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'[EE] 4-20mA reading'
2004\10\25@185712 by Omer YALHI

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There are a lot of industrial products that output 4-20mA (or 0-20mA).  Any
pointers on how to read them?  Good point to start?  I am planning to read
Danfoss Pressure Transmitters (4-20mA).

P.s. The PIC side is not the problem I am ok with PICs, the problem is how
to approach to read them.  Should I just put a 100 ohm resistor (so that
range is between 0.4V and 2V) in series with the 4-20mA loop and read the
voltage?

Thanks in advance,

Omer

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2004\10\25@195610 by Jinx

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> There are a lot of industrial products that output 4-20mA (or
> 0-20mA). Any pointers on how to read them?  Good point to start?

This is a pretty comprehensive general purpose circuit

http://www.piclist.com/techref/microchip/aout.htm

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2004\10\25@201303 by Bob Ammerman

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----- Original Message -----
From: "Omer YALHI" <spam_OUToyalhiTakeThisOuTspamteksan.com.tr>
To: "'Microcontroller discussion list - Public.'" <.....piclistKILLspamspam@spam@mit.edu>
Sent: Monday, October 25, 2004 7:06 PM
Subject: [EE] 4-20mA reading


> There are a lot of industrial products that output 4-20mA (or 0-20mA).
> Any
> pointers on how to read them?  Good point to start?  I am planning to read
> Danfoss Pressure Transmitters (4-20mA).
>
> P.s. The PIC side is not the problem I am ok with PICs, the problem is how
> to approach to read them.  Should I just put a 100 ohm resistor (so that
> range is between 0.4V and 2V) in series with the 4-20mA loop and read the
> voltage?

Yes and no....

A precision resistor in series with the loop is typically the way to go. You
have to be aware however, that neither end of that resistor may be at ground
potential.

Bob Ammerman
RAm Systems

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2004\10\25@202308 by Stephen R Phillips

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--- Omer YALHI <oyalhispamKILLspamteksan.com.tr> wrote:

> There are a lot of industrial products that output 4-20mA (or
> 0-20mA).  Any
> pointers on how to read them?  Good point to start?  I am planning to
> read
> Danfoss Pressure Transmitters (4-20mA).
>
> P.s. The PIC side is not the problem I am ok with PICs, the problem
> is how
> to approach to read them.  Should I just put a 100 ohm resistor (so
> that
> range is between 0.4V and 2V) in series with the 4-20mA loop and read
> the
> voltage?
>
> Thanks in advance,
>
> Omer

This is highly dependant on your Vref for your A2D convertor.
Typically for a VRef of 5V you use 1% 250 ohm resistors (that way full
scale is 5V). If you are using a lower voltage for VRef thing you
reduce you resistor as needed. (for example AVR 1.1V Ref would be a 55
ohm resistor. If you wish proper better scaling of the output than you
need to consider more. This is where a nice trim pot comes in handy.
You can adjust from a 20 to a 500 ohm load and achieve the exact full
scale measurement you want.

Stephen R. Phillips


               
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2004\10\25@204512 by Grimm, Justin

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In industry a 250 ohm resistor is used in series with the 4-20ma loop.
This gives a voltage across the resistor of 1-5V.

Cheers
Justin

{Original Message removed}

2004\10\26@025209 by Wouter van Ooijen

face picon face
> > There are a lot of industrial products that output 4-20mA (or
> > 0-20mA). Any pointers on how to read them?  Good point to start?
>
> This is a pretty comprehensive general purpose circuit
> http://www.piclist.com/techref/microchip/aout.htm

But it's an output circuit, not an input :(

And I notice that it derives the lower offset from the 5 V. I don't like
that, it needs a 'reference' quality 5 V, which is trouble if it also
must supply significant current. And at 20 mA it will burn 300mW in the
LM358, isn't that a bit too much? Even more if you feed it from a
standard 24V, even more if you include some safety (fools?) margin in
that 24V. But I like the floating feedback. I just used a current
mirror.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu







____________________________________________

2004\10\26@025211 by Wouter van Ooijen

face picon face
> P.s. The PIC side is not the problem I am ok with PICs, the
> problem is how
> to approach to read them.  Should I just put a 100 ohm
> resistor (so that
> range is between 0.4V and 2V) in series with the 4-20mA loop
> and read the
> voltage?

yes, but
- you will need a real voltage reference (not Vdd)
- add some protection and filtering
- take care not to share traces with any other currents (if you can
spare an A/D channel you can read the difference over the resistor,
instead of the voltage on the hot side referenced to ground)

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


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2004\10\26@082249 by olin_piclist

face picon face
Grimm, Justin wrote:
> In industry a 250 ohm resistor is used in series with the 4-20ma loop.
> This gives a voltage across the resistor of 1-5V.

I would think the LED of an opto coupler would be better.  That gives you a
fairly fixed approximately 2V drop, and deals with the isolation issue very
simply.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
____________________________________________

2004\10\26@090239 by cisco J. A. Ares

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Olin Lathrop wrote:

>Grimm, Justin wrote:
>  
>
>>In industry a 250 ohm resistor is used in series with the 4-20ma loop.
>>This gives a voltage across the resistor of 1-5V.
>>    
>>
>
>I would think the LED of an opto coupler would be better.  That gives you a
>fairly fixed approximately 2V drop, and deals with the isolation issue very
>simply.
>
>  
>

There are ways of doing that kind of insulation, but this one would be
dependant on many other variables such as temperature.

Normally those 4-20 mA sensors, if they are of that 2-wire type, you can
put one of the terminals of the shunt resistor to ground with no problem
at all. If it is a 3-wire, then you'll have to know if it is a current
source or a current drain.  I've seen most of them as sources, so again
no problem in connecting one shunt resistor terminal to ground.

Then you should give a minimum of insulation (impedance) with an
operational amplifier (unit gain voltage follower will suffice, but this
and the quality of the op-amp will affect your precision, though).

If you really need insulation (uncommon, but...) there are modules from
Analog Devices for example, that are specially made for this.

You may take a look at some application notes at the Analog Devices web
site, as well as at Maxim's.

Francisco

____________________________________________

2004\10\26@092209 by Spehro Pefhany

picon face
At 08:22 AM 10/26/2004 -0400, you wrote:
>Grimm, Justin wrote:
> > In industry a 250 ohm resistor is used in series with the 4-20ma loop.
> > This gives a voltage across the resistor of 1-5V.
>
>I would think the LED of an opto coupler would be better.  That gives you a
>fairly fixed approximately 2V drop, and deals with the isolation issue very
>simply.

Generally unsuitable for sensor/transmitter side applications- we generally
want high accuracy & stability and that's not possible using optos without
using special linear parts and closing the servo loop on the LED side-
which requires a galvanically isolated supply.

It might be okay for crude output devices that live inside an external
control loop, with aging/drift compensation.

Best regards,

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




____________________________________________

2004\10\26@093653 by Bob Ammerman

picon face
The problem comes when there are multiple 'receivers' on the 4-20ma loop. In
this case you are very likely to find that you can ground neither terminal
of the loop. What you can do is put a precision resistor in the loop and
then use a differential input to read the voltage across it. This will only
work if the common mode voltage is within the range supported by your input
device.

One way to handle higher common mode voltages is to add a relatively high
impendence voltage divider to ground on each side of the measurement
resistor. Then take your differential voltage from the centers of the
voltage dividers. Apply gain as needed.

Bob Ammerman
RAm Systems

{Original Message removed}

2004\10\26@103714 by cisco J. A. Ares

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So...

http://www.maxim-ic.com/appnotes.cfm/appnote_number/823

... this might help. That beauty does exactly what you described, 2%
precision.

Francisco


Bob Ammerman wrote:

{Quote hidden}

> {Original Message removed}

2004\10\26@104517 by Dal Wheeler

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face
cp.literature.agilent.com/litweb/pdf/5989-0286EN.pdf
describes using a linear optocoupler as a 4-20ma isolation buffer.  These
seem to be 8-10 bit resolution.

Clare LOC111 and family are also nice.

-Dal

{Original Message removed}

2004\10\26@104928 by olin_piclist

face picon face
Spehro Pefhany wrote:
>> I would think the LED of an opto coupler would be better.  That gives
>> you a fairly fixed approximately 2V drop, and deals with the isolation
>> issue very simply.
>
> Generally unsuitable for sensor/transmitter side applications

Yeah, I thought this was a digital current loop receiver.

*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
____________________________________________

2004\10\26@113516 by Omer YALHI

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face
>You may take a look at some application notes at the Analog Devices web
>site, as well as at Maxim's.
>
>Francisco

I did look at Analog devices site, found an app note on outputting 4-20mA
which is nice; but not reading it.  I'll check out maxim's site as well.
Because 4-20mA is a standard industrial output, I am thinking there should
be a single chip where I can connect it to the loop and communicate with it
serially or similar.  Is my thinking wrong?

Regards, Omer

____________________________________________

2004\10\26@152419 by Peter L. Peres

picon face

On Mon, 25 Oct 2004, Bob Ammerman wrote:

> A precision resistor in series with the loop is typically the way to go. You
> have to be aware however, that neither end of that resistor may be at ground
> potential.

I think that you will find that it is possible to ground one end of the
resistor at the reciver if the loop is 'clean' (potential free) at the
sensor. An additional small protection resistor goes in the +24V line to
the sensor, in case the wire insulation is chaffed off.

Peter
____________________________________________

2004\10\26@220823 by Chen Xiao Fan

face
flavicon
face
Hi,

There are a lot of applications which need Galvanically isolation, especially in petrol-chemical industry. My company is specialized in
this process automation field and the device called KFD2-STC4-1 is doing this task nicely but may be a bit overkill since it is not
cheap. You can refer to the website: http://www.pepperl-fuchs.com

Technical specifications:

Transfer characteristics
Deviation at 20 °C / 4 ... 20 mA
= 20 µA incl. calibration, linearity, hysteresis, loads and fluctuations of supply voltage
Temperature = 20 ppm / K
Frequency range bandwidth at 0.5 VSS-signal 0 ... 7,5 kHz (-3 dB)
bandwidth at 0.5 VSS-signal 0,3 ... 7,5 kHz (-3 dB)

Electrical isolation
Input/Output basic insulation according to DIN EN 50178, rated insulation voltage 253 Veff
Input/Power supply basic insulation according to DIN EN 50178, rated insulation voltage 253 Veff
Output/Power supply basic insulation acc. to DIN EN 50178, rated insulation voltage of AC 50 V

For the explosion proof application, the Ex device KFD2-STC4-Ex1 is also
available with ATEX approval and EN50020 safe insulation. The isolation
requirement is much higher than what AD and MAXIM device can offer so you
have to use specially made transformer to achieve this.
Both device are also SMART capable, which mean they can transmit the modulated digital signal (SMART transmitter) as well.

Xiaofan

{Quote hidden}

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2004\10\26@224629 by Bob Ammerman

picon face
>> A precision resistor in series with the loop is typically the way to go.
>> You have to be aware however, that neither end of that resistor may be at
>> ground potential.
>
> I think that you will find that it is possible to ground one end of the
> resistor at the reciver if the loop is 'clean' (potential free) at the
> sensor. An additional small protection resistor goes in the +24V line to
> the sensor, in case the wire insulation is chaffed off.
>
> Peter

You may find that you are not the only receiver on the loop.

Bob Ammerman
RAm Systems

____________________________________________

2004\10\27@145049 by Peter L. Peres

picon face

On Tue, 26 Oct 2004, Bob Ammerman wrote:

>>> A precision resistor in series with the loop is typically the way to go.
>>> You have to be aware however, that neither end of that resistor may be
>>> at ground potential.
>>
>> I think that you will find that it is possible to ground one end of the
>> resistor at the reciver if the loop is 'clean' (potential free) at the
>> sensor. An additional small protection resistor goes in the +24V line to
>> the sensor, in case the wire insulation is chaffed off.
>>
>> Peter
>
> You may find that you are not the only receiver on the loop.

That's what I meant by 'clean' loop. You may not be the only receiver but
you may be the one who is taking the shortcut ;-)

Peter
____________________________________________

2004\10\27@162818 by steve

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> Transfer characteristics
> Deviation at 20 °C / 4 ... 20 mA
> = 20 µA incl. calibration, linearity, hysteresis,
> loads and fluctuations of supply voltage

Would that be considered a typical 4-20mA device spec (0.1% f.s. total error) or is that a highly spec'd part ?

Steve.



___________________________________________

2004\10\27@172010 by Spehro Pefhany

picon face
At 09:27 AM 10/28/2004 +1300, you wrote:
> > Transfer characteristics
> > Deviation at 20 °C / 4 ... 20 mA
> > = 20 µA incl. calibration, linearity, hysteresis,
> > loads and fluctuations of supply voltage
>
>Would that be considered a typical 4-20mA device spec (0.1% f.s. total
>error) or is that a highly spec'd part ?
>
>Steve.

No, that's a fairly typical spec. for a sensor loop.

Over what range of temperature etc. that they actually meet the spec varies.

Best regards,

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





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