4 - 20 mA transducer question

[michaelR]

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.

 

[default]

You don't say if you have a power supply . . . The two wire
transducer doesn't output 4-20. It current regulates a power source
and allows between 4-20 ma to flow based on zero and span of the
process being monitored.

So you do need a power source and resistor to develop the voltage
across that you will input to your voltage monitor. Resistor value
would be (calculated) based on the voltage input to the PLC.

On Sat, 02 Aug 2003 15:10:30 GMT, michaelR <mjrx@comcast.net> wrote:

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.

-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
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[Ben]

Spehro Pefhany <speff@interlog.com> wrote in message news:<lmlniv06sieou7jkpmssbjiitck6tjdqpa@4ax.com>...

On Sat, 02 Aug 2003 15:10:30 GMT, the renowned michaelR
mjrx@comcast.net> wrote:

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.

A 249R 1% load resistor will give you close to 1~5V for 4-20mA.

Best regards,
Spehro Pefhany

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

 

[Herman Family]

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


"Gerald Newton" <senior_citizen@alaska.com> wrote in message
news:1059863182.778227@prawn...

"Spehro Pefhany" <speff@interlog.com> wrote in message
news:lmlniv06sieou7jkpmssbjiitck6tjdqpa@4ax.com...
On Sat, 02 Aug 2003 15:10:30 GMT, the renowned michaelR
mjrx@comcast.net> wrote:

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.

A 249R 1% load resistor will give you close to 1~5V for 4-20mA.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the
reward"
speff@interlog.com Info for manufacturers:
http://www.trexon.com
Embedded software/hardware/analog Info for designers:
http://www.speff.com

That is typically how it is done. 1-5 vdc analog to digital converters
can
be purchased that will plug into your PC!

 

[no spam]

daestrom wrote:

"Ben" <bengerhamin@yahoo.com> wrote in message
news:7b8c4003.0308021702.432a6927@posting.google.com...


Spehro Pefhany <speff@interlog.com> wrote in message


news:<lmlniv06sieou7jkpmssbjiitck6tjdqpa@4ax.com>...


On Sat, 02 Aug 2003 15:10:30 GMT, the renowned michaelR
mjrx@comcast.net> wrote:



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.


A 249R 1% load resistor will give you close to 1~5V for 4-20mA.

Best regards,
Spehro Pefhany


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.




You may want to put the resistor across the the PLC voltage input instead.
By running the 4-20 mA signal through the leads from the transducer to the
PLC, you gain one of the advantages of said current loops, noise immunity.

Putting the resistor across the transducer means the leads from it are
carrying 1-5VDC to the PLC. The PLC input is *probably* a very high input
impedance and any noise picked up in the leads will affect it. By putting
the resistor at the PLC end, its low impedance will help reduce the effects
of any noise induced in the leads.

daestrom

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

 

[Russell Powell]

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.

"michaelR" <mjrx@comcast.net> wrote in message
news:3F2BD452.7D38F2DC@comcast.net...

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.

 

[JOE KEITER]

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
"michaelR" <mjrx@comcast.net> wrote in message
news:3F2BD452.7D38F2DC@comcast.net...

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.

 

[Cornholio]

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.


On Sat, 02 Aug 2003 15:10:30 GMT, michaelR <mjrx@comcast.net> wrote:

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.

 

[Walter Driedger]

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.

"Cornholio" <dennyh@newsguy.com> wrote in message
news:a30hjvkqj8q08ipm6joduj2rv7732bjcsb@4ax.com...

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.

On Sat, 02 Aug 2003 15:10:30 GMT, michaelR <mjrx@comcast.net> wrote:

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.

 

[SBC-NNTP]

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: http://www.iso.org/iso/en/ISOOnline.frontpage

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


"Walter Driedger" <walter@driesmithdger.ca> wrote in message
news:Yeh_a.706741$ro6.14681043@news2.calgary.shaw.ca...

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.

"Cornholio" <dennyh@newsguy.com> wrote in message
news:a30hjvkqj8q08ipm6joduj2rv7732bjcsb@4ax.com...

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.

On Sat, 02 Aug 2003 15:10:30 GMT, michaelR <mjrx@comcast.net> wrote:

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.

 

[Bruce Durdle]

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:

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: http://www.iso.org/iso/en/ISOOnline.frontpage

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


"Walter Driedger" <walter@driesmithdger.ca> wrote in message
news:Yeh_a.706741$ro6.14681043@news2.calgary.shaw.ca...


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.

"Cornholio" <dennyh@newsguy.com> wrote in message
news:a30hjvkqj8q08ipm6joduj2rv7732bjcsb@4ax.com...



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.

On Sat, 02 Aug 2003 15:10:30 GMT, michaelR <mjrx@comcast.net> wrote:



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.

 

[Gym Bob]

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


"SBC-NNTP" <djbartlett@EvilGeniusTM.com> wrote in message
news:yiN_a.1020$Z%4.41@newssvr25.news.prodigy.com...

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: http://www.iso.org/iso/en/ISOOnline.frontpage

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


"Walter Driedger" <walter@driesmithdger.ca> wrote in message
news:Yeh_a.706741$ro6.14681043@news2.calgary.shaw.ca...
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.

"Cornholio" <dennyh@newsguy.com> wrote in message
news:a30hjvkqj8q08ipm6joduj2rv7732bjcsb@4ax.com...

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.

On Sat, 02 Aug 2003 15:10:30 GMT, michaelR <mjrx@comcast.net> wrote:

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.

 

[Ursa Major]

"Bruce Durdle" <bmdurdle@clear.net.nz> wrote in message
news:3F3BE195.9000502@clear.net.nz...

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.

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.

 

[Walter Driedger]

"SBC-NNTP" <djbartlett@EvilGeniusTM.com> wrote in message
news:yiN_a.1020$Z%4.41@newssvr25.news.prodigy.com...

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.

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.

 

[Walter Driedger]

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.

"Ursa Major" <ursamajor@bigbear.com> wrote in message
news:5Kc%a.123066$It4.51612@rwcrnsc51.ops.asp.att.net...

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.

 

[Spehro Pefhany]

On Sat, 16 Aug 2003 04:30:51 GMT, the renowned "Walter Driedger"
<walter@driesmithdger.ca> wrote:

<snip>

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.

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).

http://www.logwell.com/tech/components/resistor_values.html

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
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[P_Man]

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


On Wed, 13 Aug 2003 02:19:36 GMT, "Walter Driedger"
<walter@driesmithdger.ca> wrote:

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.

"Cornholio" <dennyh@newsguy.com> wrote in message
news:a30hjvkqj8q08ipm6joduj2rv7732bjcsb@4ax.com...

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.

On Sat, 02 Aug 2003 15:10:30 GMT, michaelR <mjrx@comcast.net> wrote:

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.

 

[Gym Bob]

Much more readable, just way too long but you held my interest for most of
it....LOL

Never happy eh?

"DJ Bartlett" <djbartlett@EvilGeniusTM.com> wrote in message
news:_KG%a.2024$9_6.125985854@newssvr13.news.prodigy.com...

I believe that apologies are in order.

I'm a little embarassed that I didn't bother to watch the group a bit to
see
what the general temperature of the discussions were like and get a gauge
on
major players' personalities. I jumped right in feet first without
checking
back messages in the thread (the server I am currently using keeps an
extremely limited backlog of messages).

I also stand corrected on the ISA bit. I appear to be guilty of pretty
much
the same thing I was berating you for: using emotional pressure rather
than
information to convince the audience.

Considering that you had already given a detailed answer and then some,
and
that you obviously DO know what you are talking about (once I slowed down
enough to actually follow my assumptions up), a somewhat gruff answer
doesn't seem quite as out of place as it did at first.

However, it drives me crazy when I see something that looks like an
attempt
to supress someone trying to use their own brain to solve a problem (even
if
the approach is less-than-optimal), particularly if the question makes it
evident that the request is being put forth by someone who is relatively
new
to a discipline.

I too, am reasonably new to the subject of applied electronics (thus
missing
the ISA acronym - which goes far, in my not so humble opinion, toward
proving my point about the accessability of the information to the
uninitiated - I am NO stranger to researching appropriate standards in a
fair number of fields), though I have more than enough theoretical chops
from various areas to keep up with a whole lot more than most newbies.

I am not a young whippersnapper either. I have been opposing , for as long
as I can remember, the hard-core academic tendancy to paint pictures in
students' minds of some sort of absolute authority that can never say a
wrong thing, should never be questioned, and whose edicts apply to all
situations and always will.

This lesson is what caused so many of the great innovators to live their
entire lives practically peniless (or burned, hanged, jailed, eviscerated,
or simply overlooked long enough to ensure their ommission from the
textbooks of the next 200 years. At least one of them was made to
voluntarily drink hemlock (if the story is true), and much new thought had
been destroyed or perverted by religeous edict as heresy. I should
probably
be thankful that we mostly just snipe and belittle each other nowadays for
not having always known "the truth," no matter that this truth is kept at
the bottom of a locked filing cabinet in a disused lavatory with a sign on
the door saying "beware of the leopard, and that this truth is really just
some people getting together and agreeing to try to convince everyone else
in the world to use "our new PLC's (they're great!)" instead of "brand X
collection of YUCKIE!!! discretes" even though NEITHER approach really
improves the nature of life on earth in any measurable way.

I defend error and experiment because it is hope for new understanding...
not of an authorized panel of approved high holy experts in the field
bringing understanding to the world in the form of universal compliance
(uhhh... acceptance), but of an individual who is expressing one of the
most
innocent and benign urges that humans seem to have, ever... CURIOSITY
(yes,
I know that they blow themselves and each other up a lot with the
by-products, but not as often or with as much efficiency or intent, for
that
matter as the authorized experts seem to).

This should honestly be applauded and encouraged (the curiosity, not the
blowing up). It is the intent to innovate which is the innovation here, in
spite of the present lack of well-known understanding (easily corrected)
and
the re-invent-the-wheel nature of the intent. Think back... didn't you
also
learn by performing many of the classic experiments? All innovation MUST
be
preceded by the intent to innovate, and the temerity to think that one can
through the use of one's own faculties arrive at something as akin to the
truth as an industry expert.

You started with the same level of knowledge. We must all progress from
zero-understanding to whatever level of expertise we wind up achieving. We
shouldn't try to make others miserable for taking the steps. Wouldn't you
have agreed when you were in the same place?

Actually, I am definitely going to give you the benefit of the doubt in
light of your totally unhostile (and far less defensive than I was
expecting) response. I expect that it is your intent to help people come
to
deeper understanding of this subject while gaining some of your own. I'll
bet you didn't expect the philosophy homework as assigned by a student?

One more time, I apologize for the roughness of my response, but I am not
embarassed by my underlying opinion and I'm going to let that stand
without
retraction.

Thanks hugely for the informative bits, and since I'm as able as anyone
else
not involved in the chain of events leading up to 249Ohm resisters being
used, and am curious about the real answer, if it's ever forthcoming, I
have
a tentative guess at a possible cause...

It seems highly likely that the not-quite perfectly linear behavior of
semiconductors to actual real-world application might cause one to arrive
at
the use of a 249Ohm resister and an trimmer for fine-adjustment a slightly
better choice along some lines of thinking than the assumption of a
"theoretically ideal" 250Ohms and a trimmer for fine-adjustment, although,
I
can think of about 10 reasons off the top of my head why it doesn't really
matter, either way (all guesses).

The 250 is more popular, surely (my guess), due to the nice round, neat
package of different base-10 multiples (another guess). But some people
are
fiercely adherant to certain approaches, even if met with a good argument
to
consider alternatives (an opinion based on plenty of experience).

Alternatively: maybe some production foul-up resulted in a warehouse full
the the eyeballs with millions of precision 249Ohm resisters which some
regional marketing vice president was ordered to unload at minimum loss
"or
else" and he wheeled and dealed until enough purchasing people were
convinced that it didn't matter at the price being quoted that a certain
batch of units 10 years down the road came out of the woodwork in ones and
twos whenever someone or other got to scraping the bottom of the barrel
for
250s and found only the discount precision 249s...

feasable?

maybe the 249s were intended to adjust for the ~1.1CR time constant with a
nice round result... I'll check my math Monday.
I seem to have a mental block re: simple multiplication/division today.

I really don't want to shoot option 2 out of the water, owing to some
really
entertaining mental imagery it calls up.

The suggestions illustrate 3 examples of how unspoken standards can be
understandably, intentionally avoided in violation of common assumptions.

1). Narrow scope of possible approaches - often indirectly/directly caused
by less-than-applicable-but-inflexible regulations
2). Murphy's Law - a little social engineering can beat a similar amount
of
electrical engineering dollar for dollar (at least long enough for
everyone
to get out of the area before anyone likely to want to kill notices the
problem).
3). Man likes nice neat, easy-to-remember black/white universal truths
that
can be tallied in multiples of 10 (?? this is the part that cracks ME
up)and
grudgingly with lots of complaining, pi, e and whatever specific
relationship between values that is represented by "electronic pi" without
undue effort. Nature, stubbornly refuses to cooperate in any way that
actually makes anything easier to intuitively understand, possible, or
even
capable of being remembered or calculated with 100% accuracy, no matter
how
much effort man pours into insisting that anything less significant than
the
1st decimal is too much of a hassle to really matter, even in cases of
being
raised to a very large irrational exponent. Funny thing is... he's
right...
and proves it regularly, but he LOVES to play devil's advocate with his
neighbors...

Rant off,

I am pleased and gratified by everyone's responses (even the one that
defended walter by verbally attacking me, and the one poking fun at my not
using caps - I am frankly more distressed by not catching my numerous
typos
before sending, I drop caps when I prioritize speed and point being made
over grammatical redundancies like capitalization (I use most appropriate
punctuation unless intentionally diverging from MHA recommendation to most
efficiently make an implied statement or to lighten the mood of my writing
slightly). I rarely use spelling/grammer checkers unless I'm in definite
doubt (hee hee - JUMBO shrimp), but I am very good at recalling those
sorts
of things with a little direct attention and I don't want to ever get too
dependant on the limited ability of a machine to simultaneously pass
intentional dramatic nuance and fix 100% of grammatical and spelling
errors
with enough discernment to satisfy me).

You guys actually all seem pretty okay, in my book.

Again, Walter, thank you for posting the summary of the standards document
in question. my next post will be direct responses to your other e-mail's
questions (some of which are included in this one).

Lastly, consider yourselves lucky. I removed a good deal of my original
line
of thought re: philosophy of standards development and human individuality
and imagination as deserving further development, but not as a direct
negative commentary on anyone's behavior or lines of thinking. I'd be
happy
to share it once/if ever I clean it up and finish the line of thought, if
anyone's at all interested in that sort of thing.

DJ

 

[DJ Bartlett]

I believe that apologies are in order.

I'm a little embarassed that I didn't bother to watch the group a bit to see
what the general temperature of the discussions were like and get a gauge on
major players' personalities. I jumped right in feet first without checking
back messages in the thread (the server I am currently using keeps an
extremely limited backlog of messages).

I also stand corrected on the ISA bit. I appear to be guilty of pretty much
the same thing I was berating you for: using emotional pressure rather than
information to convince the audience.

Considering that you had already given a detailed answer and then some, and
that you obviously DO know what you are talking about (once I slowed down
enough to actually follow my assumptions up), a somewhat gruff answer
doesn't seem quite as out of place as it did at first.

However, it drives me crazy when I see something that looks like an attempt
to supress someone trying to use their own brain to solve a problem (even if
the approach is less-than-optimal), particularly if the question makes it
evident that the request is being put forth by someone who is relatively new
to a discipline.

I too, am reasonably new to the subject of applied electronics (thus missing
the ISA acronym - which goes far, in my not so humble opinion, toward
proving my point about the accessability of the information to the
uninitiated - I am NO stranger to researching appropriate standards in a
fair number of fields), though I have more than enough theoretical chops
from various areas to keep up with a whole lot more than most newbies.

I am not a young whippersnapper either. I have been opposing , for as long
as I can remember, the hard-core academic tendancy to paint pictures in
students' minds of some sort of absolute authority that can never say a
wrong thing, should never be questioned, and whose edicts apply to all
situations and always will.

This lesson is what caused so many of the great innovators to live their
entire lives practically peniless (or burned, hanged, jailed, eviscerated,
or simply overlooked long enough to ensure their ommission from the
textbooks of the next 200 years. At least one of them was made to
voluntarily drink hemlock (if the story is true), and much new thought had
been destroyed or perverted by religeous edict as heresy. I should probably
be thankful that we mostly just snipe and belittle each other nowadays for
not having always known "the truth," no matter that this truth is kept at
the bottom of a locked filing cabinet in a disused lavatory with a sign on
the door saying "beware of the leopard, and that this truth is really just
some people getting together and agreeing to try to convince everyone else
in the world to use "our new PLC's (they're great!)" instead of "brand X
collection of YUCKIE!!! discretes" even though NEITHER approach really
improves the nature of life on earth in any measurable way.

I defend error and experiment because it is hope for new understanding...
not of an authorized panel of approved high holy experts in the field
bringing understanding to the world in the form of universal compliance
(uhhh... acceptance), but of an individual who is expressing one of the most
innocent and benign urges that humans seem to have, ever... CURIOSITY (yes,
I know that they blow themselves and each other up a lot with the
by-products, but not as often or with as much efficiency or intent, for that
matter as the authorized experts seem to).

This should honestly be applauded and encouraged (the curiosity, not the
blowing up). It is the intent to innovate which is the innovation here, in
spite of the present lack of well-known understanding (easily corrected) and
the re-invent-the-wheel nature of the intent. Think back... didn't you also
learn by performing many of the classic experiments? All innovation MUST be
preceded by the intent to innovate, and the temerity to think that one can
through the use of one's own faculties arrive at something as akin to the
truth as an industry expert.

You started with the same level of knowledge. We must all progress from
zero-understanding to whatever level of expertise we wind up achieving. We
shouldn't try to make others miserable for taking the steps. Wouldn't you
have agreed when you were in the same place?

Actually, I am definitely going to give you the benefit of the doubt in
light of your totally unhostile (and far less defensive than I was
expecting) response. I expect that it is your intent to help people come to
deeper understanding of this subject while gaining some of your own. I'll
bet you didn't expect the philosophy homework as assigned by a student?

One more time, I apologize for the roughness of my response, but I am not
embarassed by my underlying opinion and I'm going to let that stand without
retraction.

Thanks hugely for the informative bits, and since I'm as able as anyone else
not involved in the chain of events leading up to 249Ohm resisters being
used, and am curious about the real answer, if it's ever forthcoming, I have
a tentative guess at a possible cause...

It seems highly likely that the not-quite perfectly linear behavior of
semiconductors to actual real-world application might cause one to arrive at
the use of a 249Ohm resister and an trimmer for fine-adjustment a slightly
better choice along some lines of thinking than the assumption of a
"theoretically ideal" 250Ohms and a trimmer for fine-adjustment, although, I
can think of about 10 reasons off the top of my head why it doesn't really
matter, either way (all guesses).

The 250 is more popular, surely (my guess), due to the nice round, neat
package of different base-10 multiples (another guess). But some people are
fiercely adherant to certain approaches, even if met with a good argument to
consider alternatives (an opinion based on plenty of experience).

Alternatively: maybe some production foul-up resulted in a warehouse full
the the eyeballs with millions of precision 249Ohm resisters which some
regional marketing vice president was ordered to unload at minimum loss "or
else" and he wheeled and dealed until enough purchasing people were
convinced that it didn't matter at the price being quoted that a certain
batch of units 10 years down the road came out of the woodwork in ones and
twos whenever someone or other got to scraping the bottom of the barrel for
250s and found only the discount precision 249s...

feasable?

maybe the 249s were intended to adjust for the ~1.1CR time constant with a
nice round result... I'll check my math Monday.
I seem to have a mental block re: simple multiplication/division today.

I really don't want to shoot option 2 out of the water, owing to some really
entertaining mental imagery it calls up.

The suggestions illustrate 3 examples of how unspoken standards can be
understandably, intentionally avoided in violation of common assumptions.

1). Narrow scope of possible approaches - often indirectly/directly caused
by less-than-applicable-but-inflexible regulations
2). Murphy's Law - a little social engineering can beat a similar amount of
electrical engineering dollar for dollar (at least long enough for everyone
to get out of the area before anyone likely to want to kill notices the
problem).
3). Man likes nice neat, easy-to-remember black/white universal truths that
can be tallied in multiples of 10 (?? this is the part that cracks ME up)and
grudgingly with lots of complaining, pi, e and whatever specific
relationship between values that is represented by "electronic pi" without
undue effort. Nature, stubbornly refuses to cooperate in any way that
actually makes anything easier to intuitively understand, possible, or even
capable of being remembered or calculated with 100% accuracy, no matter how
much effort man pours into insisting that anything less significant than the
1st decimal is too much of a hassle to really matter, even in cases of being
raised to a very large irrational exponent. Funny thing is... he's right...
and proves it regularly, but he LOVES to play devil's advocate with his
neighbors...

Rant off,

I am pleased and gratified by everyone's responses (even the one that
defended walter by verbally attacking me, and the one poking fun at my not
using caps - I am frankly more distressed by not catching my numerous typos
before sending, I drop caps when I prioritize speed and point being made
over grammatical redundancies like capitalization (I use most appropriate
punctuation unless intentionally diverging from MHA recommendation to most
efficiently make an implied statement or to lighten the mood of my writing
slightly). I rarely use spelling/grammer checkers unless I'm in definite
doubt (hee hee - JUMBO shrimp), but I am very good at recalling those sorts
of things with a little direct attention and I don't want to ever get too
dependant on the limited ability of a machine to simultaneously pass
intentional dramatic nuance and fix 100% of grammatical and spelling errors
with enough discernment to satisfy me).

You guys actually all seem pretty okay, in my book.

Again, Walter, thank you for posting the summary of the standards document
in question. my next post will be direct responses to your other e-mail's
questions (some of which are included in this one).

Lastly, consider yourselves lucky. I removed a good deal of my original line
of thought re: philosophy of standards development and human individuality
and imagination as deserving further development, but not as a direct
negative commentary on anyone's behavior or lines of thinking. I'd be happy
to share it once/if ever I clean it up and finish the line of thought, if
anyone's at all interested in that sort of thing.

DJ

 

Gym Bob wrote:

Much more readable, just way too long but you held my interest for most of
it....LOL

I gave up reading it about 1/4 way through his diatribe.
No offense toward him intended.