Forcing OFF

PLC Platform?
If it's AB, are forces enabled?
What is the load? If it's a small load (LED Lamp) connected to a Triac output, there might be enough leakage current to keep it lit.
Could it be an output module problem? yes. Is it definately? no.
 
Make sure you have forces enabled then.
With AB PLC's, you can individually force phyisical inputs and outputs, but unless you tell the controller to 'Activate' those forces, they have no effect.
 
Even though the output is off, there is enough leakage, to the point, where you will read the same voltage as when it is on. For instance, most A-B 110VAC output cards will show 110VAC, weather the output is on or off, if you are testing it with a Fluke meter, which puts hardly any load across it. If however, you use a voltprobe, with a 'leakage' button, you can apply a load with the leakage button. This will tell you for sure if it is leakage or not. This is why I still carry a voltprobe, along with my Fluke. Saves having to put a load across it, to find out for sure.
 
There are four models of ML1000 that have solid state outputs, only one with triacs, all others have contact outputs. Contact outputs do not have leakage currents. So if its a contact output it should be off unless the contact has malfunctioned. Which ML1000 is it?


Forces are permanently enabled on the ML1000, unlike the PLC5/SLC500 where they have to be specifically enabled.
 
message to yam785 ...

you said:

If however, you use a voltprobe, with a 'leakage' button, you can apply a load with the leakage button.

please post again and mention the make and model of the instrument you're talking about ... I've looked for such an animal and have not been able to find one ... thanks ...

note to all others ... I'm NOT looking for a "Wiggins" tester ... specifically, I need a "non-solenoid" tester for my purposes ...

note to original poster josesaucedo ...

as my distinguished colleagues have said, we need more information to answer your question properly ... please give us the exact model number(s) of your equipment ... you mentioned an "output module" ... most MicroLogix 1000 systems don't use "modules" ... specifically, the outputs are "built-in" to the processor ... and please be careful when you type in the numbers ... reason: you mentioned a "Micrologix100" ... it's much more likely that you actually have a "MicroLogix 1000" instead ... that's probably just a typographical error ... we all make those - but sometimes an error can change a correct answer to a wrong answer ...

we're trying to help - but we need specific detailed information in order to give you specific detailed answers ...

and one more question for you ... on the "problem" output - is the LED turned on or is it turned off? ...

finally to Bob O ... thank you for the kind compliment ...
 
Greetings josesaucedo ...



let’s consider the following two questions ...



Question 1. Suppose that you force an output on, but that output will not conduct any electricity. What does that mean?



personally, I’d say that indicates that the output is defective ... any type of output (either relay contact or solid-state) can become defective and not be able to pass (conduct) electrical current ... the act of “forcing” the output “on” is usually done as a troubleshooting step ... the idea is to remove the PLC processor’s logic from the equation ... specifically, we can overwrite any ladder logic command which might otherwise try to turn the output “off” - and thereby we can force the output to turn “on” for testing purposes ...



going deeper ... we’re assuming that the processor is actually operating in the “Run” mode ... if the processor happened to NOT be in the “Run” mode, then the outputs would be “off” regardless of whether they were “forced” or not ... (note: this could be set up to work differently in a ControlLogix system - but we’re talking about a simpler MicroLogix system in this particular thread) ...



also ... we’re assuming that electricity is actually being applied to the output’s “common” terminal ... specifically, if no electricity is being fed INTO the output circuit, then naturally there would be no electricity coming OUT OF the output circuit ... in that case, I’d say that the original question should be rephrased ... specifically, the word “conduct” means to “carry from one place to another” ... that implies that there certainly IS electricity available at the “common” terminal - but that the output circuitry is not “conducting” or “carrying” that electricity to the actual output terminal ...



so the answer to Question #1 in simplest terms: if you force an output “on” and it doesn’t conduct electricity, then the output is bad ...



now for the next question ... hang on ...



Question 2. Suppose that you force an output off, but that output still conducts electricity. What does that mean?



this one can be MUCH trickier to answer ... the most obvious answer is: if you force an output “off” and it still conducts electricity, then the output is defective ... unfortunately there are MANY cases where that obvious answer would be inadequate - and actually wrong ... specifically, there are MANY cases where an output can be forced “off” and still conduct some amount of electricity - and the output can still be PERFECTLY serviceable for use ...



consider a VERY common “solid-state” output circuit ... these are usually based on using a TRIAC device to turn the circuit either “on” or “off” ... now the tricky thing about solid-state devices such as this is that they never actually “OPEN” the circuit COMPLETELY the way that a relay contact does ... instead they simply switch to a very high resistance when the PLC tells the output to turn “off” ... so rather than picturing the TRIAC as a “switch” which can either “fully open” or “fully close”, think of the TRIAC as a RESISTOR which can be controlled (by the PLC) to act like a LOW-value resistor (when “on”) - or like a HIGH-value resistor (when “off”) ... the tricky thing is that even in the “off” state (when acting like a HIGH-value resistor) the TRIAC is still capable of conducting a small amount of electrical current ... we normally call this a “leakage” current ... now if the load (the field device) connected to this type of TRIAC output is heavy enough, the output will work correctly ... but in other cases, the output is very light and it draws very little current ... in those cases, the trickle of “leakage current” which flows through the PLC’s output circuit is enough to keep the field device turned “on” - even when the PLC has commanded the output to turn the field device “off” ... examples of common output devices which are too “light” to be used on TRIAC outputs are neon lamps, most LED-type lamps, and extremely small relay coils, AND sensitive voltage meters ...



now getting back to the original question ...



personally, I’d give my answer to this question in two parts ...



so the answer to Question #2 in simplest terms:
Part A - when using a relay-contact type of output:



if you force an output “off” and it still conducts electricity, then the output is bad ...



Part B - when using a solid-state type of output:



if you force a solid-state type output “off” and it still conducts electricity, then the output might (MIGHT!) still be good ... don’t be surprised ... the output can still be perfectly serviceable even though it does conduct a small amount of electricity even in the “off” condition ... in order to conduct a conclusive test to tell whether the output is “really good” or “really bad”, you’ll need to provide a reasonably heavy load to the output while you’re testing it ...




hopefully this helps ... reality check: if I had to guess, I’d say that your instructor is PROBABLY just looking for the following simplest-most-logical-straight-line answers:



Answer to Question #1: if you force an output “on” and it doesn’t conduct electricity, then the output is bad ...



Answer to Question #2: if you force an output “off” and it still conducts electricity, then the output is defective ...




and to tell you the truth, there are MANY technicians out there working in the field right now who don’t know any more about this subject than that ... but ... as I - and my distinguished colleagues - have been trying to explain, there can be QUITE a lot more to the story than just the simplest “one-size-fits-all” answer that many people have come to accept ... if your instructor is indeed looking for “deeper” answers, then you should certainly try to provide them ... and be thankful for the challenge ...
 
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Voltprobe info.

It might take me a day or two to find the information on the Voltprobe Ron. I've used mine so much, the info is wore off. It's an old tool, but a good one.

Bob
 
Greetings Bob ...

thanks for looking ... I've been hunting for such an animal for years ... as near as I can tell, nobody makes anything like that anymore ... I'll appreciate any information you can give me ... even if it turns up to be a wild goose chase, I still want to be able to say that I've exhausted each and every lead possible ...


basic idea: when I demonstrate the "TRIAC leakage current" effect in my classes, I use a (sensitive) Fluke meter to show the "problem" ... and then I use a small incandescent lamp to show how "loading" the circuit will give a more accurate test ... the main issue is that most plants (wisely) won’t let their technicians use a non-UL approved “test lamp” rig to test the PLC output cards ... so ... I’d love to be able to hold up “Product X from Acme Industries” and tell the students “get one of these and your problems are over” ... unfortunately I can’t find anything that’s currently available that will safely do what my home-brew “test lamps” will do ... Amprobe used to make a VT-24 with a “loading” button on the side ... I’ve called the factory and their technical support people ... they say they don’t make anything like that anymore ...



like I said, I’ll appreciate any information that you can give me ... even if it proves to be a false lead ...
 
thanks Ron and Alaric ...

the Fluke link looks like what I've been needing ... price is about $75.00 and I've just put one on order from my friendly local distributor ...

the description on the net says that it should plug into my existing Fluke (model 25) ... hopefully the search is over ...

trivia: the distributor is McNaughton-McKay ... "big" with about 30 branches ... the sales guy said the computer shows ZERO of these ever being ordered ... not sure I fully believe that - but I have no reason to doubt it either ...

and incidentally ... I remember calling the Fluke technical support department (several years ago) and asking if they made ANYthing that would do this job ... "nope - can't help you" ...

looks like another case where the PLCs.net forum has succeeded when other resources have failed ... thanks again ...

and to Bob ... if possible, I'd still like to know about the tool you're using ... just for variety ...
 
Ron Beaufort said:
the sales guy said the computer shows ZERO of these ever being ordered ... not sure I fully believe that - but I have no reason to doubt it either ...
Maybe that's why this is a "Fluke". (I always loved the name as much as the products they make.)
 

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