How to build a 4-20 mA source for testing?

[klittle]

Has anyone built something to send a 4-20 mA singinal to an analog card for testing a program? I know you can buy an source meter but i think i have enough junk laying around ( resistors, pots, ect.) to make one that would run off a 12 or 24 volt DC power supply.

[Binaural]

Originally Posted by klittle

Has anyone built something to send a 4-20 mA singinal to an analog card for testing a program? I know you can buy an source meter but i think i have enough junk laying around ( resistors, pots, ect.) to make one that would run off a 12 or 24 volt DC power supply.

Cheap but potentially nasty way:
Make a standard voltage divider with one fixed resistor and one trimpot and put a mA scale current scale on the output to the device, selecting an initial value that gives you about a 4mA output into a test load. Adjust the voltage up using a pot until you get the desired amperage through your device. Note that if you are smart, you should knock this down to the lowest before putting it on a new load, and hence it is not a great idea for a general tool, nor is it current limited.

Better way:
Make a constant current source using op-amps (look up standard diagrams on the net). A little more design time needs to be sunk but this is much more reliable for different loads.

[MikeVT]

I built one of these (see attached). I haven't used it yet. It will produce a voltage signal, or current.

[Binaural]

http://michaelgellis.tripod.com/howland.html

[klittle]

thanks i will see if i can find the stuff to build one

[jdbrandt]

This is a 'current source' problem from Basic Ohm's Law 101

[Stationmaster]

Originally Posted by MikeVT

I built one of these (see attached). I haven't used it yet. It will produce a voltage signal, or current.

MIKE!!!!!

I built 2 of these for about 30 bucks! I'm currently using them and they work GREAT!

Just put a milliamp meter in the loop and you've got QUITE the little "calibrator".

The LAST calibrator I bought complete was about 700 bucks.

Thanks,

Stationmaster

[Roy Matson]

I have a slightly different variation on MikeVT's signal source. Mine uses the same 317 regulator with switches for 1,2,4,8,12 mA. I can use this to source (4 wire) or simulate (2 wire) It also has a 10 turn Potentiometer which I can use as a rough check for thermocouple or RTD inputs. Operating on 2 x 9 volt batteries or no batteries for 2 wire simulate.
If I need 1-5 V I just put a 250 Ohm across the terminals.
I don't need a meter, just add the value of ON switches.
Roy

[JeffKiper]

Roy do you have a drawing for this I like toys. Your tester will replace my 3~21mA/ 0~10VDC generator. My little tester has saved by behind more than once. I still carry rubber bands & superglue just to cover they backside.

[Stationmaster]

Originally Posted by MikeVT

I built one of these (see attached). I haven't used it yet. It will produce a voltage signal, or current.

Originally Posted by Roy Matson

I have a slightly different variation on MikeVT's signal source. Mine uses the same 317 regulator with switches for 1,2,4,8,12 mA. I can use this to source (4 wire) or simulate (2 wire) It also has a 10 turn Potentiometer which I can use as a rough check for thermocouple or RTD inputs. Operating on 2 x 9 volt batteries or no batteries for 2 wire simulate.
If I need 1-5 V I just put a 250 Ohm across the terminals.
I don't need a meter, just add the value of ON switches.
Roy

Roy,

Your circuit sounds interesting and useful. The idea of the "ADDING" switches is great! But in MY projects, exactly EVEN milliamps won't always do, for example when I'm trying to dial in an exact simulated "pressure" regardless of the MA. Mike's pot-turned-rheostat gives me that "analog" feature that I need.

Actually, the name on the schematic is "Panic Mode", to give credit where credit is due.

Also, sometimes the DIRECTION of MA change is as important as the MA VALUE....... again, the pot that I don't have to look at to get what I want wins out.

I like the idea of YOUR ckt being battery powered, and being able to simulate 2 or 4 wire sensors....

NOOWWWWWWW..... Lets see..... couldn't we put our heads together and come up with a ckt that has the best of both WORLDS?

Stationmaster

P.S. "Pot" is okay but "******t" (A M B I E N T) gets CENSORED??????

[Tom Jenkins]

I use JBrandt's solution all the time. Simple and effective. I usually use 1 kOhm resistors and 5k Ohm multi-turn pots that are readily available. We have put a bunch of them on breadboards with terminal blocks and we can simulate an extensive system with analog I/O very inexpensively. With some jumpers you can simulate either looop powered or self powered transmitters as well.

[Roy Matson]

Jeff,
I will draw one up in the next couple of days.
Stationmaster,
Are you refering to the ******t temperature error for thermocouple? I know how to compensate for that, Yes a pot could easily be added, in 20 years I havent needed it.
Regards
Roy

[Roy Matson]

Re the ********t correction
Since when was A-M-B-I-ENT considered politicaly incorect????

[Mickey]

Originally Posted by Roy Matson

Re the ********t correction
Since when was A-M-B-I-ENT considered politicaly incorect????

Its a drug, sometimes used for recreational reasons. Not the exact spelling but close enough for this site and others I might add.

http://en.wikipedia.org/wiki/Zolpide..._use_and_abuse

[Roy Matson]

Jeff, Stationmaster,
Please drop me an e-mail so I cand send you a drawing
roy underscore matson at yahoo dot ca