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Discussion Groups | Piclist | 4-20ma Analog Current Loop to Pic A/D
A discussion group for the PICMicro microcontroller. Also called the Microchip PIC, this list is dedicated to the use and abuse of this fine, simple, microcontroller. Close to topic posts are welcome, ie. general electronics.
4-20ma Analog Current Loop to Pic A/D - kg4pid - Jan 20 20:57:54 2009
I'm trying to find a way to interface a pressure sensor that uses a 4-
20ma analog current loop and I would like to read it using a Pic A/D
converter. I have tried using google but have not found what I'm
looking for. There will be about 30 feet of cable between the sensor
and the Pic / interface circuit. The sensor is currently connected to
a device called a Labjack which is loaned to me and must be returned.
If possible I'd like to tap in to the loop and read the current and
compare it to the Labjack readings before disconnectomg the Labjack
completly. Looks like I'll need a sutable power supply also. Anyone
here have any experience in this area?
Thanks Max
------------------------------------
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20ma analog current loop and I would like to read it using a Pic A/D
converter. I have tried using google but have not found what I'm
looking for. There will be about 30 feet of cable between the sensor
and the Pic / interface circuit. The sensor is currently connected to
a device called a Labjack which is loaned to me and must be returned.
If possible I'd like to tap in to the loop and read the current and
compare it to the Labjack readings before disconnectomg the Labjack
completly. Looks like I'll need a sutable power supply also. Anyone
here have any experience in this area?
Thanks Max
------------------------------------
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Re: 4-20ma Analog Current Loop to Pic A/D - Don Pomplun - Jan 20 23:02:12 2009
At 05:57 PM 1/20/2009, you wrote:
>I'm trying to find a way to interface a pressure sensor that uses a 4-
>20ma analog current loop and I would like to read it using a Pic A/D
>converter. I have tried using google but have not found what I'm
>looking for. There will be about 30 feet of cable between the sensor
>and the Pic / interface circuit. The sensor is currently connected to
>a device called a Labjack which is loaned to me and must be returned.
>If possible I'd like to tap in to the loop and read the current and
>compare it to the Labjack readings before disconnectomg the Labjack
>completly. Looks like I'll need a sutable power supply also. Anyone
>here have any experience in this area?
>
>Thanks Max
Assuming the pressure transducer isn't powered, you'll need a voltage
source to power it. A 12v wall wart should do. Voltage level, or
regulation isn't critical. Isolation from ground IS, but that should be
easy. Just put a resistor in the current loop, whose value will drop 20 ma
to give the voltage you want. e.g. if you are feeding a 0-1 volt input,
use R = 1 / 0.020 = 50 ohms; if your input is 0-5 volt, then R = 5 / 0.020
= 250 ohm. Any wattage is OK. When you scale the pressure value, don't
forget that 4 ma (1/5 of the full input voltage, not zero) represents the
bottom pressure value). But that's only software ;=)
Don (CA)
(You need to be a member of piclist -- send a blank email to piclist-subscribe@yahoogroups.com )
>I'm trying to find a way to interface a pressure sensor that uses a 4-
>20ma analog current loop and I would like to read it using a Pic A/D
>converter. I have tried using google but have not found what I'm
>looking for. There will be about 30 feet of cable between the sensor
>and the Pic / interface circuit. The sensor is currently connected to
>a device called a Labjack which is loaned to me and must be returned.
>If possible I'd like to tap in to the loop and read the current and
>compare it to the Labjack readings before disconnectomg the Labjack
>completly. Looks like I'll need a sutable power supply also. Anyone
>here have any experience in this area?
>
>Thanks Max
Assuming the pressure transducer isn't powered, you'll need a voltage
source to power it. A 12v wall wart should do. Voltage level, or
regulation isn't critical. Isolation from ground IS, but that should be
easy. Just put a resistor in the current loop, whose value will drop 20 ma
to give the voltage you want. e.g. if you are feeding a 0-1 volt input,
use R = 1 / 0.020 = 50 ohms; if your input is 0-5 volt, then R = 5 / 0.020
= 250 ohm. Any wattage is OK. When you scale the pressure value, don't
forget that 4 ma (1/5 of the full input voltage, not zero) represents the
bottom pressure value). But that's only software ;=)
Don (CA)
(You need to be a member of piclist -- send a blank email to piclist-subscribe@yahoogroups.com )
Re: 4-20ma Analog Current Loop to Pic A/D - "Roy E. Burrage" - Jan 21 7:26:13 2009
Unless you really want to re-invent the wheel, why not just buy a
Labjack and keep using what you have, Max? They're not but about $120
or so and ship from stock in the Denver, CO area. We use them here and
have been quite pleased with their performance.
So far as the power supply, most current loops use a 24 volt DC supply
as the standard. If you interface with a Labjack you might want to
lower that to what ever is the lowest for rated use with your
transducer. Stick a temperature stable resistor, probably wire wound
would be best, in the line and then measure the voltage drop. Just be
sure that you don't exceed the ratings of either your transducer or your
Labjack.
REB
kg4pid wrote:
> I'm trying to find a way to interface a pressure sensor that uses a 4-
> 20ma analog current loop and I would like to read it using a Pic A/D
> converter. I have tried using google but have not found what I'm
> looking for. There will be about 30 feet of cable between the sensor
> and the Pic / interface circuit. The sensor is currently connected to
> a device called a Labjack which is loaned to me and must be returned.
> If possible I'd like to tap in to the loop and read the current and
> compare it to the Labjack readings before disconnectomg the Labjack
> completly. Looks like I'll need a sutable power supply also. Anyone
> here have any experience in this area?
>
> Thanks Max
>
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Labjack and keep using what you have, Max? They're not but about $120
or so and ship from stock in the Denver, CO area. We use them here and
have been quite pleased with their performance.
So far as the power supply, most current loops use a 24 volt DC supply
as the standard. If you interface with a Labjack you might want to
lower that to what ever is the lowest for rated use with your
transducer. Stick a temperature stable resistor, probably wire wound
would be best, in the line and then measure the voltage drop. Just be
sure that you don't exceed the ratings of either your transducer or your
Labjack.
REB
kg4pid wrote:
> I'm trying to find a way to interface a pressure sensor that uses a 4-
> 20ma analog current loop and I would like to read it using a Pic A/D
> converter. I have tried using google but have not found what I'm
> looking for. There will be about 30 feet of cable between the sensor
> and the Pic / interface circuit. The sensor is currently connected to
> a device called a Labjack which is loaned to me and must be returned.
> If possible I'd like to tap in to the loop and read the current and
> compare it to the Labjack readings before disconnectomg the Labjack
> completly. Looks like I'll need a sutable power supply also. Anyone
> here have any experience in this area?
>
> Thanks Max
>
------------------------------------
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Re: 4-20ma Analog Current Loop to Pic A/D - kg4pid - Jan 21 11:14:22 2009
Well the Labjack is nice, but, Labjack + software + PC (for 24 hour
monitoring) = big $. If I can interface to this pressure sensor with
a Pic, then I can build a datalogger and only use a PC to view the
data. Using just a resistor seems too easy.
Max
--- In p...@yahoogroups.com, "Roy E. Burrage"
wrote:
>
> Unless you really want to re-invent the wheel, why not just buy a
> Labjack and keep using what you have, Max? They're not but about
$120
> or so and ship from stock in the Denver, CO area. We use them
here and
> have been quite pleased with their performance.
>
> So far as the power supply, most current loops use a 24 volt DC
supply
> as the standard. If you interface with a Labjack you might want
to
> lower that to what ever is the lowest for rated use with your
> transducer. Stick a temperature stable resistor, probably wire
wound
> would be best, in the line and then measure the voltage drop.
Just be
> sure that you don't exceed the ratings of either your transducer
or your
> Labjack.
> REB
> kg4pid wrote:
> > I'm trying to find a way to interface a pressure sensor that
uses a 4-
> > 20ma analog current loop and I would like to read it using a Pic
A/D
> > converter. I have tried using google but have not found what I'm
> > looking for. There will be about 30 feet of cable between the
sensor
> > and the Pic / interface circuit. The sensor is currently
connected to
> > a device called a Labjack which is loaned to me and must be
returned.
> > If possible I'd like to tap in to the loop and read the current
and
> > compare it to the Labjack readings before disconnectomg the
Labjack
> > completly. Looks like I'll need a sutable power supply also.
Anyone
> > here have any experience in this area?
> >
> > Thanks Max
>
------------------------------------
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monitoring) = big $. If I can interface to this pressure sensor with
a Pic, then I can build a datalogger and only use a PC to view the
data. Using just a resistor seems too easy.
Max
--- In p...@yahoogroups.com, "Roy E. Burrage"
wrote:
>
> Unless you really want to re-invent the wheel, why not just buy a
> Labjack and keep using what you have, Max? They're not but about
$120
> or so and ship from stock in the Denver, CO area. We use them
here and
> have been quite pleased with their performance.
>
> So far as the power supply, most current loops use a 24 volt DC
supply
> as the standard. If you interface with a Labjack you might want
to
> lower that to what ever is the lowest for rated use with your
> transducer. Stick a temperature stable resistor, probably wire
wound
> would be best, in the line and then measure the voltage drop.
Just be
> sure that you don't exceed the ratings of either your transducer
or your
> Labjack.
> REB
> kg4pid wrote:
> > I'm trying to find a way to interface a pressure sensor that
uses a 4-
> > 20ma analog current loop and I would like to read it using a Pic
A/D
> > converter. I have tried using google but have not found what I'm
> > looking for. There will be about 30 feet of cable between the
sensor
> > and the Pic / interface circuit. The sensor is currently
connected to
> > a device called a Labjack which is loaned to me and must be
returned.
> > If possible I'd like to tap in to the loop and read the current
and
> > compare it to the Labjack readings before disconnectomg the
Labjack
> > completly. Looks like I'll need a sutable power supply also.
Anyone
> > here have any experience in this area?
> >
> > Thanks Max
>
------------------------------------
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Re: Re: 4-20ma Analog Current Loop to Pic A/D - "John J. McDonough, WB8RCR" - Jan 21 11:21:32 2009
----- Original Message -----
From: "kg4pid"
To:
Sent: Wednesday, January 21, 2009 11:14 AM
Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
> Using just a resistor seems too easy.
What is a resistor but a device for translating current into a voltage drop?
You can measure voltage with the PIC, so all it takes is sizing a resistor
to give you a range of voltages you can deal with. Ohm's Law is your friend.
72/73 de WB8RCR http://www.qsl.net/wb8rcr
didileydadidah QRP-L #1446 Code Warriors #35
------------------------------------
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______________________________
controlSUITE™ software. Comprehensive. Intuitive. Optimized.
Real-world software for real-time control. Details Here!
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Re: 4-20ma Analog Current Loop to Pic A/D - kg4pid - Jan 21 11:56:31 2009
The reason I said "Using just a resistor seems too easy." is because
all the current loop equipment I have found is expensive. I doubt
they use JUST A RESISTOR. The pressure sensor cost about $500 and
it's not mine. I don't want to burn up this pricy sensor! Wish I had
something cheap to experiment on.
Max
--- In p...@yahoogroups.com, "John J. McDonough, WB8RCR"
wrote:
> ----- Original Message -----
> From: "kg4pid"
> To:
> Sent: Wednesday, January 21, 2009 11:14 AM
> Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
> > Using just a resistor seems too easy.
>
> What is a resistor but a device for translating current into a
voltage drop?
> You can measure voltage with the PIC, so all it takes is sizing a
resistor
> to give you a range of voltages you can deal with. Ohm's Law is
your friend.
>
> 72/73 de WB8RCR http://www.qsl.net/wb8rcr
> didileydadidah QRP-L #1446 Code Warriors #35
>
------------------------------------
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all the current loop equipment I have found is expensive. I doubt
they use JUST A RESISTOR. The pressure sensor cost about $500 and
it's not mine. I don't want to burn up this pricy sensor! Wish I had
something cheap to experiment on.
Max
--- In p...@yahoogroups.com, "John J. McDonough, WB8RCR"
> ----- Original Message -----
> From: "kg4pid"
> To:
> Sent: Wednesday, January 21, 2009 11:14 AM
> Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
> > Using just a resistor seems too easy.
>
> What is a resistor but a device for translating current into a
voltage drop?
> You can measure voltage with the PIC, so all it takes is sizing a
resistor
> to give you a range of voltages you can deal with. Ohm's Law is
your friend.
>
> 72/73 de WB8RCR http://www.qsl.net/wb8rcr
> didileydadidah QRP-L #1446 Code Warriors #35
>
------------------------------------
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Re: Re: 4-20ma Analog Current Loop to Pic A/D - Lez - Jan 21 12:16:31 2009
2009/1/21 kg4pid :
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive.
anything is expensive if they can get away with it.
if it says it works at say 5k to 100k over its range it designed to
measure a, then choose a voltage and series resistor to give say 15ma
when its at like 5k, and thats the most current it will ever pass and
you shuold not 'burn it up'
personally i'd go with a small psu, and fit a 20ma fuse on its output
to be safe, and an overcurrent trip cicuit, google should turn one up
------------------------------------
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> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive.
anything is expensive if they can get away with it.
if it says it works at say 5k to 100k over its range it designed to
measure a, then choose a voltage and series resistor to give say 15ma
when its at like 5k, and thats the most current it will ever pass and
you shuold not 'burn it up'
personally i'd go with a small psu, and fit a 20ma fuse on its output
to be safe, and an overcurrent trip cicuit, google should turn one up
------------------------------------
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RE: Re: 4-20ma Analog Current Loop to Pic A/D - David Duley - Jan 21 14:02:33 2009
You should use a metal film 1% resistor. This will minimize the effect of
temperature on your measurement. If this is a loop powered 4-20 ma device
you wont burn it up. It will never draw more than 20ma. Just check the specs
to see what your voltage range is. If you want to check things first, just
use your DVM on the ma setting without a resistor. That is one of the
reasons 4-20 is used everywhere in industry. 1) it can be easily debugged
with a current meter and 2) It won't suffer from drops due to long cable
runs. 3) it is noise immune.
Regards
Dave Duley
BMT Scientific Marine Services
_____
From: p...@yahoogroups.com [mailto:p...@yahoogroups.com] On Behalf Of
kg4pid
Sent: Wednesday, January 21, 2009 8:56 AM
To: p...@yahoogroups.com
Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
The reason I said "Using just a resistor seems too easy." is because
all the current loop equipment I have found is expensive. I doubt
they use JUST A RESISTOR. The pressure sensor cost about $500 and
it's not mine. I don't want to burn up this pricy sensor! Wish I had
something cheap to experiment on.
Max
--- In piclist@yahoogroups .com, "John J.
McDonough, WB8RCR"
wrote:
> ----- Original Message -----
> From: "kg4pid"
> To: .com>
> Sent: Wednesday, January 21, 2009 11:14 AM
> Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
> > Using just a resistor seems too easy.
>
> What is a resistor but a device for translating current into a
voltage drop?
> You can measure voltage with the PIC, so all it takes is sizing a
resistor
> to give you a range of voltages you can deal with. Ohm's Law is
your friend.
>
> 72/73 de WB8RCR http://www.qsl. net/wb8rcr
> didileydadidah QRP-L #1446 Code Warriors #35
>
(You need to be a member of piclist -- send a blank email to piclist-subscribe@yahoogroups.com )
temperature on your measurement. If this is a loop powered 4-20 ma device
you wont burn it up. It will never draw more than 20ma. Just check the specs
to see what your voltage range is. If you want to check things first, just
use your DVM on the ma setting without a resistor. That is one of the
reasons 4-20 is used everywhere in industry. 1) it can be easily debugged
with a current meter and 2) It won't suffer from drops due to long cable
runs. 3) it is noise immune.
Regards
Dave Duley
BMT Scientific Marine Services
_____
From: p...@yahoogroups.com [mailto:p...@yahoogroups.com] On Behalf Of
kg4pid
Sent: Wednesday, January 21, 2009 8:56 AM
To: p...@yahoogroups.com
Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
The reason I said "Using just a resistor seems too easy." is because
all the current loop equipment I have found is expensive. I doubt
they use JUST A RESISTOR. The pressure sensor cost about $500 and
it's not mine. I don't want to burn up this pricy sensor! Wish I had
something cheap to experiment on.
Max
--- In piclist@yahoogroups
McDonough, WB8RCR"
> ----- Original Message -----
> From: "kg4pid"
> To:
> Sent: Wednesday, January 21, 2009 11:14 AM
> Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
> > Using just a resistor seems too easy.
>
> What is a resistor but a device for translating current into a
voltage drop?
> You can measure voltage with the PIC, so all it takes is sizing a
resistor
> to give you a range of voltages you can deal with. Ohm's Law is
your friend.
>
> 72/73 de WB8RCR http://www.qsl.
> didileydadidah QRP-L #1446 Code Warriors #35
>
(You need to be a member of piclist -- send a blank email to piclist-subscribe@yahoogroups.com )
Re: 4-20ma Analog Current Loop to Pic A/D - jvoytovich - Jan 22 11:19:56 2009
You seem to be comparing apples to baseballs here. The sensor itself
is expensive because they're probably using a high precision trimmed
sensor with electronics to filter and convert the voltage output into
the 4-20ma current loop standard.
If you read the specs on the sensor, it may state that the output is
also protected against "dummy" accidents: short circuiting,
over-voltage, etc. Unless you plug it in to a wall socket, it should
not be possible to destroy it with bad wiring.
If you go to the company's web site, they may have some app notes on
interfacing the sensor which would make you feel more secure about
using a resistor. And they would most likely also have support
engineers on staff that could answer questions for you. If you follow
their advice and something blows, you'll probably get a freebie from them.
The reason why they use 4-20ma is to allow for LOOOONG cable lengths
without the IR drop in the wire affecting the reading presented at the
other end. By using a relatively high voltage at the sensor (24VDC),
the sensor electronics can vary the output voltage to get the required
current flow regardless of the drop across your sensing resistor. You
can scale the resistor voltage to 5 or even 10VDC without affecting
the operation.
Good luck.
--- In p...@yahoogroups.com, "kg4pid" wrote:
>
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
>
> --- In p...@yahoogroups.com, "John J. McDonough, WB8RCR"
> wrote:
> >
> >
> > ----- Original Message -----
> > From: "kg4pid"
> > To:
> > Sent: Wednesday, January 21, 2009 11:14 AM
> > Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
> >
> >
> > > Using just a resistor seems too easy.
> >
> > What is a resistor but a device for translating current into a
> voltage drop?
> > You can measure voltage with the PIC, so all it takes is sizing a
> resistor
> > to give you a range of voltages you can deal with. Ohm's Law is
> your friend.
> >
> > 72/73 de WB8RCR http://www.qsl.net/wb8rcr
> > didileydadidah QRP-L #1446 Code Warriors #35
>
------------------------------------
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is expensive because they're probably using a high precision trimmed
sensor with electronics to filter and convert the voltage output into
the 4-20ma current loop standard.
If you read the specs on the sensor, it may state that the output is
also protected against "dummy" accidents: short circuiting,
over-voltage, etc. Unless you plug it in to a wall socket, it should
not be possible to destroy it with bad wiring.
If you go to the company's web site, they may have some app notes on
interfacing the sensor which would make you feel more secure about
using a resistor. And they would most likely also have support
engineers on staff that could answer questions for you. If you follow
their advice and something blows, you'll probably get a freebie from them.
The reason why they use 4-20ma is to allow for LOOOONG cable lengths
without the IR drop in the wire affecting the reading presented at the
other end. By using a relatively high voltage at the sensor (24VDC),
the sensor electronics can vary the output voltage to get the required
current flow regardless of the drop across your sensing resistor. You
can scale the resistor voltage to 5 or even 10VDC without affecting
the operation.
Good luck.
--- In p...@yahoogroups.com, "kg4pid"
>
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
>
> --- In p...@yahoogroups.com, "John J. McDonough, WB8RCR"
>
> >
> >
> > ----- Original Message -----
> > From: "kg4pid"
> > To:
> > Sent: Wednesday, January 21, 2009 11:14 AM
> > Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
> >
> >
> > > Using just a resistor seems too easy.
> >
> > What is a resistor but a device for translating current into a
> voltage drop?
> > You can measure voltage with the PIC, so all it takes is sizing a
> resistor
> > to give you a range of voltages you can deal with. Ohm's Law is
> your friend.
> >
> > 72/73 de WB8RCR http://www.qsl.net/wb8rcr
> > didileydadidah QRP-L #1446 Code Warriors #35
>
------------------------------------
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Re:4-20ma Analog Current Loop to Pic A/D - U-Tap Controls - Feb 6 8:24:40 2009
Quoting kg4pid :
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
Using a resistor costs you 20% resolution for a 0-5vdc A/D input.
As previously stated your input range is now 1-5 vdc.
One leg of your resistor has to be ground referenced to your A/D.
Another concern is some(many?) 4-20 ma transmitters are connected to more than
one device(receiver). Should you pick a location in the middle of the loop and
ground one leg you may cause unintended ground loop problems on the system.
This is probably more of a problem with switchmode power supplies for your pic
vs isolated(transformer or battery) operated device.
Below is a nice application from Maxim that takes the power right off the 4-20
ma loop and gives you a full 0-5VDC for your A/D.
Your insertion resistor is only 10 ohm.
Maxim app note
http://www.maxim-ic.com/appnotes.cfm/an_pk/823
Burr Brown has a RCV420 that also gives you 0-5VDC out
Notice the ground reference needed for the input sense resistor.
focus.ti.com/lit/ds/symlink/rcv420.pdf
Dennis
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> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
Using a resistor costs you 20% resolution for a 0-5vdc A/D input.
As previously stated your input range is now 1-5 vdc.
One leg of your resistor has to be ground referenced to your A/D.
Another concern is some(many?) 4-20 ma transmitters are connected to more than
one device(receiver). Should you pick a location in the middle of the loop and
ground one leg you may cause unintended ground loop problems on the system.
This is probably more of a problem with switchmode power supplies for your pic
vs isolated(transformer or battery) operated device.
Below is a nice application from Maxim that takes the power right off the 4-20
ma loop and gives you a full 0-5VDC for your A/D.
Your insertion resistor is only 10 ohm.
Maxim app note
http://www.maxim-ic.com/appnotes.cfm/an_pk/823
Burr Brown has a RCV420 that also gives you 0-5VDC out
Notice the ground reference needed for the input sense resistor.
focus.ti.com/lit/ds/symlink/rcv420.pdf
Dennis
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RE: Re: 4-20ma Analog Current Loop to Pic A/D - u_tap - Feb 6 8:24:41 2009
On Wed, 21 Jan 2009 11:02:12 -0800, "David Duley"
wrote:
> You should use a metal film 1% resistor. This will minimize the effect of
> temperature on your measurement. If this is a loop powered 4-20 ma device
> you wont burn it up. It will never draw more than 20ma. Just check the
> specs
Correct, It is quite difficult to "burn up" a 4-20ma loop. I consistently
short the outputs
across my ma Fluke meter to verify proper 4-20 ma output.
The resistor inserted into the loop must have one leg grounded.
You lose 20% resolution when inserting this resistor since your range is
now 1-5vdc
instead of 0-5vdc as previously pointed out.
Some systems may well draw more than 20 ma under fault conditions.
The owner would have the opportunity upon initial setup for the loop to
default
with a 0 ma or a 22 ma output. Most opt for the 22 ma since this failure
mode isolates the
failure to the transmitting device and not the loop. More of these options
are available as
the complexity of the devices increase. More below!
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
>
Many loop transmitters are connected to more than one receiver. If you tap
into the loop
somewhere in the middle, then ground one leg, you may inadvertently cause
ground loop
problems. This is more of a concern if you are using switch mode power
supplies than with
an isolated transformer or battery based power for your PIC.
See this Maxim App Note that takes its power from the loop and gives you a
0-5vdc output.
It only needs a 10 ohm resistor.
http://www.maxim-ic.com/appnotes.cfm/an_pk/823
Also Burr-Brown has a RCV420 that also converts 4-20 ma into 0-5 vdc.
Notice how one leg
of the resistor must be grounded.
focus.ti.com/lit/ds/symlink/rcv420.pdf
The best place to connect these devices is at the control panel where the
24Vdc loop power
first leaves before picking up any receivers.
If you want to possibly avoid setting off alarms, solder the resistor into
the uncut loop wire than cut the wire
between the resistor leads. That way you never lose your loop.
It's all a question of accuracy and convenience of installation.
Let us know what you decided!
Dennis
.
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Re: Re: 4-20ma Analog Current Loop to Pic A/D - Phil Birkelbach - Feb 6 8:24:46 2009
That pressure transmitter was designed to be installed in a harsh
environment by "Bubba." You are not going to burn it up. The worst
thing that you can expect in what you are doing, is frying the
microcontroller.
Your right the current loop measuring stuff is expensive but they use
resistors. They may have some buffers and an optical isolator or two
but these are to protect the module, backplane, PLC processor etc.
Not the pressure transmitter.
We've used 250 Ohm 1% Metal Film resistors around here for years to
convert 4-20mA to 1-5VDC anytime we want to send the input to multiple
controllers. It works great.
Phil Birkelbach
p...@petrasoft.net
http://www.madsciencenotebook.com/
On Jan 21, 2009, at 10:56 AM, kg4pid wrote:
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
>
> --- In p...@yahoogroups.com, "John J. McDonough, WB8RCR"
> wrote:
>> ----- Original Message -----
>> From: "kg4pid"
>> To:
>> Sent: Wednesday, January 21, 2009 11:14 AM
>> Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
>>> Using just a resistor seems too easy.
>>
>> What is a resistor but a device for translating current into a
> voltage drop?
>> You can measure voltage with the PIC, so all it takes is sizing a
> resistor
>> to give you a range of voltages you can deal with. Ohm's Law is
> your friend.
>>
>> 72/73 de WB8RCR http://www.qsl.net/wb8rcr
>> didileydadidah QRP-L #1446 Code Warriors #35
>> ------------------------------------
>
> to unsubscribe, go to http://www.yahoogroups.com and follow the
> instructions
(You need to be a member of piclist -- send a blank email to piclist-subscribe@yahoogroups.com )
environment by "Bubba." You are not going to burn it up. The worst
thing that you can expect in what you are doing, is frying the
microcontroller.
Your right the current loop measuring stuff is expensive but they use
resistors. They may have some buffers and an optical isolator or two
but these are to protect the module, backplane, PLC processor etc.
Not the pressure transmitter.
We've used 250 Ohm 1% Metal Film resistors around here for years to
convert 4-20mA to 1-5VDC anytime we want to send the input to multiple
controllers. It works great.
Phil Birkelbach
p...@petrasoft.net
http://www.madsciencenotebook.com/
On Jan 21, 2009, at 10:56 AM, kg4pid wrote:
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
>
> --- In p...@yahoogroups.com, "John J. McDonough, WB8RCR"
>
>> ----- Original Message -----
>> From: "kg4pid"
>> To:
>> Sent: Wednesday, January 21, 2009 11:14 AM
>> Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
>>> Using just a resistor seems too easy.
>>
>> What is a resistor but a device for translating current into a
> voltage drop?
>> You can measure voltage with the PIC, so all it takes is sizing a
> resistor
>> to give you a range of voltages you can deal with. Ohm's Law is
> your friend.
>>
>> 72/73 de WB8RCR http://www.qsl.net/wb8rcr
>> didileydadidah QRP-L #1446 Code Warriors #35
>> ------------------------------------
>
> to unsubscribe, go to http://www.yahoogroups.com and follow the
> instructions
(You need to be a member of piclist -- send a blank email to piclist-subscribe@yahoogroups.com )
Re: 4-20ma Analog Current Loop to Pic A/D - u_tap - Feb 6 8:24:57 2009
On Wed, 21 Jan 2009 11:02:12 -0800, "David Duley"
wrote:
> You should use a metal film 1% resistor. This will minimize the
effect of
> temperature on your measurement. If this is a loop powered 4-20 ma
device
> you wont burn it up. It will never draw more than 20ma. Just check the
> specs
Correct, It is quite difficult to "burn up" a 4-20ma loop. I
consistently short the outputs
across my ma Fluke meter to verify proper 4-20 ma output.
The resistor inserted into the loop must have one leg grounded.
You lose 20% resolution when inserting this resistor since your range
is now 1-5vdc
instead of 0-5vdc as previously pointed out.
Some systems may well draw more than 20 ma under fault conditions.
The owner would have the opportunity upon initial setup for the loop
to default
with a 0 ma or a 22 ma output. Most opt for the 22 ma since this
failure mode isolates the
failure to the transmitting device and not the loop. More of these
options are available as
the complexity of the devices increase. More below!
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
>
Many loop transmitters are connected to more than one receiver. If you
tap into the loop
somewhere in the middle, then ground one leg, you may inadvertently
cause ground loop problems. This is more of a concern if you are using
switch mode power supplies than with an isolated transformer or
battery based power for your PIC.
See this Maxim App Note that takes its power from the loop and gives
you a 0-5vdc output.
It only needs a 10 ohm resistor.
http://www.maxim-ic.com/appnotes.cfm/an_pk/823
Also Burr-Brown has a RCV420 that also converts 4-20 ma into 0-5 vdc.
Notice how one leg
of the resistor must be grounded.
focus.ti.com/lit/ds/symlink/rcv420.pdf
The best place to connect these devices is at the control panel where
the 24Vdc loop power
first leaves before picking up any receivers.
Let us know what you decided!
Dennis
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wrote:
> You should use a metal film 1% resistor. This will minimize the
effect of
> temperature on your measurement. If this is a loop powered 4-20 ma
device
> you wont burn it up. It will never draw more than 20ma. Just check the
> specs
Correct, It is quite difficult to "burn up" a 4-20ma loop. I
consistently short the outputs
across my ma Fluke meter to verify proper 4-20 ma output.
The resistor inserted into the loop must have one leg grounded.
You lose 20% resolution when inserting this resistor since your range
is now 1-5vdc
instead of 0-5vdc as previously pointed out.
Some systems may well draw more than 20 ma under fault conditions.
The owner would have the opportunity upon initial setup for the loop
to default
with a 0 ma or a 22 ma output. Most opt for the 22 ma since this
failure mode isolates the
failure to the transmitting device and not the loop. More of these
options are available as
the complexity of the devices increase. More below!
> The reason I said "Using just a resistor seems too easy." is because
> all the current loop equipment I have found is expensive. I doubt
> they use JUST A RESISTOR. The pressure sensor cost about $500 and
> it's not mine. I don't want to burn up this pricy sensor! Wish I had
> something cheap to experiment on.
>
> Max
>
Many loop transmitters are connected to more than one receiver. If you
tap into the loop
somewhere in the middle, then ground one leg, you may inadvertently
cause ground loop problems. This is more of a concern if you are using
switch mode power supplies than with an isolated transformer or
battery based power for your PIC.
See this Maxim App Note that takes its power from the loop and gives
you a 0-5vdc output.
It only needs a 10 ohm resistor.
http://www.maxim-ic.com/appnotes.cfm/an_pk/823
Also Burr-Brown has a RCV420 that also converts 4-20 ma into 0-5 vdc.
Notice how one leg
of the resistor must be grounded.
focus.ti.com/lit/ds/symlink/rcv420.pdf
The best place to connect these devices is at the control panel where
the 24Vdc loop power
first leaves before picking up any receivers.
Let us know what you decided!
Dennis
------------------------------------
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Re: 4-20ma Analog Current Loop to Pic A/D - Mike Warner - Feb 6 8:26:20 2009
Hello group,
I am just getting started with PICs and this is my first posting to
the list.
An industry standard at my workplace is to have the 4-20ma current
loop terminated with a 250 ohm resistor.
This yields a signal voltage of 1-5 volts DC at the indicating device,
meter, controller, PLC, recorder, etc.
>From Ohms law, E=IR;
.004 amps x 250 ohms = 1 volt
.020 amps x 250 ohms = 5 volts
Hope this is of some help.
--- In p...@yahoogroups.com, "jvoytovich" wrote:
>
> You seem to be comparing apples to baseballs here. The sensor itself
> is expensive because they're probably using a high precision trimmed
> sensor with electronics to filter and convert the voltage output into
> the 4-20ma current loop standard.
>
> If you read the specs on the sensor, it may state that the output is
> also protected against "dummy" accidents: short circuiting,
> over-voltage, etc. Unless you plug it in to a wall socket, it should
> not be possible to destroy it with bad wiring.
>
> If you go to the company's web site, they may have some app notes on
> interfacing the sensor which would make you feel more secure about
> using a resistor. And they would most likely also have support
> engineers on staff that could answer questions for you. If you follow
> their advice and something blows, you'll probably get a freebie from
them.
>
> The reason why they use 4-20ma is to allow for LOOOONG cable lengths
> without the IR drop in the wire affecting the reading presented at the
> other end. By using a relatively high voltage at the sensor (24VDC),
> the sensor electronics can vary the output voltage to get the required
> current flow regardless of the drop across your sensing resistor. You
> can scale the resistor voltage to 5 or even 10VDC without affecting
> the operation.
>
> Good luck.
>
> --- In p...@yahoogroups.com, "kg4pid" wrote:
> >
> > The reason I said "Using just a resistor seems too easy." is because
> > all the current loop equipment I have found is expensive. I doubt
> > they use JUST A RESISTOR. The pressure sensor cost about $500 and
> > it's not mine. I don't want to burn up this pricy sensor! Wish I had
> > something cheap to experiment on.
> >
> > Max
> >
> > --- In p...@yahoogroups.com, "John J. McDonough, WB8RCR"
> > wrote:
> > >
> > >
> > > ----- Original Message -----
> > > From: "kg4pid"
> > > To:
> > > Sent: Wednesday, January 21, 2009 11:14 AM
> > > Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
> > >
> > >
> > > > Using just a resistor seems too easy.
> > >
> > > What is a resistor but a device for translating current into a
> > voltage drop?
> > > You can measure voltage with the PIC, so all it takes is sizing a
> > resistor
> > > to give you a range of voltages you can deal with. Ohm's Law is
> > your friend.
> > >
> > > 72/73 de WB8RCR http://www.qsl.net/wb8rcr
> > > didileydadidah QRP-L #1446 Code Warriors #35
> > >
>
------------------------------------
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I am just getting started with PICs and this is my first posting to
the list.
An industry standard at my workplace is to have the 4-20ma current
loop terminated with a 250 ohm resistor.
This yields a signal voltage of 1-5 volts DC at the indicating device,
meter, controller, PLC, recorder, etc.
>From Ohms law, E=IR;
.004 amps x 250 ohms = 1 volt
.020 amps x 250 ohms = 5 volts
Hope this is of some help.
--- In p...@yahoogroups.com, "jvoytovich"
>
> You seem to be comparing apples to baseballs here. The sensor itself
> is expensive because they're probably using a high precision trimmed
> sensor with electronics to filter and convert the voltage output into
> the 4-20ma current loop standard.
>
> If you read the specs on the sensor, it may state that the output is
> also protected against "dummy" accidents: short circuiting,
> over-voltage, etc. Unless you plug it in to a wall socket, it should
> not be possible to destroy it with bad wiring.
>
> If you go to the company's web site, they may have some app notes on
> interfacing the sensor which would make you feel more secure about
> using a resistor. And they would most likely also have support
> engineers on staff that could answer questions for you. If you follow
> their advice and something blows, you'll probably get a freebie from
them.
>
> The reason why they use 4-20ma is to allow for LOOOONG cable lengths
> without the IR drop in the wire affecting the reading presented at the
> other end. By using a relatively high voltage at the sensor (24VDC),
> the sensor electronics can vary the output voltage to get the required
> current flow regardless of the drop across your sensing resistor. You
> can scale the resistor voltage to 5 or even 10VDC without affecting
> the operation.
>
> Good luck.
>
> --- In p...@yahoogroups.com, "kg4pid"
> >
> > The reason I said "Using just a resistor seems too easy." is because
> > all the current loop equipment I have found is expensive. I doubt
> > they use JUST A RESISTOR. The pressure sensor cost about $500 and
> > it's not mine. I don't want to burn up this pricy sensor! Wish I had
> > something cheap to experiment on.
> >
> > Max
> >
> > --- In p...@yahoogroups.com, "John J. McDonough, WB8RCR"
> >
> > >
> > >
> > > ----- Original Message -----
> > > From: "kg4pid"
> > > To:
> > > Sent: Wednesday, January 21, 2009 11:14 AM
> > > Subject: [piclist] Re: 4-20ma Analog Current Loop to Pic A/D
> > >
> > >
> > > > Using just a resistor seems too easy.
> > >
> > > What is a resistor but a device for translating current into a
> > voltage drop?
> > > You can measure voltage with the PIC, so all it takes is sizing a
> > resistor
> > > to give you a range of voltages you can deal with. Ohm's Law is
> > your friend.
> > >
> > > 72/73 de WB8RCR http://www.qsl.net/wb8rcr
> > > didileydadidah QRP-L #1446 Code Warriors #35
> > >
>
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