20+ hours banging head off wall

Discussion in 'General Electronics Chat' started by bowlingo, Oct 8, 2011.

  1. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    Hi all,

    I am trying to get a PH-CF transmitter to work with SMS controllers analogue inputs..

    Here is the transmitter

    http://www.omega.co.uk/ppt/pptsc.asp?ref=CDTX300

    As you can see the transmitter takes the value of the PH and CF and then outputs them at between 4 - 20mA I can calibrate the system without problem using a multimeter i.e 5mS (half of CF range) = 12mA (half way between 4 and 20 mA)

    They work great on there own i.e the PH side into the sms controller that is going to read the PH values or the CF side of the transmitter into the sms controller that is going to read Cf values....Until I have both on at once..when this happens the readings go crazy that the controllers read of which is due to them sharing the supplies in some fashion.

    Here is the diagram provided by the company who sold me the transmitter

    [​IMG]

    I have wired it exact and always the readings go crazy at the sms controller (but perfectly fine when wiring one or the other)

    Here is the circuit diagrams provided with the transmitter

    [​IMG]

    I have wired it in both combinations and used 3 x individual power supplies but no luck. You will notice the negatives are joined internally...I have opened the unit to confirm this. This means theres no way of seperating the supply apart from cutting the tracks inside the unit or similar. Due to the nature and affordability of this unit I am stuck with this one as I cant find anymore specific to my application that are not 3+ times the price.

    On the website I bought the transmitter from you will see this...

    http://www.omega.co.uk/ppt/pptsc.asp?ref=CCT

    This is a loop isolator..if all else fails with the wiring could this or something similar work?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Gee, it looks to me as if the upper schematic is very different from what is proposed by the lower two schematics.

    I've edited the top schematic and attached it. Note that the connections for MA1 and MA2 are different than originally shown. You really should ask the vendor about this, but I think I have it correct.
     
  3. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    Thanks Sgt...

    The people who sold me this did so on a very hit and miss basis...its been sent back to them twice they are now saying it works fine of which is true due to them working individualy...but not together.

    If I go and wire this up and its wrong could I damage anything?..theres a lot of moneys worth here
     
  4. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    I have also just noticed you have joined the 0 connections to - internally there is a connection between the power supplies - point and the 0 connections so im not sure if the connections on your edited diagram are needed?

    Thanks
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You're telling me that the negatives of the two units are wired together internally, yet that top schematic showed the positives wired together. So, that would cause them to "fight" each other like you've been seeing, getting weird readings all over the place.

    If you don't want to risk it, then wait until Monday and ask the vendor again if the schematic I've re-done is correct instead of what they had sent to you before.

    I haven't worked with these before, but just from what you've been writing and what the three documents have shown indicates to me that the top schematic is NOT correct as drawn.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I take it back - I apparently got confused somewhere.

    Ask the vendor again on Monday why these things aren't working as they should be.
     
  7. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    The problem is the only option now is to get a refund from the vendor due to them not having a clue which is to do with these units being very rare I believe (I had to wait several weeks for them to build it aparently)

    The provided diagram there tech sent me is definitely wrong...

    But the diagram that came with it shows the negatives joined internaly of which ive confirmed by opening it up and looking at the tracks against the 0 and power supply - pins.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I got confused by the way the different diagrams were drawn. I've realized since that the top schematic actually DOES match the others. I was mistaken.

    I don't know why the polarities seem to be wrong. I haven't worked with one of these type systems before.
     
  9. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    The negatives joined between the power supplies - and the 0 terminals internaly dosent of course make a difference as of your modified diagram.....
     
  10. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    This is my guess as I have no experience with these probes.

    Have you tried placing PH & Conductivity probe in fluid but in two electrically isolated(separated) containers to see if the unit works?

    The current from one probe might be interfering with that of the other.

    As far as I know the probe uses electrodes to measure the parameters in the fluid of interest. Does the specification specify a minimum separation distance between probes or mention any requirement regarding the conductivity of the fluid?

    Have you follow exactly the wiring connection of the probes and did not try to use one instead of two wires for those that you think are redundance.
     
    bowlingo likes this.
  11. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    I have tested it using seperate containers with solutions in rather than together..

    I have tried all the diagrams exact with and without the - wire issue
     
  12. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    I have just tried as of the modified diagram and using 2 x multimeters im getting 1.52 mA but I should have 11.2mA and 12mA of which I get when wiring and testing individualy.

    Thanks
     
  13. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You would need to confirm:

    1. Both the probes are connected and a voltmeter to monitor the 24V supply

    2. With either output meter is connected, the reading is correct. Same for the remaining meter.

    3. Watch the reading on one meter, the moment you complete the connection of the second meter, the mA output drops to 1.52mA. This is also true regardless of the order which meter is connected first.

    4. Does the 24V power voltage drops when this happens?
     
  14. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    With either output connected the readings are correct.

    When I use 2 x multimeters I get no drop in current when both are connected or when either one is connected (stays exact) but when I remove the multimeters and connect them to the analogue inputs of the SMS controller the readings received go haywire.

    When I connect each of them individualy to the SMS controllers they work great..The sms controllers are seperate units (one for PH and one for CF) so im not trying to mix the inputs with the SMS controllers..even the power supplies for the SMS controllers are seperate from the power supply/supplies of the transmitter

    Thanks
     
  15. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    ok...ive just been over it again a couple of times

    When both outputs are connected into 2 x multimeters the readings are as they should be (the same as when connected individualy)

    When each output is put into the sms controller individualy all is good and the readings are the same as the multimeter readings

    When I put both outputs into the 2 x seperate sms controllers one output is dropping from 4.5 - 1.52 and the other is raising from 3.88 - 7.8

    I have measured across the supply and at various points and I am getting 24V

    If something was in series the values would change like this?
     
  16. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    So the problem is the SMS controller input.

    Somehow the separate SMS controllers share one common connection, maybe its 0V or positive supply, so it will only be fine when only one is used.

    Do you have details or connection spec on the SMS controller?
     
  17. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    The problem is at the SMS controllers as they are sharing some kind of common - or + somewhere. I have found this by seperating all the input and output wiring from one and having one of the transmitters outputs connected to it. This will end up turning into a nightmare trying to find what they are sharing...

    would a signal isolator work? like this one http://www.omega.co.uk/ppt/pptsc.asp?ref=CCT

    Due to the sms controllers sharing something I am not sure if this will work
    Thanks
     
  18. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    488
    What's the SMS-controller model?
     
  19. bowlingo

    Thread Starter Member

    Jun 29, 2011
    162
    0
    The SMS controller im refering to is one of these..

    http://gsmalarmsystem.en.alibaba.com...r_RTU5011.html

    Here is the manual

    http://www.megaupload.com/?d=R7HJ999Y

    I have 2 x sms controllers one is using its analogue input for PH and the other is using its analogue input for CF as above all is well when using either the PH or CF on there own but when i am trying to use both cf and ph the readings go all over the place....this is due to the 2 x controllers using a common point within the system I have built. I have tried a power supply on each controller, a power supply on the control side of the sms controller (relays etc its switching) and one on the ph/cf transmitter...nothing seems to work. If I were to put one of these http://www.omega.co.uk/ppt/pptsc.asp?ref=CCT on the sms controllers analogue input for ph and one on the sms controllers analgue input for cf would that not work? I am wiring it exactly as the first drawing.


    I am running the PH side through 1 seperate sms controller and the cf side through another seperate controller and have them connected each individualy between AGD and AG0...I was almost jumping with joy earlier when I managed to manipulate the scale and base settings on the analogue input so it was measuring the CF spot on againsy my calibration solutions and hand help meter I was restarting the units etc and all was remaining spot on.

    What happened then was I tried to do the same with the ph..I took the probe out of ph4 buffer solution and put it into ph7 buffer solution this then made the cf probe jump from 0 (probe as in free air) to 2.5..I then put the ph probe into ph10 buffer solution and the CF reading went up to around 7 still whilst in free air.

    This tells me the cf and ph outputs are not actually seperate on the transmitter this is using the top diagram as of this post...if I can get them to seperate i.e hopefully correct the wiring I am pretty sure I will be able to manipulate the base and scale settings on the transmitters and get it to measure acurately.

    Thanks ​
     
  20. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Ok, I see exactly what your problem is.

    You are having trouble interfacing two current values, which shares a common positive, to the SMS controller, which has 4 channels but only a common at the negative end.

    With electronic component, it can be solved using current mirror to duplicate the current but unfortunate I don't know of a ready made solution. Now with the problem clearly stated, I think you will get an answer from many of our members soon.

    [​IMG]
     
    Last edited: Oct 10, 2011
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