Interfacing inputs to PLC
Interfacing inputs to PLC
(OP)
Hi,
When dealing with standard limit switches and a set of contacts on electromechanical relay, do we care what type of DI module we use (24Vdc, 125Vdc, 120Vac)?
I know that the material of contacts is selected based on the load that would be switched off, but should I be concerned if I want to wire a mechanical limit switch to a 24 Vdc input card? I would think that as long as the contact is not worn out and has good continuity when in closed position, the PLC should be able to read high when the contact closes.
There are gold plated contacts that are rated for milliamps, but the gold plating is there to primarily protect against environment (oxidation), since in low voltage application resistance that is developed due to elements is more critical than for the same contact used in higher voltage application. Am I right?
Please share your thoughts/experience.
Thanks,
CuriousElectron
When dealing with standard limit switches and a set of contacts on electromechanical relay, do we care what type of DI module we use (24Vdc, 125Vdc, 120Vac)?
I know that the material of contacts is selected based on the load that would be switched off, but should I be concerned if I want to wire a mechanical limit switch to a 24 Vdc input card? I would think that as long as the contact is not worn out and has good continuity when in closed position, the PLC should be able to read high when the contact closes.
There are gold plated contacts that are rated for milliamps, but the gold plating is there to primarily protect against environment (oxidation), since in low voltage application resistance that is developed due to elements is more critical than for the same contact used in higher voltage application. Am I right?
Please share your thoughts/experience.
Thanks,
CuriousElectron





RE: Interfacing inputs to PLC
I have seen lotsa both, even 24Vac.
I have never seen 120Vdc used, although I am sure there are DI cards that will work for that potential.
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Interfacing inputs to PLC
RE: Interfacing inputs to PLC
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RE: Interfacing inputs to PLC
RE: Interfacing inputs to PLC
As for 'special contacts' you need none what-so-ever as long as you are working above 12V. If you're below 12V you need to assess things and in some cases go with gold. (Generally all 5V and below need gold for reliability unless additional work is done.)
Keith Cress
kcress - http://www.flaminsystems.com
RE: Interfacing inputs to PLC
I was concerned that 24Vdc might not be high enough for standard mechanical contacts, but sounds like it's non-issue.
RE: Interfacing inputs to PLC
Keith Cress
kcress - http://www.flaminsystems.com
RE: Interfacing inputs to PLC
I think more and more voltage selection of devices is driven by what people have to wear to work on a live circuit for maintenance or troubleshooting. I think if you have a choice i would try to stay below the 50vdc/ac level so that your suiting up is a very low arc flash gear level.
If its some distributed network for devices in field then its driven by that network, but this would be usually 24vdc. So all these boxes or IO brick will usually be 24vdc.
Sometimes the use of 120vac is driven by customers in trying to standardize on certain parts. If its 120vac in past then they want all new devices to be 120vac.
My preference is to keep all devices distributed network with 24vdc as the voltage level.
RE: Interfacing inputs to PLC
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RE: Interfacing inputs to PLC
-AK2DM
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