Micrologix output always ON

Hi everyone,

This is my first time working with Micrologix 1200 PLC. I have my inputs and outputs wired to the PLC & on the process of testing now. One thing I notice is I get 24V on Output 3 even when the output LED for Output 3 is off. I checked the output status from rslogix and its 0. But I continuosly get 24V. i tried to toggle the output & tried force off but still I get 24V.

Not sure why this is happening? Can someone help me? Do I have just one choice - to use some other output?

Thanks
Preeya

Greetings Preeya,

I'll assume (gosh I hate that word) that your MicroLogix has "relay" type contacts ...

based on what you've posted it sounds like a "welded" contact ... this usually occurs when the output load is too heavy for the contact rating ... the resulting "arcing and sparking" acts like an electrical arc welder and burns the contacts together ...

if you have an extra output, move the wires over and readdress the program to match ...

just be sure to check and make sure that the actual field load is within the contact's rating ... if it isn't, then sooner or later you'll just burn up another one ... if the load is too heavy, then look into using an "interposing" relay to handle the extra load ...

even if the load current isn't too high, you can still run into trouble if the load is an "inductive" one ... this type of load can damage contacts (even the solid-state type) from the "inductive kickback" effect ... think about your car's 12 volt battery and the minimum 20,000 volts or so that it takes to make the spark plug fire ... that high voltage comes from the ignition coil - courtesy of the "inductive kickback" effect ... the solution to this problem is usually a "snubber" ... post again if you need more info ...

Please post the model number of the Micrologix 1200. If it a DC output, as opposed to relay output, make sure you have an appropriate load attached. Feeding any solid-state ouput into an open circuit will show output voltage even if the output is turned off (of course this depends on the meter also).

Thanks Ron & Bernie,

Sorry I did not post the number. It is a 24 I/O Micrologix 1200
(1762-L24BXB) I have output 3 connected to a control valve through a fuse. Output 4 has exact connections. I have disconnected the fuses so there is not load connected to the outputs.

- When I measure the voltage across output 3 & 4 terminal in respect to DC-, when both outputs are logically off; I get 24V on output 3 & oV on output 4.
- When I force the outputs on or toggle the bits; I still get 24V on output 3 & 24 V on Output 4.

When I connect the load, I notice the same thing on output 3 - always 24V. Output 4 works fine.

Is output 3 damaged? I am using different outputs now (4&5) but not sure why this happened?!

Since the BXB is a relay output I'd say welded contacts as the symptom in agreement with Ron. I would vote for the 'inductive kickback' as the most likely cause.

I have this problem quite frequently on ML1000's that I installed before I used my brain and put in dry contact relays immediately after the output. Generally I have motor starters connected to mine, and the inductive inrush current is enough to weld them up.
Anyway I never install any PLC without a slimline relay on all outputs.
By the way, I have replaced the relays in the ML1000 OK and they might be the same in the ML1200. They were Omron relays, but I can't remember the cat#. Just desolder and resolder on the new one.

I believe your problem is in your wiring. The L24BXB has both DC as well as relay outputs. You need to look at the manual to wire it correctly. In particular, do you have a DC common wire terminated to the COM2 terminal?

See this catalog, page 46:
1762-um001-en-p

I agree with the previous guy, you must check your I/O grouping, as in the manual it will show how the I/O grouping goes together, they are kinda funny with micrologix. check rockwell website for the I/O wiring for that model.
D.M

Well once again all the thought process here has complicated the troubleshooting process. I have read all the post including your first querry. My first thought is to dumb up the PLC! Erase all memory, take the battery out and let the PLC DIE! Then re-apply power with no program in the PLC. Let me know (I say me but I mean this forum) tell us if you still see the same thing!

just a Bitmore

Post script:
You guys with all the advice are good, but please let the PLC get back to square 1!!!! Then after that, if the output is still stuck, a simple test program would prove that! One thing I really don't like about all the advice is... most of it is not fundamental! Loads really don't have to do with much of the world now, yes it can happen that you blow an output, but get real, your contenplating worse case senario here, that in my opinion, scares more people than anything else!

Bitmore said:

I have read all the post including your first querry.

Click to expand...

Are you sure 'bout that, Bit?...

In Preeya's first post, he stated "One thing I notice is I get 24V on Output 3 even when the output LED for Output 3 is off".

I think we can safely assume that it's NOT program related... :nodi:

beerchug

-Eric

With all these things that preeya has checked it is safe to say that the PLC output 4 is bad (relay or not).

Solenoids can be nasty, people often underestimate how much current they draw, and how difficult it is to break a DC current on an inductive load.
There are two remedies:
1. Snubbers.
2. Heavy duty relay, solid state relay or contactor in place of the relay output from the PLC.
Depending on your application it can be enough with snubbers.

When done with that, there is one very important thing to do:
Check your PLC program that it cannot repeat itself rapidly so that the output pulses as well. If breaking an inductive DC current is bad, then breaking followed immediately by making a contact on an inductive DC current is WORSE.
Such behaviour can also be caused by an unreliable input signal if it decides in the program that the valve must be ON.
If your application allows it, you can add delay timers to the output signal, making it impossible for it to cycle rapidly.

Thanks..

Thanks for all your replies guys.. Yes its true Micrologix 1200 L24BXB has both relay & FET outputs, see here

I am using Output 3 (Group 2) which is a FET output. I have COM 2 connected to DC-. I don't think there is any problem with the wiring as the control valve connected to this output was working fine before this problem occured. I am connecting this output to the valve directly with a fuse in between. Is this safer?

Also I am pretty sure there is nothing wrong with the PLC program. As I said the output LED is off while I receive 24V! I also receive 24V when the output LED is on..

Not sure why this happened & how to prevent this from happening again..

The fuse is OK, but it will not protect the next output from being damaged.
I have seen 24VDC outputs eaten by the solenoids they were connected to. I normally put a medium sized relay between 24VDC and "regular" solenoid sizes. The solenoids I use normally have snubbers built right in.
How large is the solenoid ?

Hi Jesper,

The valve I have connected is a 24V, 40W solenoid valve. I am not really sure if it has a snubber built in. Do you recommend I add a diode IN4004 parallel to the load? although it is not a relay output from the PLC?

THanks,
Preeya

I would definitely add the diode if you aren't sure the solenoid contains one. The voltage surge on field collapse will be pretty nasty with a solenoid that large.

Also, 24 VDC/40W will draw about 1.67A of continuous current, which is just slightly over the 1.5A per point rating for the FETs. My guess is you would be OK with this but you are RIGHT at the edge.

Keith


P.S. I removed the reference to the ASCO valve. Where did that come from??