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Current Loop PIC - looking for info how to build it

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neuralc

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Hi,

Where I can find definitions to build an interface for a curent loop (4-20 mA) and a PIC micro?

ThX


NeuralC
 

suiram

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

Stallion

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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...

Bitburner
 

neuralc

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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


NeuralC
 

Stallion

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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

BitBurner
 

neuralc

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Hi Stallion,

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

ThX

NeuralC
 

neuralc

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Nobody have related material ? ...

ThX

NeuralC
 

T

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

msmax

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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
 

penrico

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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.
 

msmax

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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.
 

neuralc

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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.


ThX

NeuralC
 

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