Hi there,can we can just discuss about low cost solutions for generating 4-20 ma current?

Directly, probably not but all it takes is the inclusion of a 110R resistor per channel giving a 0.4 – 2.2V range or a 220R to give 0.8 – 4.4V

Depending on the ADC on the micro, this would work just fine (110R for the 3V3 micros and 220R for the 5V micros), this changes if you apply a real VREF but that’s easy math to figure out the required R value to match

Hope this Helps

Peter

I've never used this sort of current loop myself, so this is only an attempt to be helpful -- YMMV.

There's an article about Current Loop at Wikipedia that might have useful background and links: http://en.wikipedia.org/wiki/Current_loop.

The Cypress PSoCs have "high drive" outputs, but I don't know if they go as high as 20 mA.  I suspect you'll need to make a current source using an external transistor or op amp: http://en.wikipedia.org/wiki/Current_source#Simple_transistor_current_sources

The nice thing is it can detect outages as 0 reading is not 0 current, it is 4mA (Just enough to drive the remote sensor and nowadays a micro, ADC, DAC etc) and as it is a current system, can tolerate quite a range of wiring resistance in the loop without affecting the readings (Within the limits of the MAX Volts of the loop source)

I worked with 4-20mA for years with British Gas for instrumentation of all sorts of sensing (Pressure, Flow, Temperature etc)

Ahh the memories

Thanks Peter and john for your valuable answer.

Being specific,here actually i am referring to Electrical programmable transducers.In which we presents electrical quantities such as voltage,current,active power,reactive power,frequency,phase angle in form of either current or voltage.so can you suggest some solution in which I can integrate both voltage output and current output on single channel.

Also I want add that although 4 mA is  lowest point. some times you need to provide 0-20 mA range for some quantities.

electrical programmable transducers follows IEC 60668 standards.

parth pandya

Unless a device has independent power it is will not support  the current going to 0 as some of that 4mA is actually used to power the electronics at the sensor end and in some industrial environments there are devices called “Intrinsically safe Barriers“ or “Zener barriers” that is used to limit the power available to a sensor in for example a gas prone environment.

See this PDF, it has a nice schematic of such a barrier http://www.mpelectronics.com/pdf/app_note_for_stahl_barrier.pdf

Typically the 4-20mA loop is sourced from a 24V supply (In the safe zone) as is the supply typical for most industrial instruments

So as I said, going to zero is not normally an option and going below 4mA is typically taken as a fault condition

Like everything, there are always exceptions to this (uPower sensors running from a coin cell for years and sealed in an explosion proof housing for instance) that would not require power from the 4-20mA loop to power the sensor but this still does not allow for the common understanding that below 4mA is typical of a fault condition and to choose otherwise would limit the use of the solution

Hope this helps

Peter

Dear peter,

I am a firmware developer and i have not seen transducers in working environment.but I want to make my doubts clear.

In above image you can see that 0 to 10 kv reprented by 0 to 20 mA.

parth pandya

I did say there are always exceptions and also that the sensor would have to be powered by other means, all the sensors I have worked with for British Gas where loop powered and hence there could not be a zero current. I did not say it did not exist. but I am glad your on top of it, it has been quite a few years and I am sure there are many advances since I was in the field (Literally ). Also my experience is based on the gas industry, looking at that DIN Rail mount module I would guess your in the Power industry, it looks to be some kind of power meter / remote sensor (I have not looked up the part, just guessing )

btw, TI has a number of chips designed to work directly with 4-20mA (or 0-20mA ) loops, you should check them out, but you can also create the loop driver with not much more than an op amp or two and a few other components

I also found this, it may be of interest: http://forum.arduino.cc/index.php?topic=134213.0

Peter

The device in your picture has multiple outputs - the 0 - 20mA output is quite different from and not compatible with the industry standard 4-20mA signalling. In 4-20 4mA is zero and 20mA is full scale - since the transmitting device expects to draw power from the loop the current can never drop to zero.

The CEWE device in your picture needs power from another source to work.

Are you trying to design a transmitting device ?

MK

Hi Michael,

We are trying to design electrical programmable transducer.

Hi peter,

You are right it is powered from external source.

parth pandya

Is this a commercial, home or student project ?

How many do you want to make ?

MK