Very basic question from new learner.

Hello,

Forgive the basic question, but I've been having a hard time finding a clear answer to this online or otherwise. I'd really appreciate any help here!

This is still a theoretical problem at the moment. Let's say I've got a PLC operated machine that's running on a 480vAC power circuit and a 120vAC control circuit, and we're relying on the PLC's internal power supply (transformer/rectifier) to convert that control power to whatever we need.

I want to wire a start PB into this PLC. I know one side goes to an input terminal on the appropriate module. But what does the other side of the start pb connect to? Is there a neutral return on the module to connect to?

Once again, forgive my ignorance. I really appreciate any help!

The answer is dependent on what kind of input the PLC uses. Voltage could be 120 VAC, 24 DC, or something else. It will also depend on whether the configuration is sinking or sourcing. Look for a schematic of an actual PLC input.

Regardless of the voltage you choose, there will be a terminal on the PLC's input module for the common side of the circuit. For a 120 VAC circuit, you connect the AC neutral to that terminal. In a 24 VDC circuit you connect either the +DC or -DC to that terminal, depending on whether you use a sourcing or sinking input module

What is the difference between sourcing and sinking modules? Is that a matter of polarity in a DC circuit? If so, which is which, and is that something that is reversible or more a matter of how each DC PLC is built?

Again, thank you for your responses! I'm excited to figure this whole PLC thing out!

Concerning my original question, I clearly need to take a closer look at our control panels at work to see if I can be more specific about the PLC in question.

Thank you!

A sinking input module has the -DC wire connected to its common terminal. The +DC goes to the switching devices in the field.
A sourcing input module has the +DC wire connected to its common terminal. The -DC goes to the switching devices in the field.
Nowadays many DC input modules can be wired as either sourcing or sinking. Depending on the brand and model, there may be limitations to that. Some modules may require that once you decide on sourcing or sinking, all of the field devices must be wired the same way. Some may allow a mixture of different types of field devices. You have to read the module's data sheet to be sure.

Baccarat said:

.......Again, thank you for your responses! I'm excited to figure this whole PLC thing out!

Concerning my original question, I clearly need to take a closer look at our control panels at work to see if I can be more specific about the PLC in question.

Thank you!

Click to expand...

May I respectfully suggest that you first take some basic electrical courses? From the questions you’ve asked so far it seems you could use them. Also, please make sure you don’t go anywhere close to the inside of an energized control panel.

I applaud your curiosity but there is a good chance you could hurt yourself or others.

Baccarat said:

This is still a theoretical problem at the moment. Let's say I've got a PLC operated machine that's running on a 480vAC power circuit and a 120vAC control circuit, and we're relying on the PLC's internal power supply (transformer/rectifier) to convert that control power to whatever we need.

Click to expand...

I just thought that this paragraph needed to be addressed. You would not typically obtain your IO System control power source from the "PLC Power Supply".

The PLC power supply provides multiple DC voltages for operating the CPU, the backplane and the IO modules. (note that these components may be integrated into a single unit in smaller "brick" type PLCs) Some PLCs allow access to the PLC power supply for loop excitation power on analog modules or even for discrete IO points, but that is not the primary function of the PLC power supply. Better designs will typically employ external transformers or DC power supplies to provide power for the IO circuits. There is some risk involved in exposing your PLC power supply to field wiring. PLC IO modules typically feature photo isolation between the field wiring electronics and the backplane electronics so as to protect the backplane electronics from exposure to field devices.

Your question states that it has a 120V control circuit.

That leads me to believe the buttons/switches etc are more than likely 120VAC. Assuming that is the case, 120VAC hot will be wired through a fuse or small breaker upstream of the button. Neutral is wired to a "Common" or "N" terminal on the input card of the PLC (or input terminal section of a brick PLC). Then, when the button is pressed, it applies 120VAC to the input it is wired to, and you should see the corresponding LED for the Input point light up.

I much prefer working with 24V systems for electrical safety reasons, however, you don't have much of a choice when working on a system someone else has already installed.

Always pay close attention to the voltage ratings of both the PLC's I/O points, and the devices connected to them to avoid damaging equipment, and hurting yourself.

Hi, and welcome to the forum!! I wish you the best in your endeavor. I really like programming, hope you do too. Ron Beaufort has a real good article and diagrams about sink/source. I copied it and have it on my laptop, so when I get stuck in the field, I can look at it instead of trying to remember. It's attached, hope this helps.

saultgeorge said:

Hi, and welcome to the forum!! I wish you the best in your endeavor. I really like programming, hope you do too. Ron Beaufort has a real good article and diagrams about sink/source. I copied it and have it on my laptop, so when I get stuck in the field, I can look at it instead of trying to remember. It's attached, hope this helps.

Click to expand...

I have a pretty good grasp of sinking and sourcing, but this Word document is great for putting it into simple terms.

Mr. Baccarat,
Frequently the PLC manual (or the I/O manual) will have diagrams for 'how to'
wire various things to the PLC. And it does vary (some) from PLC to PLC.

Some PLC's have a built in power supply and will offer connections enabling
one to use that to work Inputs and Outputs. Others will require a separate
24 Vdc supply which is used to power both the PLC and the I/Os.

I just finished installing a little project with an Omron Programmable relay.
This 'baby PLC' runs off 120 Vac, and the inputs are 120 Vac, and the outputs
are relay contacts.

This is a fascinating field to be working in.
Poet.