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'[PIC] 4-20 mA current Loop'
1999\04\21@150219 by Bob Drzyzgula

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Can anyone point me to a circuit and/or PIC code (MPASM,
preferably, but I can translate...) to interface with
a 4-20 mA current loop with a PIC?  Preferably something
that used the built-in ADC on a PIC16C7x, but an example
that used an external ADC would still be useful.  I've got
a current transducer (F.W. Bell PC-50) that I want to read
with a PIC.

TIA,
--Bob Drzyzgula

--
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Bob Drzyzgula                             It's not a problem
spam_OUTbobTakeThisOuTspamdrzyzgula.org                until something bad happens
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1999\04\21@190907 by Tony Nixon

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Try putting a 250R resistor on the output to ground. That should give you a
signal of 1 to 5 volts which can be read by the PIC.

You may have to be careful that the signal does not go above 20mA.
Sometimes a 4-20mA circuit can deliver more that 20mA.

Regards

Tony

1999\04\22@121055 by Wagner Lipnharski

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Tony Nixon wrote:
>
> Try putting a 250R resistor on the output to ground. That should give you a
> signal of 1 to 5 volts which can be read by the PIC.
>
> You may have to be careful that the signal does not go above 20mA.
> Sometimes a 4-20mA circuit can deliver more that 20mA.

A more realistic industrial environment resistor value in this situation
is something low as 10 to 20 Ohms.  Even that several current loop
generators can supply up to 20 or 25 Volts, is not a guarantee that
everyone would do it.  Sometimes there are other equipments (with
resistors in series online) already connected on that loop. Don't forget
about the wires resistance and bad contacts, they exist and this is main
reason to use current loops, in environments where those losses exist.
Steal 5 Volts from the loop is somehow inappropriate when you can do the
same measurement with just 200 mV.

One of our equipment produced to monitor and calibrate current loops
(4-20mA), was planned to use 20 Ohms shunt resistor. We received a round
and loud "NO" from the customer, his maximum allowed resistance, later
specified, was just 10 Ohms.

1999\04\24@064802 by Dr. Imre Bartfai

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Hi,
I have used a simple 250 ohm resistor and I read it with ADC of the PIC.
(20 ma * 250 ohm = 5 Volts). It is a simplest thing anyway. However, 4 mA
means 1 Volts. But it does not matter...
Imre


On Wed, 21 Apr 1999, Bob Drzyzgula wrote:

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