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Old December 15th, 2006, 09:54 AM   #1
staleyt
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Reading Current on 1756-IF16

This is my first time dealing with PLC hardware, so forgive me for a possibly simple problem.

I'm trying to take a 0-50mV signal from a welder and feed it into an Allen Bradley 1756-IF16 analog input. The input module is configured to for differential inputs. The 0-50mV signal represents 0-1250A coming from a welder, and I have taken the wires directly off of the input and output of the analog display on the welder.

I have successfully wired one welder up so I can read the current on the computer, but when two welders are wired into the same module, I run into problems. Both welders must be on in order for the displays to show a nonzero value, and when they are both on, the values I am reading seem like they are low.

The low voltage on the current meter is the voltage being outputted by the welder, which can vary from 0-70V. Could this be a problem, even though the signal is only being compared to something 0-50mV higher than it?

Thanks to anyone who may be able to help!
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Old December 15th, 2006, 12:11 PM   #2
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I am not sure but, I think that module is NOT an isolated module. That means that all the inputs have to use the same voltage source for the inputs. You need an isolated card to use more than one voltage source. If I had manuals on this PC I would look for you.

Since you got one to work and the problem surfaced when you attached the other. This could be it?

Look at the wiring diagrams in the manual really close. Make sure that this is not your problem.

One of the guys that know for sure will be along soon am sure.

Good luck

RSL
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Old December 15th, 2006, 01:31 PM   #3
Ron Beaufort
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Greetings staleyt ...



and welcome to the forum ...



your first problem is that the module you’re using is not rated for 0 to 50 mA ... instead, its highest rated current is 20.5 mA ... this could cause a “pegged-needle” type of issue after your welder gets up to about half its current capability ... worse case: you could conceivably damage the module with the higher current ...



you said that you’re new at this - so here’s a tip ... besides using the software (RSLogix5000) to configure the module for “differential” connections, you also need to make sure that the wiring on the module’s terminal block is also done correctly for a “differential” setup ...








the “jumper wires” that I’ve highlighted above are the connections that seem to give most people the most trouble ...



suggestion: if all else fails, you might want to look into a 1756-IF6I module ... this one has “isolated” inputs ... the specified input range is still going to be about 20 mA - not 50 - so you’re really going to have to do something about that issue before you go much further ... different companies make “signal conditioners” that can take in one particular range - and then put out another range ... the guys here on the forum can help with that idea if you need some guidance ...



and incidentally, if you have the RSLogix5000 software, then the user manuals for most of these modules are probably already installed on your computer ... look under “Help - Online Books - ControlLogix Manuals” ... the “Specifications” chapter is probably the first thing you’ll need to look at ...



good luck with your project ...
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Old December 15th, 2006, 01:39 PM   #4
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Ron - he said "mV", not "mA".

It sounds like he is picking the signal off a meter shunt. I would suggest a transducer to step the signal up to a more usable range, such as 0-5V, or 0-10V.
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Old December 15th, 2006, 01:55 PM   #5
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You're right, Tom. It's a 0-50 millivolt signal that represents a 0-1250 amp current.

I thought I would need a transducer, too, but I can read one signal just fine. I can also read two signals when they are on different modules. Right now, I have one welder's current meter wired to a 1756-IF16 in single-wire mode, and another to the same model in differential mode, and I have no problems. But, I would like to have both of these signals wired to the differential model because the virtual ground would be different for possible new signals. Sorry if this doesn't make sense.

Thanks for the responses so far, everyone! And Ron, I really got a kick out of your signature.
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Old December 15th, 2006, 01:56 PM   #6
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Quote:
Ron - he said "mV", not "mA".
duh! ... thanks, Tom ... how did I miss that? ... I've been trying to get over a BAD cold for the past week or so - but I didn't know that my brain was THAT fried ...

sorry for the bum steer, staleyt ... please just disregard everything I said ... except for the "welcome to the forum" ...
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Old December 20th, 2006, 10:39 AM   #7
staleyt
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Are there any suggestions for my problem? Please see the first post, and thanks for any help!
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Old December 20th, 2006, 10:49 AM   #8
Mickey
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As Tomalbright subjested I would use something like this
see TRY,TRX millvolt range -50 to 1000mv
http://www.miinet.com/products/sg_temperature.shtml

There are other brands as well.
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Old December 20th, 2006, 10:55 AM   #9
staleyt
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The low signal isn't the problem, though; the problem is that I can't have two signals on one module. I can read one signal just fine.
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Old December 20th, 2006, 11:47 AM   #10
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I don't have the details, but I believe there is a limit to the potential difference between two individual signals on the same non-isolated module. I haven't yet found it in the manual.


Could it be that your two signals are excceeding this limit?

EDIT: I am probably wrong about this, I can't find any mention in the book...

Last edited by OkiePC; December 20th, 2006 at 11:53 AM.
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Old December 20th, 2006, 01:31 PM   #11
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OkiePC is on the right track...

The 1756-IF16 is not islolated. Wiring it in differential mode doesn't make it isolated either. The manual shows internal power at +15v and -15v relative to the RTN terminal. All RTN terminals are wired together. That means all signals have to be in that same 30v window at all times. I'm betting your two welder signals have significant voltage difference between them. (i.e. no common reference)

Buy signal isolators for each welder for two reasons. First, they will allow you to wire the signals to the same card. Second, they will allow you to use the full range of the input card. At 50mv you are only using 1% of the range even with 0-5v selected.

My recommendation is to buy an isolator that will take your 0-50mv and convert it to 4-20ma. You can also get one that is powered from the 4-20ma loop. This will allow you to install the converter in the welder and run a single twisted pair to the PLC. Put the loop power supply in the PLC panel.

Good luck,
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Old December 20th, 2006, 09:14 PM   #12
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I mentioned above, "I think that module is NOT an isolated module"

You need to isolate the signals to read them both. The 4-20 ma route would be my choice too.

RSL
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