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PICList Thread
'Current to Voltage Converter'
1996\10\28@081222 by Rob BRUCE-BRAND

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I'd appreciate some ideas on a precision, temperature-stable current
to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
(which I will then feed into a Voltage-to-freq converter and read
with a uP).

Many thanks
Rob Bruce-Brand
Durban Electricity

1996\10\28@103453 by tjaart

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Rob BRUCE-BRAND wrote:
>
> I'd appreciate some ideas on a precision, temperature-stable current
> to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
> (which I will then feed into a Voltage-to-freq converter and read
> with a uP).
>
> Many thanks
> Rob Bruce-Brand
> Durban Electricity

What sort of temperature swings are you expecting? If not too severe, a
resistor will do the job just fine.

You could also charge a cap and measure the time it takes to 'trip'
(change to high) your uP's input. This will work well on a PIC. (Of
course)
--
Friendly Regards

Tjaart van der Walt
______________________________________________________________
|  Another sun-deprived R&D Engineer slaving away in a dungeon |
|WASP International GSM vehicle tracking and datacomm solutions|
|           +27-(0)11-622-8686 | http://wasp.co.za             |
|______________________________________________________________|

1996\10\28@124334 by Bob Fehrenbach

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Rob BRUCE-BRAND <spam_OUTBRUCE-BRANDRATakeThisOuTspamDURBAN.GOV.ZA> wrote:
>I'd appreciate some ideas on a precision, temperature-stable current
>to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
>(which I will then feed into a Voltage-to-freq converter and read
>with a uP).

A 249 ohm 1% resistor works for me.

--
Bob Fehrenbach    Wauwatosa, WI     .....bfehrenbKILLspamspam@spam@execpc.com

1996\10\28@140831 by Scott Dattalo

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Rob BRUCE-BRAND wrote:
>
> I'd appreciate some ideas on a precision, temperature-stable current
> to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
> (which I will then feed into a Voltage-to-freq converter and read
> with a uP).
>

If your planning to ultimately send your current signal to a "voltage-
to-frequency" converter, then all you will need are two resistors.
This is because most V/F converters are really current-to-frequency
converters (eg AD652, LM231, etc). Let's take the AD652 for example.
It has a full range current input of 1ma. So you would need to
divide your full range of 20ma by 20. (Actually, I would divide by
a slightly larger number to allow overanging).

4-20ma-->
----------+------+
         |      |
3.8 to 19 /      /  0.2 to 1ma
   ma    \      \
     R1  /      /  R2
         \      \
         |      |
        ----    +---- To "virtual gound" in V/F converter
       / / /

Use current division to find the resistor values:

 I(R2) = I_in * R1/(R1+R2)
  1ma  = 20ma * R1/(R1+R2)
  R2   = 19 * R1


The other equation relates to the maximum drive voltage of the
4-20ma circuit. In other words, the voltage developed at the input
is
V_in = I_in * (R1||R2)
     = I_in * (R1*R2)/(R1+R2)
     = I_in * 19/20 * R1

And, Vin must be less than the maximum voltage of the 4-20 ma driver,
so:
 V_max > V_in(max) = 20ma * 19/20 * R1
       > (19 volts)*R1
or
    R1 < V_max/(19 volts).

There are obviously other design considerations that I'm intentionally
ignoring, but I think this will get you going. (The other considerations
are 1: temperature stability of the resistors, 2: over voltage
protection
especially when your circuit is not powered, 3: Calibration, 4: Compen-
sation for the low end 0.2 ma offset current (software). I'll be glad
to address these issues off line, Rob.)

Scott

1996\10\28@145951 by ernhard Schweighofer

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On Mon, 28 Oct 1996, Rob BRUCE-BRAND wrote:

> I'd appreciate some ideas on a precision, temperature-stable current
> to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
> (which I will then feed into a Voltage-to-freq converter and read
> with a uP).
>

I think you are looking for a resistor: U = f(I) = R * I   :-)

If you want to convert 0-20mA to 0-5V you choose a value of about
250 Ohm. But be sure to get a resistor with a low TEMPCO (oh, what's just
the english expression for 'Temperatur - Koeffizient') ?). I think
precision  metal-film resistors have a TEMPCO of about 20 ppm/(degree
kelvin) or even lower. Hope that's good anough.

Q'uapla

PS.: I hope you understand what I wrote, because I couldn't find all the
words I wasn't sure about in my 'Langenscheidts'-dictionary :-(

Bernhard Schweighofer alias schweigispamKILLspamsbox.tu-graz.ac.at
(Student at Graz University of Technology, Austria)

1996\10\28@185001 by Robert Lunn

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>> I'd appreciate some ideas on a precision, temperature-stable current
>> to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
>> (which I will then feed into a Voltage-to-freq converter and read
>> with a uP).
>>
>
>If your planning to ultimately send your current signal to a "voltage-
>to-frequency" converter, then all you will need are two resistors.
>This is because most V/F converters are really current-to-frequency
>converters (eg AD652, LM231, etc).

       Scott took the words out of my mouth.

       I would recommend the AD654 for this purpose (it's an
       8-pin package).  The data sheet is on the web.

___Bob

1996\10\29@185125 by Customized Controller Solutions

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Use a fixed voltage as your supply or exciter voltage.  24 V seems to be
standard.  Then calculate the resistance value to get a current flow of
20mA.  Read the voltage drop across the resistor.  The drop will vary
according to the current drain.  Your transducer should be in line with
the resistor and the other end of your supply voltage.  That is the current
to voltage converter portion of your circuit.  The use a 4046 P.L.L. for
a V.C.O. and read the freq on your PIC.

Brent

Brent Pollock
Customized Controller Solutions
.....ccsKILLspamspam.....ewol.com

  From the simple to complex, We provide custom tailored solutions
  at reasonable prices.

Embedded Control. Digital and Analog Electronics and Communications.
Process Control,...

At 05:17 PM 10/28/96 +0200, you wrote:
>I'd appreciate some ideas on a precision, temperature-stable current
>to voltage circuit to convert e.g. a 4-20mA signal to a 0-5V signal
>(which I will then feed into a Voltage-to-freq converter and read
>with a uP).
>
>Many thanks
>Rob Bruce-Brand
>Durban Electricity
>

1996\10\29@204838 by Jack Rutherford

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Are you wanting to convert a 4-20mA to a 0-5V or a 1-5V?

**************************************************************************


{Quote hidden}

Jack Rutherford
@spam@jrutherKILLspamspampreferred.com

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