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Current Loop PIC - looking for info how to build it

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Full Member level 4
Nov 6, 2001
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Where I can find definitions to build an interface for a curent loop (4-20 mA) and a PIC micro?



Expensive, but easy way: AD421 (Analog Devices)+ any PIC you want.

Use a PWM output of the pic through an R-C network to filter it in to a voltage controlled current source using an op-amp. It is cheap and easy...

Look at the Microchip web site for getting an anlog output from a pic.

Look at National or fairchild op-amp data sheets for voltage controlled current sources.

Good Luck...


Hi all,

suiram, the AD421 it's a great IC, the problem is that it's too much expenciv (about 22 EURO!) ThX

Stallion, the tips that you give are for produce a 4-20 loop output from a PIC right? But what I want is a input to the PIC. ThX


Oooops, sorry.

Well, then I would use a 220 ohm resistor to GND.

4mA = 0.88V
20mA = 4.4V

You could also use a couple of op-amps to get 0-5V insted of 0.88 - 4.4V. This method works well.

Another alternative, but more expensive, is a Burr-Brown part, RCV420. (I think that is right) Anyway, it gives you 0-5V out and an error flag for <4mA.

Any PIC with an ADC will do the job. The PIC16F870 is pretty cheap.

Good Luck...

I hope I helped this time. lol


Hi Stallion,

But, we can put a 220 Ohm resistor on the loop? There are no specification for input impedance?



Nobody have related material ? ...



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<font size=-1>[ This Message was edited by: THE BORG on 2001-12-20 03:46 ]</font>

Hi Neuralc,

You have to check the max/min loads for your equipment which sources the 4-20mA signal.

I read this type of signals from Infrared flame detectors or IR-gasdetectors which can source up to 500 Ohms max.

A new feature in this are HART devices which uses the same wires for a 485 like data transmission to a handheld interrogator.
If you like to use this protocol then the load resistor should not be less then 250 Ohms.

Regards, Msmax

Hi Neuralc,

If max load is your problem, you can put a lowest resistor as possible like 1ohm or 0.47ohm, then using a opamp you can amplifier the signal to measure with PIC.

I really don't think a load resistor of 1 Ohm is desired for industrial type of equipment.

First of all: what equipment do you have?
Datasheets? Describe your application a bit more. We really like to help, but we only can help if you provide us with good description of what you need.

Generally spoken the load resistors for 0-20mA devices are around 200-500 Ohms.

If you like to read the value with an analogue input of a PIC then Use a 200 Ohms 1% resistor for load. Normally spoken these devices have an overrange and max source of 25mA. In this setup you can read 0-5V at the input of the PIC. Use some protection circuit if interfacing field wiring.

Hi all,

The application it's not for a specific equipment. It's to interface automation sistems like Si.emens S.7 with Analog Output Cards, All.en Bradle.y , ...

So the idea it's close a loop of one of this outputs and get the current values.



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