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Old January 13th, 2009, 08:34 AM   #1
parvezl
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2-wire and 4-wire sensor wiring for analog module

Hi,

Is it possible to connect 2-wire sensor and 4-wire sensor together in an analog module of a PLC, where COM terminal is only one single. For eg, i am using twido TWD AMI 4LT module.

I need to connect 3 loop powered sensors and 1 externally 230VAC powered sensor.

Is there something wrong here in the connection ???

Regards,
Foo
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Old January 13th, 2009, 12:49 PM   #2
abdallah mohasseb194
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4-wire sensor wiring

Am not aware about your analog card but from my previous experience in automatic controll i think you are right because all y0u need in case of non-two wire transmitter is to share the common-return between transmitter and receiver good luck
abdallah
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Old January 13th, 2009, 02:27 PM   #3
BobB
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Be extremly careful using 2 wire non-isolated sensors/transmitters into analogue cards of this type - they can interfere with each other.
YES!! I have been caught and now only use isolated sensors/transmitters.
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Old January 13th, 2009, 05:00 PM   #4
panic mode
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bob can you elaborate please?
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Old January 13th, 2009, 05:05 PM   #5
Roy Matson
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It looks perfectly OK to me
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Old January 13th, 2009, 05:41 PM   #6
mellis
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The wiring is fine as long as all the transmitter outputs are isolated like BobB said. By isolated, I mean neither transmitter output terminal is referenced to ground. For the 4-wire transmitter the output terminals may not be referenced to the incoming power either.

High quality 2-wire instruments are isolated. It's been a while since I ran into one that wasn't. I think if I did come across one today, that not being isolated pretty much says "poor quality".

4-wire instruments are also usually isolated, but I still find ones that aren't. If I had a problem with mixed 2-wire and 4-wire instruments, I'd disconnect the 4-wire signals first to try to find the offending transmitter. "Low cost" drives both DC and AC sometimes have analog speed signals that are referenced to the incoming line. I recall one incident where a 0-10vdc speed signal was actually 90-100vdc when referenced to ground. Hooking that one up as shown would definitely let some smoke out.
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Old January 13th, 2009, 05:59 PM   #7
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I believe either it will not work or you will cause interference. The reason I say this is the 230 VAC 4 wire transmitter you are using produces its own 24VDC and therefore has it's independent DC common. In your drawing you are using the DC common of one power supply to act as the return for a different 24VDC power supply. Please let me know how you made out with this.
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Old January 13th, 2009, 06:12 PM   #8
danw
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Quote:
Originally Posted by mellis View Post
I recall one incident where a 0-10vdc speed signal was actually 90-100vdc when referenced to ground. Hooking that one up as shown would definitely let some smoke out.
If it were me, I'd connect the three 2 wire transmitters as shown; chances are they are isolated and all will work fine.

But before connecting either output wire from the 4 wire device, I'd check each of the output wires for common mode voltage with respect to the 0v terminal on the Siemens, to see how much common mode there is. Hopefully, less than a couple volts.

I, too, have had problems like Mellis cites, with 4 wire outputs. I never trust a 4 wire output. His example has wildly excessive, damaging common mode, but other times it's 15Vdc common mode, enough to saturate the other inputs, while not destroying the input card.

Dan
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Old January 13th, 2009, 09:33 PM   #9
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Explanation
I often use 4-20ma inputs into an analogue card in swimming pool systems for reading temperature, Ph, FAC, TDS etc. Normally there is only 1 temperature input and the rest are driven, isolated outputs from a chemical controller. I normally use a loop powered head mounted RTD transmitter (also called a hockey puck transmitter).
PLC analogue cards are usually isolated from the PLC bus but the inputs are quite often not isolated from each other and have a common negative connection.
Had a job where I required 3 x 4-20ma inputs to read temperature - used hockey pucks - readings all over the place - they interfered with each other via the common negative connection.
Advice - use isolated transmitters and it does not happen - or has not happened to me anyway. It is possible to buy isolated hockey pucks by the way - my client had only run 2 wires to each transmitter - he then had to run 4 wires to each transmitter when I changed them to isolated hockey pucks - +/-24VDC and 4-20ma return. There is a big difference in price by the way but not worth the agro.
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Old January 14th, 2009, 10:10 AM   #10
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Personally I would add an isolator for the 4 wire circuit and unless the SMPS is only about 200mA and short circuit protected I would add some fuses.

Bryan
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Old January 15th, 2009, 03:51 AM   #11
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The loop powered sensors are 3 nos of RTD, with 4-20 transmitters which are head mounted. the 4-wire transmitter is a Mettler Toledo M-300 conductivity transmitter.

The problem i am facing now is that, when i connect the 4-wire transmitter, all my 2-wire transmitters give max reading at 11 to 12 mA instead of 20mA.

The same configuration when connected with Mitsubishi FX2N-4AD does not cause a problem. Where all 4 commons are different for each AI. Is it because of that?

Can someone clarify?

--
Foo
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Old January 15th, 2009, 05:09 AM   #12
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So either the M-300 is pulling the SMPS down or there is a ground loop problem. Ground loop problems never seem logical until you draw everything out and calculate currents and voltages. The answer is as I wrote above, add a signal isolator between the M-300 and the PLC and hopefully your problems will go away. I had similar issues with two guided radar level sensors that worked fine with only one connected but gave bad readings if both were used. Adding signal isolators fixed the problem, something like this from Status.

Bryan
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Old January 15th, 2009, 05:10 AM   #13
Ken Moore
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Quote:
Mitsubishi FX2N-4AD does not cause a problem. Where all 4 commons are different for each AI
Yes, if all of your AI's were isolated you would not have the problems you are experiencing.
Fast solution (as already mentioned) would be installing a signal isolator on the 4-wire device connections.
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Old January 15th, 2009, 02:09 PM   #14
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Read my last post - I have been caught - I no longer use non-isolated transmitters!!
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Old January 18th, 2009, 09:34 PM   #15
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Quote:
Originally Posted by BobB View Post
Be extremly careful using 2 wire non-isolated sensors/transmitters into analogue cards of this type - they can interfere with each other.
YES!! I have been caught and now only use isolated sensors/transmitters.
Hi Bob,

Can you please post part number of the 2-wire device that you had problem with? I think the confusion is part of possibly different naming convention. I asked for explanation of above statement since I did not understand how is it possible that two wire transmitter is not isolated (well it's not after it's installed of course, but it can be wired either as sink or source and I don't see how two-wire sensor can be cause of trouble). The problem with shared commons is normal for 3 wire and non-isolated 4-wire devices (but 2-wire...?). The explanation in second post suggest that original device was replaced with 4-wire transmitter (not 2-wire device).

Quote:
Originally Posted by BobB View Post
Explanation
...... Had a job where I required 3 x 4-20ma inputs to read temperature - used hockey pucks - readings all over the place - they interfered with each other via the common negative connection.....I changed them to isolated hockey pucks - +/-24VDC and 4-20ma return....
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