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Interfacing inputs to PLC

Interfacing inputs to PLC

Interfacing inputs to PLC


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.


RE: Interfacing inputs to PLC

Depends on the industry (24Vdc or 120Vac). Both voltages work fine.
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

AC usually isn't as hard on contacts as DC. Higher voltages also act to clean off the contacts during switching.

RE: Interfacing inputs to PLC

120 VDC is usually used in electrical substations. I have used both 24 VDC and 120 VAC using standard contacts without any issues. If of concern, wire the input such that when the signal is off, the contact is closed. This is mostly used on alarms. If there is a failure in the field, most likely the circuit will open up and signal an alarm. Tends to be fail safe.

RE: Interfacing inputs to PLC

It's usually the most 'logical' voltage for the system. You normally use what's already there to keep things less expensive and more dependable. Preferably it's a low 'safe voltage' like 24V DC or AC but it doesn't pay to install a power supply or transformer just to create that control voltage, as installing either adds expense and reduces reliability.

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

Thank you for all the replies, guys!

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

Answer your question: 24vdc is not a problem with contacts on relays or limit switches or other devices.

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

The only place I have run into problems with contact resistance in field contacts is when people put TTL logic level circuits through external contacts. At 5VDC and low energy, a film will eventually build up on the contacts and there is insufficient energy to "burn" it off. I ran into this in elevator controllers, where the computer needed to know for sure if contactors were closed or not, and they used the TTL logic right off of the board, 7mA at 5VDC, going to auxiliary contacts on the contactors. We tried gold flashed contacts, bifurcated contacts, cross-hatched contacts, wiping contacts, nothing was reliable enough. We finally convinced the controller mfr to run their board level signals to an op-amp and monitored a 24VDC signal, no problem after that.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Interfacing inputs to PLC

In low voltage (TTL), low current (<10mA) applications I have sometimes added a resistor in parallel to the device being switched to pull more current (10's of mA)through the contacts.


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