micrologix 1200 output has voltage

Buelligan

Member
Join Date
Apr 2011
Location
New Mexico
Posts
11
I have a micrologix 1200 in one of my machines, it's about 2 years old now, recently one of the outputs shows voltage (+24vdc) at all times, even when the output is not true.
I can cycle the run button on and off, and see the output light go on and off, but when I measure voltage at that output terminal, there is +24 there at all times, it does not switch off.
I am suspecting the PLC has internal short or something of that nature, and have ordered a new one.
Any thoughts?
 
Is it a solid-state output or a relay output ?

Some solid-state outputs will "leak" a very small amount of current, so you can detect it at the output with a meter, especially if there is no load connected.

If it's a relay output, then you have very likely proven that the internal contacts have "welded" together. Disconnect the relay group source voltage wire to determine that for sure, to prove you're not back-feeding 24V to the terminal.
 
If you know the MicroLogix model number and the Output number you can look up whether it's solid-state or relay.

MicroLogix 1200 model numbers with a "W" have exclusively Relay outputs, while ones with an "X" have a mix of relays and solid-state outputs. I would have to get out the wiring diagrams to tell you which terminals connect to which types of outputs.

An interposing relay is generally a very light-duty load, but if it's operated frequently the contacts in the MicroLogix internal relay could eventually weld together due to the back-EMF voltage created when the interposing relay coil turns off, especially if there's no reverse-bias diode or other back-EMF snubber across the output.
 
So if its connected to a relay that enables the VFD, does it disable the VFD if you turn the output off? I have seen this many times with solid state outputs as well as when they fail, they fail closed.

When testing solid state outputs I will use one of my old meters, like a wiggie that will put enough of a load to test it correctly.
 
The micro logix is a: 1762-L24BXB C


When the output is turned off, I see the output light go off, but the relay is still on, and the vfd continues to run, driving the table to its mechanical stops, and then the belt drive starts slipping.
Hitting the Estop stops it, but when I pull the Estop out, it restarts.
All this with the output off.
There is still +24vdc at the output terminal.
 
"1762 MicroLogix 1200 System, MicroLogix 1200, 24V dc power, (10) standard 24V dc and (4) fast 24V dc digital inputs, (5) relay outputs, (4) standard 24V dc FET outputs, (1) fast 24V dc FET output"

It looks like that model has multiple types of outputs. Do you know output you are tied to? What type of coil does the relay have?

I know it goes without saying, but do you have any spare outputs you can connect to?
 
Your PLC has both relays and solid state relays (FET's) which one is the VFD connected to?

If solid state see Ken's post.
 

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