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

yes it can, however it will not reach 10 Volts, and as the resistance in series is 560 ohms it will also not reach the 20 mA, however you can change the resistors to get correct resukts.
It is simple why it is not done as a simple block battery will not be able to deliver this 20 mA for long, a better solution is to use 2 block batteries.
 
yes it can, however it will not reach 10 Volts, and as the resistance in series is 560 ohms it will also not reach the 20 mA, however you can change the resistors to get correct resukts.
It is simple why it is not done as a simple block battery will not be able to deliver this 20 mA for long, a better solution is to use 2 block batteries.

The two batteries are in series, so they are both delivering 20mA @ 9V.

I'm sure you've not extended battery life at all...
 
Ryan,

Welcome to the forum first of all!

I set mine up to run on 3 - 9V batteries, in series, but i have never used it that way. My experince has found that almost always, when dealing with analog signals, there is a 24VDC power supply to borrow power from. I put 24VDC input banana jacks in the project box, to supply the electronics in the box.

Mike
 
For the guy like me who would rather buy than build, I'm re-publishing my review of a cheap 4-20mA source that I posted at
http://www.plctalk.net/qanda/showthread.php?t=107307
several weeks ago.

I bought five of the simulators (on ebay, from China for about $10 each) shown in the graphic images below and just finished testing them out-of-the-box. (use 4-20mA source as a search term)

front_view_2.jpg


back_view2_sm.jpg


Observations:

a) the digital display reads to one digit to the right of the decimal point, ie, to 0.1. The sales description mentions 0.01, but the hundredths digit is not displayed. Some of the ebay listing photos do not show powered displays so it isn't obvious what the display resolution is.

b) The generated signals are off by a 0.1mA or 0.2mA (measured with a calibrated mA calibrator), so the device is not a calibrator, it's an indicating source. There are no adjustments (pots or otherwise) to correct zero or span.

That doesn't bother me because I am looking for cheap 0-20mA source and if I need an exact mA value I'll put meter in the circuit.

c) I use two 9V batteries in series as a day-to-day power supply. The field connections are on the Phoenix Contact style removable connector which makes testing and disconnecting the battery 'pack' easy. (the higher the voltage the greater loop resistance it can drive.
According to the specs, 12V will drive 350 ohms, 24V will drive 800 ohm; so 18V will drive 600 ohms.

d) I give them a thumbs up for the price at $10-12 each. They are perfect for sourcing a mA signal.
Just be aware of the display resolution and the inaccuracy of the displayed value (it's an approximation) to see if it meets your needs.
 
Last edited:
Here is the 4-20mA source that I built...

Using the LM317 datasheet as well as some of the schematics posted earlier in this thread as a reference, I finally got around to putting together a PCB for this.

I included a DIP switch to individually switch in different value resistors that set currents of 1, 2, 4, 8, and 12mA. So any combination of switches can be activated to easily simulate the full 4-20mA range in +1 mA increments.

The switch for the 12mA setting is actually adjustable via a potentiometer for the full 4-20mA range so I am not necessarily limited to discrete 1mA steps.

I included some test points and a shunt so I can easily clip a mulitmeter in series without actually unhooking any wires.

if anybody is interested, I'd be happy to post my schematic and parts list. I even have a few extra bare PCBs available if there is any interest.

board_.jpg 12mA_.jpg
 

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  • 20mA_.jpg
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Using the LM317 datasheet as well as some of the schematics posted earlier in this thread as a reference, I finally got around to putting together a PCB for this.

I looked and did not see a post from you in this thread, its 10 years old... so are you saying it took you 10 years to build it? I think you should of spent the money and just bought one :)

But since you revived the thread we now have a new one that does +/- 10VDC and 4-20ma its nice and comes with a case and leads https://www.plccable.com/new-updated-analog-simulator-and-generator-with-lcd-0-10vdc-0-22ma/
 
I have several of these ALP2 that GIT posted. They are on every service van we have.

I had a analog output card on a PLC go bad. Plc said 20 mA the valve only moves 75%. Customer wanted to replace the valve. I grabbed my tester and proves it was the output card and not the valve.
 

I have that one, and it works fine using a phone charger battery pack (whatever you call them). Took a while to figure out the 'Chinglish' instructions, and I never did figure out the supposed battery inside, but all the same, simple and cheap and effective.
 
I have had a couple of the Chinese versions and my issues was the accuracy sometimes off by a few % I also hated carry a seperate power supply... I used mine a lot when working on Spirax controllers for a customer, they had them from the first floor to the 9th so having a battery built in was great when troubleshooting them and checking calibration, not having to look for power was the choice for me, I also had a Fluke 707's and they are very nice but cringed at the cost.
 

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