4-20ma loop question.

Just stumbled across this forum and hopefully someone can help.

I have a two wire 4-20 ma transmitter connected to an analogue input on a plc.
On rare occasions, using a Fluke process meter, I can "add" approx. one milliamp to the circuit without disconnecting the loop.

I have other uses for the process meter, so I would like to build a cheap circuit, which I can permanently wire in parallel with my analogue input so when I switch it "ON" it will add one ma to the loop.

Any bright ideas ?

Why? If its just to increase the value in your PLC why not do it logically in the PLC?

Old prehistoric Sattcon plc, don't have the software or program or leads and don't want the hassle, hence my original plan will get me out of trouble until, I get around to replacing the lot.
So want it done as per original post.

I missed the evening news, but I doubt Kirchoffs Law was revoked.

Where is the additional 1mA coming from, that the AO regulation circuit can't 'see' that addtional 1mA and adjust for it?

An analog output (AO) drives an analog input (AI) somewhere. Analog inputs have a fixed input shunt resistance to IR drop the current.

Conventional AO circuits regulate the voltage to maintain the desired current level in the circuit and do so as long as the loop resistance remains within driveable limits of the voltage supply.

So altering resistance will not alter the total loop current (except for momentary transient adjustments in the circuit).

Altered resistance could divide the loop current into two paths, one across the AI, one bypassing the AI, but that would decrease the current at the AI, not increase it. And a fixed resistance would not always offer a constant current of 1mA because the AO's voltage varies to produce the desired current.

I'm very curious about
- whether you meter is it a calibrator meter that can source an current, voltage or quasi resistnacel, or is it a read-only meter?
- where your meter is inserted in the AO circuit to obtain an additional 1mA
- where does each lead attach?
- what function is the meter set to? Read ohms?

- exactly where the 1mA appears;
- on the meter? or
- on the AI?

Does the meter read
- 1mA? or
- loop current? or
- loop current + 1mA?

When the meter creates this extra 1mA, what does the AI read?
- 1mA? or
- loop current? or
- loop current + 1mA?

Maybe put one of these across the transmitter?

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6284232

Pablox said:

"add" approx. one milliamp to the circuit without disconnecting the loop.

Click to expand...

Without disconnection anything ? So you're placing it across the plc input how / where ?

Would it not be as simple as placing a resistor in parallel with his sensor? Assuming a supply voltage of 24v, and a current of 0.001A, then a 24k resistor is all that's needed. Place the resistor in series with a switch (gold contacts because of the low currents involved) and connect in parallel with the sensor. If you're using another voltage, just use this handy Ohm's Law calculator to determine what resistance you need:

http://www.ohmslawcalculator.com/ohms_law_calculator.php

The resistor you choose should be a 1% precision or better device. You may want to add a trimmer pot in series with your resistor to fine tune the current if exact accuracy is needed.

-rpoet

danw said:

I missed the evening news, but I doubt Kirchoffs Law was revoked.

Click to expand...

I did see the evening news, and you are right, Kirchoffs Law was neither revoked or mentioned, in fact Gustav didn't get a look in.

danw said:

very curious about
- whether you meter is it a calibrator meter that can source an current.

Click to expand...

It sure is.

This one to be exact
http://www.fluke.com/fluke/usen/process-calibration-tools/loop-calibrators/fluke-789.htm?PID=56133

danw said:

what function is the meter set to? Read ohms?

Click to expand...

Nope m/a output

danw said:

Does the meter read
- 1mA? or
- loop current? or
- loop current + 1mA?

Click to expand...

It reads 1ma

danw said:

When the meter creates this extra 1mA, what does the AI read?
- 1mA? or
- loop current? or
- loop current + 1mA?

Click to expand...

The AI reads loop current + 1mA

theDave2 said:

Without disconnection anything ? So you're placing it across the plc input how / where ?

Click to expand...

http://assets.fluke.com/video-PTOOL/3846747_ptools_Fluke_789.html

I have done this umpteen times, there is no problem.
Eg if a process is in operation, and say needs a pressure reading of 2 bar before the next step in the plc program is executed, and for whatever reason I got 1.9 bar, using the process meter I can "add" the difference", the sequence will be completed, and I can then repair leak or whatever.

My question from the beginning, is :
Can I achieve the same result without using my current source i.e Fluke 789 and without interfering with the Plc program using relatively cheap hardware.

Pablox said:

Eg if a process is in operation, and say needs a pressure reading of 2 bar before the next step in the plc program is executed, and for whatever reason I got 1.9 bar, using the process meter I can "add" the difference", the sequence will be completed, and I can then repair leak or whatever.

My question from the beginning, is :
Can I achieve the same result without using my current source i.e Fluke 789 and without interfering with the Plc program using relatively cheap hardware.

Click to expand...

Put a signal conditioner inline, and then tweaks its scaling temporarily. If it's always the same 1ma that you need to add, then maybe you can find a unit with presets, so you can simply flip a switch.

http://www.wici.com/technical/manuals/wici_pdf/cat_pdf_files/DR4380A_Catalog_Page.pdf
http://www.omega.com/ppt/pptsc.asp?ref=DRF_SERIES&ttID=DRF_SERIES&Nav=

Pablox said:

My question from the beginning, is :
Can I achieve the same result without using my current source i.e Fluke 789 and without interfering with the Plc program using relatively cheap hardware.

Click to expand...

Why not add a "Manual Sequence Advance" bit in the plc program that you can trigger from the HMI for such an event. Then you won't need any hardware at all.

Of course, you'd want it to be password-protected...

OZEE said:

Why not add a "Manual Sequence Advance" bit in the plc program that you can trigger from the HMI for such an event. Then you won't need any hardware at all.

Of course, you'd want it to be password-protected...

Click to expand...

see post 3 m8.

You can make a 1mA constant current source using a LM317L and a single 1.2K resistor. Look on page 9 for the circuit, under "adjustable current source". Instead of the potentionmeter, use the resistor.
Put the circuit in paralell with your 2 wire sensor and switch it in and out of the circuit as needed.
http://www.national.com/ds/LM/LM317L.pdf

This is most likely how the calibrator induces an additional milliamp (yeah, the arrows go positive to negative).

The transmitter's output cannot 'see' the 1mA current from the calibrator across the AI resistor; that 1mA returns to the calibrator's supply. So the transmitter's AO is happy with its current output; it doesn't attempt to cut back because of excess current in the loop.



A resistor cannot be put in parallel with the existing analog input shunt resistor because
a) that would just decrease the AI's input resistance.
b) The 24V is NOT across the resistor. The transmitte's AO regulates the current in the loop by dropping whatever voltage is needed to get the loop current it wants. So a 24K resistor would not get 24V across it.



I think you can can add a milliamp by putting ~24K ohm resistor as shown below. The blue circuit has 24K + 250 ohms across the DC power supply.

Thanks for the replies, looks like I posed a bit of a puzzler.
Will have to investigate some of the replies further when I order a few bits.
The solution submitted by RDS looks perfect to me using the LM317L and a little variable resistor should give me a useful "adjustable" current source costing less than a dollar.

RDS said:

You can make a 1mA constant current source using a LM317L and a single 1.2K resistor. Look on page 9 for the circuit, under "adjustable current source". Instead of the potentionmeter, use the resistor.
Put the circuit in paralell with your 2 wire sensor and switch it in and out of the circuit as needed.
http://www.national.com/ds/LM/LM317L.pdf

Click to expand...

Just a thank you m8, only got around to trying your solution today, it works 110%, amazing solution and an amazing little device that LM317L.
Tiny, cheap and works a treat.
Thanks again m8, and thanks to all others who tried to help. :site: