0-10 VDC signal to 4-20 mA?

Good day to you all,

First of all, I'm new to this forum as I'm seeking your help from experienced programmers and electrotechnicians (for both professional or hobby)

I'm facing a challenge, I work on a merchant vessel and a while ago some electrician has installed a 0-10VDC pressure transmitter and a control unit which only works with a 4-20mA sensor.

The Uopen (open contact voltage) or the PLC is 22VDC, the pressure transmitter has a separate power supply set to 24VDC. To change the signal from 0-10VDC to 4-20mA a converter is installed (consisting of op amps I assume?).

This converter gets power from the same 24VDC supply as the pressure transmitter, the + connection input is coming from the 0-10VDC signal wire from the transmitter and the - signal input comes from the - of the 24VDC power supply.

On the converter module there are 2 potmeters, 'zero' and 'gain'. The 4-20mA connection open contact voltage is 10VDC. When I measure mA between the output signal contacts I can properly adjust zero to 4mA and see signal increase as the input voltage increases. However, here is my problem, when I connect the output of this module to the input of the control PLC I get a steady current of 22mA that barely changes while changing the zero and gain of the converter, and also the voltage over the input contacts has decreased to 15VDC.

Now I know the use of mA signal can have two main purposes; receiving a signal, or sending a signal to e.g. an actuator. I suspect that this converter module is designed for sending control signals instead of particularly transforming a V signal to mA signal for receiving a sensor value. I also know that transforming mA signal to V is relatively easy as installing a resistor of 250 Ohms for 0-5V (from 0-20mA) and 500 Ohms for 0-10V will do the trick. However, I have found that the other way around is not that simple. I lack the knowledge and experience in solving this matter myself, especially with the limited access to the internet while at sea.

Right now I'm seeing a few options causing this problem:
- System is wrongfully connected
- Converter is malfunctioning
- PLC is malfunctioning
- Converter is not designed for this purpose.

I really appreciate your thoughts and ideas on this, I'm always eager to learn so this is a good opportunity for me to improve my knowledge on this matter.

Questions....
Do you have a part number to the converter you are using.
How do you have it hooked up?
What is the PLC and cards (I/O) part numbers

When measuring current you need to be in series, have you tried this when you are connected to the PLC? also the conversion from V to mA resistance depends on the PLC brand and model you are using, some the 250 ohm is a good number but not for all.

Also welcome to the forum

Unfortunately I have not found a model/type number of the converter nor the PLC, if it's alright with you I'll make a drawing and gather pictures of the set-up in a few hours.

PLC input 4-20 mA + goes to - on the converter and + of the converter goes to - of the PLC input. I've measured the current in series by disconnecting the - of the converter and the + of the PLC and connecting the Amp meter in between.

When measuring current from + of the converter to - of the converter I get a nice and steady 4-20mA over 10VDC. However, when creating a loop with the PLC I get strange values. Maybe the signals don't cooperate?

At this moment there's no additional resistor installed, if this would be a solution for the situation I will try it as soon as I can.

I'll post additional information in a few hours

Step 1 is obviously read the manuals and wiring diagrams for the PLC and converter.

If I was guessing at your problem, I'd say PLC +input goes to + of the converter, PLC -input goes to - of the converter.

Vincenturion said:

I'll make a drawing and gather pictures of the set-up in a few hours.

Click to expand...

We like pictures and drawings...

Just remember when measuring current you need to complete the loop, not like voltage when you can just put the probes on the contacts, both will change with you have the load of the PLC in the loop

Your step 1 is obvious yes, but as I can't find anything about the converter and the control unit has a superficial manual, I don't get any further. That's why I'm here nevertheless.

AustralIan said:

If I was guessing at your problem, I'd say PLC +input goes to + of the converter, PLC -input goes to - of the converter.

Click to expand...

Changing the polarity of the terminals only changes the direction of the current, not the flow (22mA both situations), already tried that but thanks for your suggestion.

If the voltage to mA converter is not an isolating type then you can have problems when the two separate DC power supplies do not have their zero volt commons tied together.

I would try feeding the pressure transmitter and converter from the 22V PLC power supply so everything shares a common supply. If that works, then I'd revert to the original and tie the zero volts together

I know that things can be different on ships so I answer with caution.

The bit that stands out in your description is that the transmitter has a separate 24V power supply. Unless the 0V or minus legs of the transmitter and PLC are at the same voltage you can get a ground loop problem. If you have tested the circuit and know that the supplies are not referenced to earth then ignore this suggestion. Otherwise, as a test connect your meter set to volts between the PLC minus volts and the transmitter supply minus volts.

Then also referring to the separate 24V power supply. In a loop there must be a voltage to drive the loop, sometimes it can be supplied from the PLC, sometimes it can be supplied by the transmitter, it shouldn't be supplied by both. You know that the transmitter is supplying it because you get a reading on a passive multimeter. Make sure that you aren't also trying to supply the loop from the PLC.


Saffa types quicker than I do!!

Very often Analogue cards can have either active or passive loops for 4 wire transmitters the supply is sourced from a power supply to the +&- of the 4 wire sensor, 2 wire sensors require a supply either from the analog card or a supply to the +ve lead of the 2 wire sensor with the -ve lead feeding the + of the card, the -ve of the card is then connected to the -ve of the power supply, check the type of analogue card as it could be active or passive.
fitting an active card to a 4 wire sensor could cause max current to flow.
Suggest you read this it shows the types of connections
https://www.plcacademy.com/plc-analog-input-output/

geniusintraining said:

We like pictures and drawings...

Click to expand...

https://ibb.co/TmxpR79
https://ibb.co/CPb1dmm
https://ibb.co/ysj9QBY
https://ibb.co/1My470Y
https://ibb.co/7ncRCG6
https://ibb.co/FwpNqtD
https://ibb.co/XY2h5n1
https://ibb.co/jRWGMTX
https://ibb.co/Gs2LsJH
https://ibb.co/v1S0JrB
https://ibb.co/b5HkSpL
https://ibb.co/56Lqjp5
https://ibb.co/4JBcBMX
https://ibb.co/PZVK9vW

I hope these photos shed some light on my situation and on what I've tested so far. My apologies if my schematic diagram doesn't contain the official symbols, I'm not trained in that. The controller is made by an Italian company named fourgroup, type FULL-APP1.

And you have it wired like this? https://ibb.co/FwpNqtD

Also I did not see the PLC unless thats what the app2 is, then its someones microcontroller and unless you have their software/cable your in trouble

I think your going to need to get a hold of the manufacture

In this photo, https://ibb.co/TmxpR79
the label on the converter reads -20_0_20mA

-20_0_20mA is not the same as 4-20mA.



What makes you think the converter's output is 4-20mA output?

Can you test the converter?


Do you have variable DC power supply?

Can you power the converter, connect the variable DC supply output to the input of the converter and measure the current output of the converter and prove that
0V input = 4mA output
5V input = 12mA output
10V input = 20mA output
?

danw said:

In this photo, https://ibb.co/TmxpR79
the label on the converter reads -20_0_20mA

-20_0_20mA is not the same as 4-20mA.



What makes you think the converter's output is 4-20mA output?

Can you test the converter?


Do you have variable DC power supply?

Can you power the converter, connect the variable DC supply output to the input of the converter and measure the current output of the converter and prove that
0V input = 4mA output
5V input = 12mA output
10V input = 20mA output
?

Click to expand...

As "DanW" says i'd be careful can you check the OEM documentation on the signal level. -20-0-20mA would look like the sort of input you get from a differential pressure transmitter. Is there a specific OEM pressure transmitter recommented?

A 0-10v PT is not going to be easy to convert to 4-20 or even 0-20mA you may get a barrier or a converter but it may be better getting the correct sensor.

Googling 0-10Vdc to 4-20ma converter
brings up dozens of pages of choices.

danw said:

Googling 0-10Vdc to 4-20ma converter
brings up dozens of pages of choices.

Click to expand...

Dan, I think 4x4 was talking about the PT it self, not a signal converter/conditioner, as you pointed out if the signal from the PT is -20_0_20mA now trying to convert it to 0-10 then 4-20 is going to be a mess.

I think I would push back on the person that designed it and make them supply something that will work not that you need to dream up all these work arounds.