# 4 - 20 mA transducer question

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I am trying to use a two wire 4 - 20 mA pressure transducer to a PLC analog input, but the PLC is a voltage input. Suggestions? Thanks in advance.

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I do agree with this answer but I would try to find a 250 ohm precision resistor if possible. Install this resistor across the + and - terminals in parrallel with the transducer and you should have no problems. Could you buy a 4-20ma current input card? I do not know what kind of PLC you are working with or if that would even be possible for you. This could be a bad idea of mine because I work for a large company and do not know the equipment you are working with.

Good Luck, Bengerhamin

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There are a number of relays which will convert from 4-20 ma to 1.5v, or some other voltage range. I think we used some "Action Pak" device for this a while back. It may be necessary to use the relay if distance gets to be an issue.

Michael

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The OP said "2-wire" which means a DC power source is needed to power the transmitter. He probably needs an external power supply (24 VDC is typical) and the voltage input of his PLC should be isolated. Then he can connect as follows:

250 ohm precision resistor across PLC voltage input Power supply positive to 4-20 ma transducer positive. 4-20 transducer negative to PLC positive VDC input Power supply negative to PLC negative VDC input
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Practicallly all transducers use current. This is to prevent voltage drop from the transducer to the measuring circuit. Use a precision resistor at the circuit to convert the current to voltage.

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Put a 10 ohm resistor across the input terminals to the PLC. Then the voltage will be 10 times whatever the current is, ie 40 to 200mV.

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No, this is a silly suggestion. PLCs, and other control systems, have standard signal values. 1-5 VDC is one of them. They are defined by ISA and are used world wide. 40 to 200mV is definitely not. Inventing your own in the face of international standards is very bad engineering unless you really have sound reasons for considering your idea an improvement. It's like designing an appliance that runs on 178 volts, 98 Hz.

Walter.

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not meaning to be rude or anything, but your response seems to indicate a fair amount of irritation at the idea of attempting to use "non-standandard signal values" (i.e. 40 to 200 mV) to drive a response in a non-specific circuit, but you are not offering any clear alternative to the suggestion, nor do you seem interested in obtaining information that might clarify the request enough to give a workable answer.

again, pardon my saying so, but such behavior does little to positively influence the condition resulting in your (assumed) irritation, and if anything, seems most likely to leave at least one other person irritated and hostile, as well... not only at you, but at the information that you are providing, since you are using this information as evidence of your subject's inadequacy, and are not even bothering to give specific enough alternatives to establish whether or not you actually know what you are talking about, or are just venting a desire to tell someone they are wrong, that their reasoning is bad, and make the implication that everyone else in the world knows the "right" answer.

if your irritation has reasonable foundation, your response does not. invoking hostility in the people around you is much more likely to result in direct moves to INCREASE the level of your irritation that would not have been attractive had you not presented your views in an unhelpful-seeming manner, even if that is through just inventing facts which contradict yours and undermining any positive effect your knowledge might have by confusing your audience and making it evident that you are involved in an emotional exchange (which makes anything further that you might say, potentially totally inaccurate or even dangerous for the same reasons).

do you suppose that it would be more efficient to offer a less-vague and more clearly a means to a solution than "1-5 VDC is one of them" and "they are defined by ISA and are used worldwide," which really sounds very much more like reasons for irritation than any usable method of mollifying it? does it not also seem likely that the ISO (not ISA, although I might be willing to refrain from treating your own mistake as evidence that you are an idiot, and offer, instead that you probably just hit the wrong key... all the way on the opposite end of the keyboard - you must have a dvorak layout... wait... no, the O is STILL on the opposite side of the keyboard... maybe you have a twitch or something) PLC signal values are just a teensy bit less accessable to the ininitiated than, say the peak-to-peak and oscillation rates of local wall current? your likening of the two to underline the level of wrong that you are responding to, is not exactly what i would call an accurate illustration of this, particularly since ISO standards are frankly, ignored ALL THE TIME (particularly in America, where ISO standards are, very often, the WRONG standard to follow).

This method of sort of "teaching through public embarrassment" really just gives cause for everyone around you to actively supress any urges toward imaginative experimentation or the pursuit of new knowledge through unorthodox applications (which is where the majority of new knowledge and significant design improvements come from). you do NOT want people who are working with electricity and invention to be resistant to the idea of friendly (not to mention helpful!)collaboration, information-sharing, the pursuit of ever better approaches to existing problems, and most of all, CONFIDENCE IN A PRESENT ABILITY TO BE CAREFUL ENOUGH TO AVOID ENDANGERING PEOPLE OR THEIR PROPERTY THROUGH PREVENTABLE OVERSIGHT

I do agree with your implied assertion, however: that it is sound practice to research applicable standards, simply to gain an understanding of how to shortcut a great deal of trial-and-error by STARTING with an acceptable approach and a high degree of probable (nowhere NEAR 100%) compatibility with the work of others, as well as with common troubleshooting techniques, but it should NEVER be suggested that it is BAD to diverge from these standards. It is merely BETTER to add the expertise behind these standards to one's own work, which will, out of necessity or out of curiosity, almost always diverge from SOME standard, and truly cannot be an improvement on existing implementation without doing so.

This is far from being a BAD thing, but reinventing the WHOLE wheel is time-consuming, when one can far more easily simply make refinements to the shape and construction material of the spokes, and the chances of successfully improving on the design when taken all at once is much lower.

Often reinventing the wheel is a great way to learn a lot of stuff that people may have forgotten. Discouraging this, particularly among the neophyte, is the BAD thing here.

Just out of curiosity, WHY didn't you provide links to the sources of your critical information (and the method of correcting the "error")?

I pretty much, just think you were trying to be superior without actually BEING as superior as you purport to be. In any case it is worse than useless to hoard your information. What good is it, if it doesn;t benefit anyone?

It's this easy:

this is a tremendous area of information: as the introductory page states, the ISO has published over 13,000 standards worldwide. there is absolutely no cause for chagrine at not having perfect recall of every single one of them.

DJ

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Back off buddy - Walter has a considerable amount of experience in the instrumentation field.

And he did not make a typing mistake - the ISA (International Society for Automation) has long been a standards setting body worl-wide in the paractical aspects of automation. The ISO has little input in the instrumentation area.

Bruce.

SBC-NNTP wrote:

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I know you can type the key because you used caps on a whole word.

"non-standandard

techniques,

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And on occasion Walter does appear to be an arrogant, inflexible dinosaur who enjoys taking people to task for imaginative (if somewhat unorthodox) thinking. DJ, I think, made his point rather well.

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"non-standandard

The circuit was quite specific. The information provided was quite complete: How to get a 4-20 mA signal into a PLC that has a voltage input. I had given a very clear alternative in an earlier post, "You are expected to place a 250 Ohm resistor at your input. 0.1% precision resistors for this purpose are readily available."

The relevant specification is ISA 50.1, Compatibility of Analog Signals for Electronic Industrial Process Instruments. It can be summarized in a direct quote of the following section:

3.3 Receivers 3.3.1 The standard current signal shall be 4 to 20 mA, dc (a 16 mA span). 3.3.2 The standard voltage signal shall be 1 to 5 V, dc (a 4 volt span).

It can be summarized even more briefly in a quote from my earlier post. I really saw little value in directing the original questioner to a \$27US document than can be summarized in one sentence. (It was very high tech in

1975.) All PLC vendors follow it.

Please tell me how changing this range could be considered any form of innovation? Using digital communications, that would be innovation. But that was not the original question.

Walter.

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Big Bear,

Ouch! But allow me a bit of defence ...

1 - I try to be concise. Sometimes this comes out as arrogant. I'm not really. Well maybe a little bit. I'm much nicer in person. (You did say "appear to be".)

2 - Complicated is not the same as imaginative or clever. Simple, if it works, is imaginative and clever.

3 - Dinosaur. Well, all I can say is that I'm not extinct yet :-)

Now, I have a question. Why do some people use 249 ohm resistors while the mathematical solution would seem to be 5 volts / 20 mA = 250 Ohm? I've got a drawer full of 250 Ohm precision resistors but I've never seen a 249.

Walter.

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I'm an electronics guy, an instrument designer, Walter, and 249R is an EIA standard E96- and E192-series resistor value (and 250R is not). This is typically what's stocked by electronics distributors in both

1% tolerance and 0.1% tolerance. *I've* got a drawer full of 249R axial lead 1% parts and SMT 0805 1% parts, but have seldom seen a 250R0. Nor do my suppliers show a 250.0 ohm in either 1% or 0.1% tolerance as a standard product! Using standard values, two 499R in parallel would get you a bit closer, and four 1K00 would be bang-on (plus tolerance).

250.0 ohm must be a non-standard value made special for the process-control industry. That's not expensive particularly, you just have to order a bunch of parts (thousands) from a manufacturer, pay much more per part, or buy them from someone who did one of those.

Best regards, Spehro Pefhany

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Us Technicians love it when engineers do CrAzY stuff and don't bother to even make a print of it. It really helps us out out with lots of overtime pay. The plant managment, well we all know what they think like... A nice 250 ohm resistor to bring 4-20 ma to 1-5 v would be the standard thing to do. BUT you do what you want and we'll be here to pick up the pieces when ya leave.

P_Man

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150R in series with 100R is also bang on and both are in the E6 through E96 ranges. 250R is not an uncommon value for ultra precision resistors (like 0.01%) but that would be expensive and likely overkill.
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I gave up reading it about 1/4 way through his diatribe. No offense toward him intended.

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