Best practices

For the common.. if we are using 24 Vdc , do you usually use the 24V+ or 0v- ?? what about with 220 Vac ?? phase or neutral ?

thanks !

0V- is the common in 24vdc

220vac is usually a phase to phase voltage so there is no neutral. but a typical 120 vac system would have a neutral which would be the common

joaco1993 said:

For the common.. if we are using 24 Vdc , do you usually use the 24V+ or 0v- ?? what about with 220 Vac ?? phase or neutral ?

thanks !

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Define 'common.' Do you mean common as it relates to the power supply, or common as it relates to I/O module common or switching N.O, N.C, COM(mon)

We usually use 24VDC and 0VDC for DC power. Some customers want +24 and -24 which is misleading to me, but hey the customers always right.
For 220 or 480 will have L1 L2 L3 and sometimes N if they are using a neutral.

common i refer to the I/O..

24+ and 24- ?? how is that.. are there any power supp that admit that ??

joaco1993 said:

common i refer to the I/O..

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It would depend on the devices (I/O module and connected devices.)

joaco1993 said:

24+ and 24- ?? how is that.. are there any power supp that admit that ??

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Probably a USER NAMING CONVENTION, (but as Jax said -- misleading.)
Otherwise, I would assume a dual rail PSU and would expect it to have +24V, 0V, and -24V.

ok thanks for answering.. one last question. When you say +24V and 0V.

You actually have a difrerence of 24V between the + and - of the power supp.

But why 0V ? is the (-) grounded in the power supp and thats why you say 0v ??

Or is it just a naming convention also..

0V is the common naming convention, as it is 0 volts to ground, but some customers, as JaxGTO says, call it -24V since it is the negative side of the 24 volts, but truly -24V would be below the ground and 48 volts below the positive side.

Welcome to the start of a lot of confusion.

joaco1993 said:

common i refer to the I/O..

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It would depend on the devices (I/O module and connected devices.)
For an OUTPUT module, assume that the signal on the COM terminal is the signal that gets switched to the output terminal(s.) What signal is COMMON (connected) to both, the output module and the power supply...
When I say OUTPUT MODULE, I mean the smallest portion of the output module that has an individual (ISOLATED) COM terminal.
Some output modules have multiple COM terminals - the COM terminals MAY OR MAY NOT be internally connected.

Assume that we have these two devices RS4N-DE and RS6N-DE.
They are two of Automation-Direct's offering of small, multi-relay din-rail mountable modules.
They are quite similar, but distinctively different. See discussion of sourcing/sinking.
For the RS4N-DE, you would connect the +24V to the OUTPUT module's COM terminal,
and then the output module would switch the +24V to the RS4N-DE.
In this arrangement, you would connect the 0V to the RS4N-DE COM.

For the RS6N-DE, you would connect the 0V to the OUTPUT module's COM terminal,
and then the output module would switch the 0V to the RS4N-DE.
In this arrangement, you would connect the +24V to the RS6N-DE COM.

I do not have an example of connecting the NEU to COM in an AC circuit.

Aabeck said:

0V is the common naming convention, as it is 0 volts to ground, but some customers, as JaxGTO says, call it -24V since it is the negative side of the 24 volts, but truly -24V would be below the ground and 48 volts below the positive side.

Welcome to the start of a lot of confusion.

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Some people use GND, 0VDC, COMMON interchageably.
It really depends on the context of the circuit at hand.

Say I have an embedded controller circuit, I may say 5V, 3.3V and GROUND. What I REALLY mean is 5V, 3.3V and 0V.

ok thanks all for answering ! i had this doubt for a long time and starting to get confused.

I like to say 24vdc+ and 24vdc- since the + and - go along with the terminal discriptions on the power supply.

Common, Ground, 0V, and neutral are not the same.
Using them interchangeably is neither accurate nor is it good practice.
If you want to always improve in your career and be better at what you do then don't follow practices just because you see them being repeated.

Here is a good document to follow for grounding and some conventions:
http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1770-in041_-en-p.pdf

"Common" is just a catch-all term for any conductor that is shared by multiple devices. It could be the ungrounded or the grounded conductor, depending on what the application is.

For example, a PLC input will use the grounded conductor (negative in a DC circuit, neutral in an AC circuit) as the Common, because all of the input channels have the grounded conductor "in common." On a PLC Output card, the ungrounded conductor is common because all of the output channels in a given group will share the ungrounded conductor "in common."

FactoryTalktotheHand said:

For example, a PLC input will use the grounded conductor (negative in a DC circuit, neutral in an AC circuit) as the Common, because all of the input channels have the grounded conductor "in common." On a PLC Output card, the ungrounded conductor is common because all of the output channels in a given group will share the ungrounded conductor "in common."

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FT, please rewrite this paragraph to include the ability to have sourcing/sinking inputs and sourcing/sinking outputs.
Not all Input modules use the 'grounded' conductor as a common, and not all output modules use the 'ungrounded' conductor as common.