potential free contact in a plc circuit.

[Basu]

what is a potential free contact in a plc circuit?

 

[Andrew Evenson]

It's a relay contact

Basu,

A potential free contact is just a relay contact. Unless you put a potiential across the contacts, there isnt any there.

Andrew

 

[Nigel Betteridge]

This sounds like "volt free contacts". On the assumption that it is, then it means switch or relay contacts which have no voltage applied to them. Put another way, the contacts are not referenced to a voltage.

Without any exceptions that I know of, all relays have volt free contacts. This means that you have the option to control load voltages which differ from the control (coil) voltage. eg a relay with a 12Vdc coil can control a 230Vac load.

As supplied, most control switches are also volt free.

I would guess that your enquiry is referring to the input requirements of your PLC.

Ihope my guesses are of help to you

 

[Kidblue]

We normally provide volt/potential free contacts via 'interposing' relays on our PLC outputs (normally to interface with other vendors equipment, where they provide the control voltage, we provide the logic)......



Hope this helps!

 

[Tom Jenkins]

"Potential free" or "dry contacts" are used when the device being controlled has its own power supply. For example, most VFDs have a 24 VDC power source in the drive that is used to control starting the drive. The external contact used to start the drive is a dry contact, and it switches the voltage supplied by the VFD to let it know it should begin running. Closing the dry contact connects the drive input to the VFD's 24 VDC and powers the drive input. The internal devices sense the voltage, start the drive, and the current and voltage is returned to the VFDs own power supply "-" through internal circuitry.

The term "potential free" is only in reference to the control contact when it is not connected to an external device. Obvioulsy, during operation, there must be potential or voltage passing through the contact or nothing can happen.

This term is meant to differentiate between a dry contact, powered extenally, and a contact that is connected to a voltage source inside the control system that is sent to an external device. An example of this would be a PLC output that uses the PLC's own 24 VDC power, and when the contact closes this voltage is connected to an external "unpowered" load such as a relay coil.

 

[Werner]

We normally provide volt/potential free contacts via 'interposing' relays on our PLC outputs (normally to interface with other vendors equipment, where they provide the control voltage, we provide the logic)......



Hope this helps!

I agree on this one. Potential free does not mean there is no voltage over the contact. From the schematics point of view there is no power on the contact because the power is deliverd by somebody else. Since this power supply is not in the drawing you can consider this potential free.
We also use this kind of contacts to give information to systems that connect to our system. For example "Line running" or "Stop". The contacts can be dangerous when the external party switches 230VAC with the relay contact. So when the panel is powerd off the power from the external party must also be switched off. (Or it must be marked/protected properly)

 

[UncleBuck]

Some potential free I/O or Dry contacts share common or Neutral.

Make sure not to mix and match voltages if the common is shared.

You can also get isolated output "dry contacts" which means there is no other common shared.

Sharing commons can be problematic unless you do it carefully. Dry or not; the common side of the circuit is the most frequently neglected and can cause your potential free contact to have some potential you do not want.

 

[rsdoran]

Semantics or actual definitions?

Some potential free I/O or Dry contacts share common or Neutral.

Make sure not to mix and match voltages if the common is shared.

You can also get isolated output "dry contacts" which means there is no other common shared.

Sharing commons can be problematic unless you do it carefully. Dry or not; the common side of the circuit is the most frequently neglected and can cause your potential free contact to have some potential you do not want.

This is contrary to just about everything that has been stated. A potential free contact will not share a common ...if it did then it would not be a potential free contact.

Its my experience that these are used when combining two (or more) machines to create a system...example: unwind, printing press, and re-wind. Run permissives would be used to establish a start sequence..ie re-wind is started first then press than unwind. In other words potential free contacts are used to "relay" a signal that will not have a potential reference at the point of use.

Some of the examples stated I think would be called "interposing relays", that separate different voltages within a system. In those cases a common could possibly be bonded or shared.

 

[Jeff Chambers]

Hello,

Would you expect to see a fuse in the power being fed to the "dry contac", or is there a fuse on the card itself (A/B system)

 

[mordred]

If your asking about the 1747 rack cards I have never come across a fuse on the cards usually just in the power supply however its often good practice to add a fuse particularly if its an output