces the input/output cards. Is my thinking correct ?

basically, yes ...

In addition, this where (sinking) NPN and (sourcing)PNP is determined ..yes?

basically, yes ... just keep in mind that the "sinking" and "sourcing" concept applies only to DC circuits – not to AC ... so, in the figure below,

SWITCHES C and D are "sourcing" inputs ...
MODULE B is a "sinking" module ...
MODULE D is a "sourcing" module ...
LAMPS G and H are "sinking" devices ...

sounds like you're getting the picture ... just apply everything that you know (or think) to the figure below and see if it all works out ...

.

 

[mellis]

I do understand about only specific voltage can only be used on a device or equipment requiring that voltage.

I am understanding this is logic rather than physical, pertaining to the PLC.

I'll rephrase my question: I want to turn on a two pole contactor with a 110 volt coil. 110 volts is the control voltage to do this.
How is the PLC wired to make this happen?
The input voltage is what?
The output voltage will be the same?

So, I would wire 110 volts to a terminal on the PLC, when all of the bit boxes are true the output terminal for that circuit will energize the lighting two pole contactor's coil.

Another device may have 24 vac control circuit. I would use the same procedure.
And so...

OR...

Is there just one control voltage used for the entire PLC ? Eg. 24vdc. And all the outputs are 24vdc coil relays and the 110 volt lighting circuit runs through a set of dry contacts to the lighting contactor?

I think I'm getting a handle on where you are coming from, but I could be wrong. See if this helps at all...

The power supply for the PLC processor is independant of the power for inputs and outputs. It's purpose is to power the backplane of the PLC chassis and the cards in the chassis (just the "logic" part of those cards). It does not power the circuits that input devices and output devices are wired to. It's usually 5vdc, 12vdc, 24vdc. Consider it "Internal Power".

For 120vac input cards, you bring a 120vac neutral (from a breaker panel) to the input card. The 120vac hot goes to the devices you are monitoring with the input card. The switch leg from each device goes to one of the input channels on the input card. If it helps you to visualize it, think of the PLC input as the "light bulb" in a simple switch and light circuit.

You can use the same circuit for one or several input cards as you see fit.

For 120vac output cards, you bring a 120vac hot to the output card. The 120vac neutral goes to the devices you are controlling with the output card. One channel of the output card goes to each device. You can think of the PLC output as a "contact" wired to control a device. The output may actually be a contact, or a triac depending on what you select for an output card, but you will wire it pretty much the same.

You can use the same circuit for one or several output cards as you see fit. They can even be the same circuit as the input cards, but they do not have to be.

If your system is really small, the PLC power, the inputs and the outputs could all be on the same circuit, but they do not have to be. Each I/O card is able to be powered independantly.

24vdc inputs and outputs are similar. You need a 24vdc power supply for the field wiring. The PLC does not supply this.

The one exception to this rule... Analog output cards. The 4-20ma or 0-10v signal is usually supplied from the backplane power.

Any clearer?

 

[Icekold]

It's becoming crystal clear..thank you and everybody contributing with your help. Once the concept and basics can be understood the whole process becomes clearer. The last few days many of my questions have been answered. I never thought a site like this would and could be so helpful...I learned that also. And I appreciate everyone's knowledge. PLCs now fascinate me more than before. The functionality and diversity is powerful. In my field, HVAC, these will definitely become more preferred.

 

[Lancie1]

In my field, HVAC, these will definitely become more preferred.

Hmmm...HVAC is one of the most difficult areas to use PLCs. The reason is that many HVAC systems use 24 VAC as the control voltage. That is a very rare voltage to find in PLC input modules. Many PLC inputs will accept a range of DC voltages and some accept 120 or 240 volts AC, but not low-voltage AC.

There are some PLCs that have 24 VAC PLC inputs, and there are special-purpose HVAC controllers that use nothing but 24 VAC.

 

[Icekold]

Thank you for all your help. Since last posting, I have taken the advice here and educated myself and with greater understanding of the plc terminology and functions and hardware. I am at a point of buying the plc and related hardware. I am tending toward Omron and wondering is their CX-Software Suite is difficult to use as a beginner and user friendly ?

 

[Ron Beaufort]

it was my understanding that one of your primary requirements was a PLC system which offered "security" for your program code ... specifically, you are developing a program which you regard as "proprietary – trade secret" intellectual material ...

if those "security" requirements are still of concern to you, then I recommend that you ask the forum to help you compare the Omron products against the Allen-Bradley products along those specific lines ...

more specifically, to compare the SECURITY features of the Allen-Bradley ControlLogix and/or CompactLogix platforms - against the SECURITY features of whatever Omron platforms you're been considering ...

just to nail down the idea:

IF (that's a big IF) you are still concerned about the possibility (indeed the PROBABILITY) that other competing vendors will quickly take advantage of your custom-designed algorithms, then "ease of use" might be an extremely minor issue – when compared with the security features that each PLC system may (or may NOT) provide ...

DISCLAIMER: I have ZERO experience with Omron – so I personally can't help you with this piece of the puzzle ...

as far as I know, it is quite possible that Omron might indeed have an equal (or perhaps an even better) handle on the "security" aspect of your requirements – but I'm thinking primarily of Allen-Bradley's ability to support custom-designed Add-On Instructions – and to allow you to password protect the contents of those instructions before you roll your programs out to your customer's sites ...

I'm sure that the forum members can tell you whether or not Omron supports that degree of "security" for your program code ...

I wish you the best of luck with your project ...