4-20mA to voltage conversion...

[John Fields]

On Sat, 03 Apr 2004 20:20:02 GMT, "Roger Gt" <not@here.net> wrote:

"Rodo" wrote
: Hi all...


This thread is closed......

Really!

---
Excuse me?...

A thread is closed when it dies, not when someone declares that it's
closed.

--
John Fields

 

[John Fields]

On Sat, 03 Apr 2004 20:17:47 GMT, "Roger Gt" <not@here.net> wrote:

Try a 4-20 loop over a three hundred mile distance some time.
Even with lenkirk we used 300Volt compliance for it. But a volt
per mile was not all that bad (two repeaters.)

---
That was just stupid; you should have used FM or DTMF.


--
John Fields

 

[Roger Gt]

X-No-Archive: yes
"John Fields" wrote
: "Roger Gt" <not@here.net> wrote:
:
: >"Rodo" wrote
: >: Hi all...
: >
: >This thread is closed......
: >
: >Really!
Yes!

: Excuse me?...
You farted? We didn't notice!

: A thread is closed when it dies, not when someone declares that
it's
: closed.

This one is dead, same thing!

 

[Roger Gt]

X-No-Archive: yes
"John Fields" wrote
: "Roger Gt" wrote:
:
: >Try a 4-20 loop over a three hundred mile distance some time.
: >Even with lenkirk we used 300Volt compliance for it. But a
volt
: >per mile was not all that bad (two repeaters.)
: ---
: That was just stupid; you should have used FM or DTMF.
: John Fields

State of California stupid? You have something there, they
specified the interface!

 

[petrus bitbyter]

"Rodo" <dsp1024@yahoo.com> schreef in bericht
news:j57bc.16036$BE3.9384@nwrddc03.gnilink.net...

Hi all...

I'm trying to convert a 4-20mA current to a voltage I can use with an MCU.
I
thought a simple 250 ohms resistor could do the job but it didn't .... the
voltage across the resistor is fine (5v). The problem is with respect to
the
common GND of the rest of the circuit, the resistor terminals are at 32v
and
27v.

I can not load the 4-20mA line with any more resistance or divert any
current to GND. I tried an optoisolator but the 4mA is enough to turn on
the
LED so I get very little voltage swing at the output (phototransistor).

I thought of an op-amp but I can't think (or find) the right
configuration.
Any suggestions ?

Thanks

That 4-20mA question seems to repeat every semester in a different shape.

An opamp can be used but you require with a high CMRR. A MAX4198 may be a
good choice.

You can find nice chips to solve this problem at once. MAX472 for instance.
Linear also offers some good solutions. Texas has the INA138/168. A little
Google on "high side current measuring" will reveal more.

A "standard" way to convert a "floating"current to a common related voltage
is shown below. (Look with fixed font.)



high ___ low
----------+----|___|----+------------
| R1 |
.-. |
| | |
| |R2 |
'-' |
| |
| |
| |

| |
Q1 |-----+---| Q2

/| | |\
| | |
| +-----+
| |
+----+ |
| | .|
\| | |/
Q3 |--+------| Q4
<| |>
| |
Vout------+ |
| |
.-. .-.
| | | |
| |R3 | |R4
'-' '-'
| |
------------+-------------+-------------
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

In this circuit R1 is the current sense resistor that can have any
reasonable value. The voltage across R2 is equal to the voltage across R1.
So R1 sets the current through Q1 wich causes the output voltage across R3.
You can use an opamp to subtract the 0-4mA part.

petrus


---
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[John Fields]

On Sat, 03 Apr 2004 22:12:11 GMT, "Roger Gt" <not@here.net> wrote:

X-No-Archive: yes
"John Fields" wrote
: "Roger Gt" wrote:
:
: >Try a 4-20 loop over a three hundred mile distance some time.
: >Even with lenkirk we used 300Volt compliance for it. But a
volt
: >per mile was not all that bad (two repeaters.)
: ---
: That was just stupid; you should have used FM or DTMF.
: John Fields

State of California stupid? You have something there, they
specified the interface!

---
Then, of course, they're stupid as well...

--
John Fields

 

[John Fields]

On Sat, 03 Apr 2004 22:12:11 GMT, "Roger Gt" <not@here.net> wrote:

X-No-Archive: yes
"John Fields" wrote
: "Roger Gt" <not@here.net> wrote:
:
: >"Rodo" wrote
: >: Hi all...
:
: >This thread is closed......
:
: >Really!
Yes!

: Excuse me?...
You farted? We didn't notice!

: A thread is closed when it dies, not when someone declares that
it's
: closed.

This one is dead, same thing!

---
Really? I would have sworn it was still alive...

--
John Fields

 

[YD]

On Fri, 02 Apr 2004 05:44:47 GMT, "Rodo" <dsp1024@yahoo.com> wrote:

Hi all...

I'm trying to convert a 4-20mA current to a voltage I can use with an MCU. I
thought a simple 250 ohms resistor could do the job but it didn't .... the
voltage across the resistor is fine (5v). The problem is with respect to the
common GND of the rest of the circuit, the resistor terminals are at 32v and
27v.

I can not load the 4-20mA line with any more resistance or divert any
current to GND. I tried an optoisolator but the 4mA is enough to turn on the
LED so I get very little voltage swing at the output (phototransistor).

I thought of an op-amp but I can't think (or find) the right configuration.
Any suggestions ?

Thanks

Since your input is floating you'll need a differential amplifier to
get the signal referenced to local ground. Below is a suggestion:



10k
___
10k +---|___|-------+
___ | |
-O------o---___|---o |
| | |
| | |\| |
.-. +-----|-\ |
Iin | | | >------o-----O+
| |250 +-----|+/ Vout
'-' 10k | |/| O-
| ___ | |
+O------o--|___|---o |
| |
.-. ===
| | GND
| |10k
'-'
|
|
===
GND
created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de


This will give a 1 to 5 V output, you'll have to deal with the 1 V
offset by your own.

- YD.

--
Remove HAT if replying by mail.

 

[Roger Gt]

X-No-Archive: yes
"YD" <yd.techHAT@techie.com> wrote in message
news:ahqu601q79v33gufo4igeviangucn0ts2e@4ax.com...
: On Fri, 02 Apr 2004 05:44:47 GMT, "Rodo" <dsp1024@yahoo.com>
wrote:
:
: >Hi all...
: >
: >I'm trying to convert a 4-20mA current to a voltage I can use
with an MCU. I
: >thought a simple 250 ohms resistor could do the job but it
didn't .... the
: >voltage across the resistor is fine (5v). The problem is with
respect to the
: >common GND of the rest of the circuit, the resistor terminals
are at 32v and
: >27v.
: >
: >I can not load the 4-20mA line with any more resistance or
divert any
: >current to GND. I tried an optoisolator but the 4mA is enough
to turn on the
: >LED so I get very little voltage swing at the output
(phototransistor).
: >
: >I thought of an op-amp but I can't think (or find) the right
configuration.
: >Any suggestions ?
: >
: >Thanks
: >
:
: Since your input is floating you'll need a differential
amplifier to
: get the signal referenced to local ground. Below is a
suggestion:
:
:
:
: 10k
: ___
: 10k +---|___|-------+
: ___ | |
: -O------o---___|---o |
: | | |
: | | |\| |
: .-. +-----|-\ |
: Iin | | | >------o-----O+
: | |250 +-----|+/ Vout
: '-' 10k | |/| O-
: | ___ | |
: +O------o--|___|---o |
: | |
: .-. ===
: | | GND
: | |10k
: '-'
: |
: |
: ===
: -1Volt reference......
: Here related the output to the 0 to 4 volts you are trying to
achieve.
: you will probably need to provide some trim to get it exact.

: GND
: created by Andy´s ASCII-Circuit v1.22.310103 Beta
www.tech-chat.de
:
:
: This will give a 1 to 5 V output, you'll have to deal with the 1
V
: offset by your own.
:
: - YD.

 

[Tony Williams]

In article <j57bc.16036$BE3.9384@nwrddc03.gnilink.net>,
Rodo <dsp1024@yahoo.com> wrote:

I'm trying to convert a 4-20mA current to a voltage I can use
with an MCU. I thought a simple 250 ohms resistor could do the
job but it didn't .... the voltage across the resistor is fine
(5v). The problem is with respect to the common GND of the rest
of the circuit, the resistor terminals are at 32v and 27v.

I wonder if you could 'fly' a capacitor using
PhotoMOS switches. It would have a slow response,
but good CMV rejection and low comp count.

250R 4-20mA
----<-----+--/\/\--+----<--
| |
+ +
Sw1\ \Sw2
+ +
| C + |
+---||---+
| |
+ +
Sw3/ /Sw4
+ +
| |
| +--->ADCin
|
+------------0v

SW1/2/3/4 are 4x PhotoMOS switches, and I haven't
bothered to sketch-in their primary LEDs.

SW1+2= Closed, Sw2+3= Open; C is disconnected from
the ADC and connected across the 250R. Vc= I*R.

Sw1+2= Open, Sw3+4= Closed; C is disconected from
the R and connected to the ADC, which measures I*R.

PhotoMOS switches can be slow to come OFF, so there
has to be a time delay between every pair coming OFF
and the next pair being turned ON. But since you
have a uP this should be no problem to implement.

The size of the C depends on the input leakage of
the ADC and how long the Convert time is. Probably
the C would be in the region of 0.1uF to 1uF.

Some protection would be useful to guard against
wrong switch operation, and an opamp could be used
before the ADC, mainly as a sacrificial protector
of the uP.

Leave the uP to deal with the 4mA offset, bearing
in mind that the lack of the 4mA is an intentional
indicator of an open-circuit or unpowered loop.

--
Tony Williams.