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 Author Topic: 2-wire 4-20mA PIC Iterface  (Read 8982 times) 0 Members and 1 Guest are viewing this topic.
Hexer379
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 « on: January 18, 2008, 04:20:28 04:20 »

Hello @all

i have a problem , with a 2-wire 4-20mA interface ...
this interface is for a "Pressure measurement" ...
the device has an Input of 24V DC ...

i have no idea for an interface to 0-5V AD-Input .

thanks for Help ...
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shaiqbashir
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 « Reply #1 on: January 18, 2008, 05:47:32 05:47 »

you have to use PIC ADC channels for this task. You need to convert voltage from the pressure sensor into current and then convert that current into voltage again at ADCs input.
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FriskyFerret
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Put it in, take it out.

 « Reply #2 on: January 18, 2008, 08:01:31 08:01 »

The 4-20 ma current loop is very common in industry and rather simple to work with.

The pressure sensor will let a current pass through it that is equal to the pressure it senses. The pressure sensor, also known as the 'transmitter' in current loop applications, will let exactly 4 mA of current flow when it senses its minimum pressure and 20 mA when it senses its full-scale pressure.

A simple example: A current loop pressure sensor rated for 0-10 PSIG has no pressure applied to its input port. It will let exactly 4 mA of current flow through the loop. If you then apply 10 psi, it will let 20 mA of current flow. If 5 PSI of pressure is applied, the transmitter will allow 12 mA of current to flow in the loop.

The way you convert the current to voltage is by inserting a resistor in the current loop. The most popular resistor value for this purpose is 250 Ohms, since it will produce a 1-5Vdc signal when 4-20ma flows through it!

Simply measure the voltage across the resistor --> which is exactly equal to the current --> which is exactly equal to the pressure at the transmitter. Choose a 1% metal film type resistor for stability is accuracy is important.

You really need to Google '4-20ma current loop' and understand the simple theory of industrial current loops or you're gonna burn that nice pressure sensor out.
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Taner
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 « Reply #3 on: January 18, 2008, 01:39:02 13:39 »

Here is a circuits of converter
4-20mA to 0-5V

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free
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 « Reply #4 on: January 18, 2008, 06:28:13 18:28 »

You can use XTR115
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FriskyFerret
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Put it in, take it out.

 « Reply #5 on: January 18, 2008, 09:31:31 21:31 »

Quote
Here is a circuits of converter 4-20mA to 0-5V

Circuit sets zero and gain. This unnecessarily complicates the circuit. Do the span and gain in software. There is no need to buffer the voltage off the resistor since the output impedance is sufficiently low for most any A/D converter.
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Hexer379
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 « Reply #6 on: January 20, 2008, 09:25:02 09:25 »

Hello ...

thank you all very much !!! ...
i have test it with a 250Ohm Res. and a Z-Diode with 5,1V to protect the ADC Input ...

IT WORKS !!!

Thanks ...
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super_iori
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 « Reply #7 on: April 09, 2008, 01:25:39 01:25 »

Hello...
I am using the chip xtr115 or xtr117, and I want to know if anyone have used it b4?
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anemon06
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 « Reply #8 on: October 27, 2008, 05:47:44 05:47 »

I need 4-20 mA to 0-10 V converter circuit. Figure is 4-20mA to 0-5V. How is this schmetics converted 0-10V.
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Taner
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 « Reply #9 on: October 30, 2008, 02:29:06 14:29 »

This circuit is wrong by me,
look here:
http://www.sonsivri.com/forum/index.php?topic=4092.0

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