Switch question 1

[Lc85]

Hi,
We have a control panel which contains a plc. The panel provider is not providing a pressure switch but they have 3 terminals +, sw,- . The sw terminal is input into the Plc. The plus is providing 24V and the - is ov

My question is can we use a change over contact type switch with common, no ,nc terminals on the arrangement they have provided?

Thanks

 

[jraef]

Seems as though what they are doing is allowing you to use it Sink or Source to your PLC inputs, which may be selectable. Some people prefer Sinking inputs, some prefer Sourcing inputs, your supplier extricated themselves from the controversy by allowing users to connect it either way.

Now YOU will have to decide based on your preferences, and if you have none, you will get numerous opinions of which way to go... let the battle commence!

"Will work for (the memory of) salami"

 

[OperaHouse]

I would like to add one caution. PLC inputs are very low current. Actual mechanical switches are not reliable when used with these low current inputs. Over time the contact surfaces will oxidize and not pass current. It may take 5-10 years for the problem to show up so you just may not care, but it will happen some day. I have seen many commercial products with failed switches where the engineer was not aware or just didn't care.

 

[flexoprinting]

Job security for me OperaHouse, locating/replacing is what I do. But sink or source, I'm thinking 6 to 1 half a dozen to another, does it really matter?

 

[Skogsgurra]

Swim or sink in the source?
Pls ignore.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[dpc]

Actual mechanical switches are not reliable when used with these low current inputs.

I think this is a bit of an overstatement. There must be millions of switches out there serving as PLC inputs. Somehow the large majority of them continue to function. The contact oxidation and corrosion can be largely overcome by 1)using high quality switches suitable for their service and 2)using a high enough wetting voltage to punch through any oxidation. In power plants and other critical facilities, 125 V dc is commonly used for wetting voltage and this helps tremendously. It is more of any issue when 12 V or 24 V is used as the wetting voltage.

The contact oxidation can be an issue, but it's only one of many.

 

[itsmoked]

The oxide busting voltage for switches is typically about 10V. This means 5V circuits will have problems unless the switches have gold contacts. 24V will remove this as an issue. If the switches get flaky on 24V they have failed - reaching end-of-life.

Lc85; It appears no one has answered your question.

Yes. You can use any switch you want since you have more than 10V involved. To use a normally open switch hook it from your given "+" terminal to the "SW" terminal.

FYI the setup they've given you is to use something like an optical switch where it's an active electronic device. You'd feed it the power it needs to function by hooking it's "+" and "-" terminals up to the PLC ones you've listed, then the electronic switch's output to the PLC "sw" input.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[OperaHouse]

I disagree with that. Along with a minimum voltage, manufacturers specify a minimum current to break through the oxide layer. Most opto inputs will not supply enough. Silver contacts are the worst offender. Those with wiping action fare much better. Recently replaced a pair of sealed switches that were working om 24V optos. Connecting them to a 120V lamp for a few cycles and they were good to go for other applications. It doesn't happen that much but you should always design so an open switch reverts to a safe condition.

 

[LionelHutz]

My take on those terminals is that you are to use a switch that requires 24VDC power and has a NPN output. That seems to be a fairly typical connection in the sensor world.

You can't just hook the switch up either way (sourcing or sinking) unless you also switch the logic inside the PLC.