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.
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
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
I have to interface 4-20 ma D/Ps to a building automation system at
validated FDA sites and the standard we employ is for interface to a 0-10VDC
input is a 24VDC external PS and a 499 ohm resistor giving a range of 2-10
VDC; use a NIST source to calibrate the controller input
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.
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
Back off buddy - Walter has a considerable amount of experience in the
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
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:
3.3.1 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.
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.
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
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.
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.
150R in series with 100R is also bang on and both are in the E6 through E96
250R is not an uncommon value for ultra precision resistors (like 0.01%)
but that would be expensive and likely overkill.