how to get a constant Voltage

Sid2286 · Joined: 12 Aug 2010 Posts: 119

I have an 4-20ma input, i put a 10ohms resistor wrt grd and get 40mV and 20mV respectively and give that to ADC and display value. everything is fine till here, now i have the same current which is going ahead to some unknown load, how do i display the values in this case?
Please give some feedback.

thanks!
sid

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

Sid2286 · Joined: 12 Aug 2010 Posts: 119

Well somewhat close.. But not exactly what i meant!
I wasn't getting the name.. I want to build a loop indication.. That means I get 4-20mA, display the PV on certain range and that loop is further connected to plc or HMI.

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

Sid2286 · Joined: 12 Aug 2010 Posts: 119

Yes, its quite confusing. Is there a way of programming PIC wherein I get voltage across Rsense as input and then Rsense is further connected in series to whatever external load ?

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

temtronic · Posted: Mon Aug 08, 2011 10:42 am

You could put a 250r resistor in the 4-20ma loop and have a differential amplifier measure the voltage across it. With proper resistor selection for the diffamp, it'll output say 1 volt for 4ma, 5 v for 20 ma. This output can easily be read by the PICs ADC.
There are commercial chips/modules that do this but for a one-off project and as a learning tool, it's best to do it yourself.

Sid2286 · Joined: 12 Aug 2010 Posts: 119

Accepted that the maximum load will be 250r at the plc. But it won't be necessarily 250r it may be less...in that case the readings will be different!

Ttelmah · Joined: 11 Mar 2010 Posts: 19513

The whole point about 4-20mA drivers, is that they produce a precisely defined _current_. If the loop is producing 12mA (say), then a 100R resistor in the loop, will always measure 1.2v _whatever else is in the loop_ (within the limits of the driver's ability to generate voltage). There should be a figure from the manufacturer of the device driving the loop, giving the maximum resistance it is able to drive. Typically something like 500R. Provided the total resistance in the loop does not exceed this, the current will remain the same whether you insert a 100R resistor, or a 200R resistor, and whatever other devices are in the loop.
You choose either the resistor itself, or the gain of the op-amp between the sense resistor, and the PIC's ADC, to give the voltage you require. Remember that the PIC's 5v supply is generally not accurate as a Vref, so if you use (say) a 2.5v Vref, and a 100R resistor, you can read from 0 to 25mA - a lot of 4-20mA devices use a higher current like 24mA, as an 'error' indicator, and if you can read to 0mA, you can detect that nothing is driving the loop. You'd then have 655counts for the 4-20mA range, and limited error detection as well.

Best Wishes

Sid2286 · Joined: 12 Aug 2010 Posts: 119

Thanks temtronics and Ttelmah. I got what is required but i would like to know if the way I'm programming is right!
Now lets say I've 4mA input, I use a diff amplifier and get say 1.5V, I use a read_adc() function and find out its value as 255. Now again I read the value for 20mA and find the voltage as 3.5v and adc value as say 800. So I use these, 255 as my min and 800 as max, I calibrate on a scale of range 0-100. Is this the right way to do?

Sid2286 · Joined: 12 Aug 2010 Posts: 119

Please any replies or suggestions :(

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Industry standard 4-20 mA loop indicators like E+H RIA141 https://portal.endress.com/wa001/dla/50002285073/000/05/TI106Ren_1108.pdf
are loop powered and are designed with a low resistance shunt (only limited voltage drop) and a Z-diode like voltage limiting circuit to supply the processor and display, both connected in series. In my opinion, the design requirements are quite obvious, when you think about it.

Sid2286 · Joined: 12 Aug 2010 Posts: 119

I tried using a differential amplifier but it seems to be not working! It works for a certain condition(without any load..when load is connected to ground), however when the load resistance changes...the voltages at TP1 and TP2 changes with respect to ground. and thus, my differential output changes giving no constant adc input for 4mA and 20mA.

My circuit diagram is as follows:

http://www.freeimagehosting.net/71122

Ttelmah · Joined: 11 Mar 2010 Posts: 19513

Hopefully R6 is not as shown in the picture?. 10K!.... Won't work.
U1B, is not needed, or doing anything really.
There is a potential problem, if the 4-20mA driver is not fully floating, depending how the ground in the PIC circuit relates to the 4-20mA driver circuit voltages, and the common mode range of the op-amps.

Best Wishes