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PICList Thread
'[EE] 4-20mA current sensing'
2006\03\03@110721 by Eduardo Garcia

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Hi all,
   Can anybody show me the best way to read a 4-20mA sensor?
   In my design I am using PIC16F4680 with a lot of A/Ds available and I
think that it can be useful, but don't known how.

   Any url link or help will be very appreciated.

Regards,
Eduardo Garcia


               
_______________________________________________________
Yahoo! Acesso Grátis - Internet rápida e grátis. Instale o discador agora!
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2006\03\03@114236 by alan smith

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resistor across the current loop and feed to the A/D.  Calculate the resistor such that it is within range of your A/D reference.  Been there, done it, works well.

Eduardo Garcia <spam_OUTegnascimentoTakeThisOuTspamyahoo.com.br> wrote:  Hi all,
Can anybody show me the best way to read a 4-20mA sensor?
In my design I am using PIC16F4680 with a lot of A/Ds available and I
think that it can be useful, but don't known how.

Any url link or help will be very appreciated.

Regards,
Eduardo Garcia



_______________________________________________________
Yahoo! Acesso Grátis - Internet rápida e grátis. Instale o discador agora!
http://br.acesso.yahoo.com

2006\03\03@131235 by Jon Chandler

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The resistor is the way to go.  The only tricky part is locating the
resistor in the circuit so that one side of it is connected to common, as
all the A/D convertors will be tied to the same common point.

Jon



At 08:42 AM 3/3/2006 -0800, you wrote:
{Quote hidden}

2006\03\03@134636 by Spehro Pefhany

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At 10:11 AM 3/3/2006 -0800, you wrote:
>The resistor is the way to go.  The only tricky part is locating the
>resistor in the circuit so that one side of it is connected to common, as
>all the A/D convertors will be tied to the same common point.
>
>Jon

Keep in mind that most 4-20 loops can have significant overcurrent (20-50%
sometimes) as an indication of sensor failure etc. Zero is also possible,
of course, under failure conditions.

If grounding is an issue, it may be necessary to isolate the inputs, which
can get fairly involved and/or expensive.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2006\03\03@141334 by Eduardo Garcia

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Thanks Allan, Jon and Spehro!

Now I see how simple can be it.

Is there any circuit "downloadable" from internet with these protections or
more complicated circuits?

Regards,
Eduardo Garcia

{Original Message removed}

2006\03\06@073654 by o, Edson Jose

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> Is there any circuit "downloadable" from internet with these protections
> or
> more complicated circuits?
>
> Regards,
> Eduardo Garcia
>


In this link
http://www.national.com/an/AN/AN-20.pdf
you can read at page 6 about "Current-to-Voltage Converter".


Saudações,
Edson.


2006\03\06@111156 by Richard E. Teague

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part 1 1426 bytes content-type:text/plain; (decoded 7bit)


>    Can anybody show me the best way to read a 4-20mA sensor?

 

>    Any url link or help will be very appreciated.



______________________________________________________________



Hi Eduardo,



Try the TI INA117. Here is a link to the datasheet



http://focus.ti.com/lit/ds/symlink/ina117.pdf



Look at page 8 diagram (a). The internal precision resistor

network gives this chip a common mode voltage range of +/- 200v,

so it doesn't matter where it goes in the current loop.



The only downside to the INA117 is that you will need at least

+/- 10v supply. I usually use an ICL7660 or a MAX232 to

generate a low current negative supply for apps like this.



TI will send you overnight samples of the INA117 and MAX232

at no charge.



I have attached a gif of an input protection circuit for the

PIC A/D to limit the input voltage to approx. Vcc +.3/gnd -.3

in case your loop uses a 120% signal to indicate sensor burnout.

For diodes the 1N5817 would be a good one, or if you can deal

with surface mount, the Central Semi CMPSH-3S are great.

That is a surface mount package with 2 diodes series connected.

If you would like a few of those, just send me your address

off list, and I will mail you a few.



An input protection circuit like this ia a good idea ANY time

that a PIC input connects to the outside world.



Good luck,

Richard



part 2 26312 bytes content-type:image/gif; (decode)


part 3 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2006\03\06@123338 by Spehro Pefhany

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At 10:11 AM 3/6/2006 -0600, you wrote:

>I have attached a gif of an input protection circuit for the
>
>PIC A/D to limit the input voltage to approx. Vcc +.3/gnd -.3
>
><snip>



>An input protection circuit like this ia a good idea ANY time
>
>that a PIC input connects to the outside world.

Do keep in mind that Richard's circuit will indeed limit the voltage
to Vdd + 0.3, but that Vdd is not guaranteed to behave itself.

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
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2006\03\06@143350 by Eduardo Garcia

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Hi Richard!

Thank you so much!

I will try it!


Eduardo Garcia

----- Original Message -----
From: "Richard E. Teague" <EraseMEk5btpspam_OUTspamTakeThisOuTcox.net>
To: "'Microcontroller discussion list - Public.'" <piclistspamspam_OUTmit.edu>
Sent: Monday, March 06, 2006 1:11 PM
Subject: RE: [EE] 4-20mA current sensing


{Quote hidden}

--------------------------------------------------------------------------------


> -

2006\03\06@161406 by Jinx

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I tried this circuit with Maxim samples

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

Works well, although they did leave off pin1 (Shutdown) of the
MAX472, which needs to be grounded. I've been using it with
a 0 - 5000mm depth sensor and an F88 and it's pretty good,
almost to the mm. For the 249R I have 240+10 1%, and 3k9
for the 3k89. The sensor's source is unregulated 12V from an
SLA

2006\03\06@161838 by Jinx

face picon face
> a 0 - 5000mm depth sensor and an F88 and it's pretty good,
> almost to the mm

BTW, before anyone points it out - the F88 ADC can't resolve to
1 in 5000 but you can see sub-bit voltage on a DVM

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