Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

4-20mA loop calibrator

Status
Not open for further replies.

Overclocked

Member
I want to build a 4-20mA loop calibrator for work. Ive decided it will be powered by a battery (9V) but can also be loop powered to withstand up to 30V. I originally looked into using a LM317 as a constant current source, But 1) I am worried about accuracy and 2) The resistors I would need are hard to get (62 Ohms and 312 Ohms) in the accuracy I would want them, so Ive decided to build a Constant Current source using a op-amp. I would like the calibrator to be accurate to atleast 0.03% or better, since the A/D converters that are used use 15 bits. Ideally I would like each box to cost $5 to $10 each.

What would I need to look for in a Op-Amp that's to be used as constant current source? I would use a Zener diode 0r Voltage reference that is atleast accurate, or more accurate than I need. Resistors would have to be 0.1% Tolerance or better.

Or does any one have suggestions for LDO's that will be just as accurate?
 
Need more information.
Do you want to output 4mA and 20mA or must it output all currents from 4 to 40?
0.03% can not come from several 0.1% resistors + 0.1% reference + (?).
I looked up some 4-20 machines and fount: 1% to 0.2% and 0.1%.
Also found 0.05% "linear" which is not the same thing.
Your 15 bits is not accuracy but resolution.
 
Need more information.
Do you want to output 4mA and 20mA or must it output all currents from 4 to 40?
0.03% can not come from several 0.1% resistors + 0.1% reference + (?).
I looked up some 4-20 machines and fount: 1% to 0.2% and 0.1%.
Also found 0.05% "linear" which is not the same thing.
Your 15 bits is not accuracy but resolution.

Opps! That's what I meant. I will use a switch to switch between 4mA and 20mA, which should be as easy and switching between resistors.
 
For a voltage reference, the TL431C is rated for 0.2% tolerance at 25C and 1% over the temperature range -40C to +85C.
 
Look at the MAX6126. At $6.00 for one. 2.5V 0.02% Lots of money, but one of the best.
Also see ISL21090

You can get real good resistors.

Choose an opamp with low input current.
I would choose a MOSFET not a bipolar transistor.

Draw up a schematic and lets see.....
 
Essentially, this is what Im thinking of. RL wont be there on board, it will be off board. Values have not been picked other than my power source (which is 2 9V batteries) and Reference voltage. RS sets the Current out, in this case Vref/Rs. I chose 4.096V because it gives me a larger range of resistances to choose from.
 

Attachments

  • 4-20loop source.png
    4-20loop source.png
    4.7 KB · Views: 240
The 4.096V needs to be across RS. What you have places 9-4.096 across RS.
Here is something I got from an011. There is an error around VR1 but the rest looks good.
D1 is there to make pin-4 of op-amp 0.6V below ground. Many amps do not work well if the inputs/outputs get too close to the power supply voltage.
**broken link removed**
**broken link removed**
 
Hmm, Well Ive reduced that schematic down to the bare basics, ie without all the fluff, as far as my understanding goes. I dont need it to work at low input voltages, so I removed R2, R3 ,R4 and VR1. Instead I will change R6 (Called out as R1/R2 in my schematic to vary the current). However, it might be more beneficial to use the same method via a switch to vary the current as the original author did, as I am finding out that the resistors needed aren't going to be avaiable anyway. For a 4.096V Reference, I would need 204.8 Ohms and 1024 Ohms for 20 and 4mA respectively. I am finding out that the closest value is 205 Ohms and 1020 Ohms with 0.1% Accuracy.

I also removed D1, but Im slightly unsure about *not* keeping it, only because it does serve to bias the negative side slightly to make the op-amp float.
 

Attachments

  • 4-20loop source Ver 1.1.png
    4-20loop source Ver 1.1.png
    5.1 KB · Views: 243
You can always use a second resistor in series or in parallel with a standard value sense resistor to trim it to the custom value you want.
 
Looks good.
I would keep the 4mA resistor in all the time and then switch in a 16mA resistor for a total of 20mA. (simple switch)
You could have a three position switch for 4mA, 0mA, 20mA. (center off switch)
 
I am going to cheat and suggest a Calex Programmable Current Source. The model 930 is a nice board to work from. Keep in mind the lower the voltage you use the lower the compliance voltage you have to work with. Think the load resistance of whatever you hang out there. Anyway, a Calex 930 or similar module and one of those sensor and display modules off Ebay or Amazon for a few bucks. Tis way you need not make a board.

Ron
 
You know, I looked more at the calibrators at work. How can they be accurate down to 0.001 mA or +/- 0.015% (speced out the in datasheet) when the best components are accurate to 0.02% or more?
 
Last place I worked, we had a cell site alarm card that had 20 mA I source and sense channels (designed to operate over 600 feet wire). I don't remember the exact spec but it was no where near the accuracy your shooting for, so I gotta ask. What is the intended application for your current loops? Is there some particular reason the accuracy is so critical?
 
Last place I worked, we had a cell site alarm card that had 20 mA I source and sense channels (designed to operate over 600 feet wire). I don't remember the exact spec but it was no where near the accuracy your shooting for, so I gotta ask. What is the intended application for your current loops? Is there some particular reason the accuracy is so critical?

Not really. Just a exercise in precision. The simulators we have eat up batteries like crazy, and recently I had to do something that involved letting my program do stuff over night, so I figure why not use it as a opportunity to make something
 
Overnight fix: One time I wrote a program, ran it overnight and the problem was fixed in the morning. It opened and closed a relay over the GPIB bus every 30 sec or so on a power supply.
 
Not really. Just a exercise in precision. The simulators we have eat up batteries like crazy, and recently I had to do something that involved letting my program do stuff over night, so I figure why not use it as a opportunity to make something

I can relate to that. We had a pair of Fluke 725 Multifunction Calibrators that were handheld. While they were very handy and nice using them for things like overnight current testing was a pain as they ate 4 AA Alkaline batteries. They it was a pain in the butt to drag out a large calibrator just to have a small current to run a valve or something. I finally found a cheap unit and while it wasn't a Fluke it had a plug in wall wart. :)

Ron
 
Couldn't you just use a 6v wall wart? Assuming 4 AA batteries

Likely yes, I never bothered. I just looked for something that ran on 120 VAC 60 Hz. The company had deep enough pockets plus the lab guys got another new piece of test equipment.

Ron
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top