latched input

[bigheadted]

anyone have any experienced of inputs being latched on on plcs and suggestions to the possible causes. I have heard that poor neutral faults can cause this but cant really see how

 

[Skogsgurra]

It surely can/could. Especially if your input is directly into a CMOS gate without opto-coupler. There is a set of parasitic SCRs in the CMOS structure and they can (or rather, could) be turned on if more than a certain amount of current (a few tens of mA was sufficient) was forced into the input pin.

But that is history. Gone. No problem anymore. A badly designed system that relied on connection to the neutral to reduce input voltage could cause that phenomenon if you lost the neutral. So that part of the story is also plausible.

But do not use that as an explanation for problems in a modern PLC. All major CMOS manufactorer have "Latch-Up-Hardened" inputs nowadays.

Gunnar Englund

http://www.gke.org

 

[bigbillnky]

bigheadted

I know 120 volts into a DC input card will permanently latch the input. I saw it done TWICE in a row by the same technician. He did not appreciate the humor I found in the situation, but we ALL make mistakes (especially me at times).

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[Skogsgurra]

There is a thin line between latching and frying...

Gunnar Englund

http://www.gke.org

 

[dpc]

If you have longs runs of cable between the contact, the voltage source and an ac input module, in some cases the capacitance of the conductor is sufficient to trick the input circuit into thinking the contact is still closed after it opens.

The capacitive reactance forms a parallel path with the remote contact.

Also, at least in the past, driving an input directly from a solid-state output module can cause the input to latch.

 

[bigheadted]

skogsgurra
thanks for your reply. the input was 24vdc i actually disconnected the wire to it. could you explain the cmos bit a little further for me
thanks

 

[Skogsgurra]

Have a look at this link:

http://www.ece.drexel.edu/courses/ECE-E431/latch-up/latch-up.html

The PNP-NPN transitors connected as shown are the equivalent of a thyristor. Once you trigger it, it will conduct until it melts or the supply voltage is switched off.

Gunnar Englund

http://www.gke.org

 

[dpc]

For dc input - as far as my previous post - never mind....