Need Help -Replacement parts for 4-20 ma loop controller

Discussion in 'General Electronics Chat' started by Tarmizi Hamid, Jun 7, 2015.

  1. Tarmizi Hamid

    Thread Starter New Member

    Jun 7, 2015
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    Hi there, im repairing one of my client's equipment. Please refer to picture below. my problem is i need to replace the black components. the components is combination of relay which is uses 4-20ma as control current to complete the secondary circuit which is 15vac converting to 5vdc. can you guys help me out here. im stucked. i cant find any replacement component such as ssr or conventional relay to replace this item. this is custom made from China.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you know the function of the original, then you should be able to duplicate it, if it is a custom made component then you need to show a little more of the circuit, that particular picture of the board shows very little without knowing what and where the connecting pins do.
    Max.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    Is there any writing or printing on the other side of the black module?
     
  4. Tarmizi Hamid

    Thread Starter New Member

    Jun 7, 2015
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    No. thats the problem.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    I think you're stuck with working out the schematic from the PCB tracing on the back of that board, which looks like it shouldn't be too hard. Do you know the pinouts of those connectors?
     
  6. upand_at_them

    Active Member

    May 15, 2010
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    4 to 20mA...Sounds like a car's oxygen sensor.
     
  7. panic mode

    Senior Member

    Oct 10, 2011
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    I am with wayneh, you need to know functionality of the unit and
    replicate it using whatever you have at disposal.
    4..20mA transducers/signal converters are very common.

    what I find odd is:
    1. you refer to it as 'relay'
    2. it uses analog 4..20mA loop signal to 'convert 15VAC to 5VDC'

    my guess is that loop signal is just converted to PWM for example. 15VAC is odd too...
     
  8. Tarmizi Hamid

    Thread Starter New Member

    Jun 7, 2015
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    here is the diagram. [​IMG]

    thats how the all six pins connected. 2 pins for the 15vac. 2 pins are for signal a signal b, both are 4-20ma, the last 2 pins is the ouput which will produce 0-5vdc to activate the ssr. The 4-20ma also have voltage around 1-5v.
     
    Last edited: Jun 17, 2015
  9. Tarmizi Hamid

    Thread Starter New Member

    Jun 7, 2015
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    this is the circuit from the manufacturer.
    [​IMG]
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Its an ancient serial protocol that dates back nearly to the telegraph stations in the wild west before they had wireless.

    I think it was used on some teleprinters and a few early serial I/F ASCII RTTY printers.

    May even have been used for the old tikka-tape machines.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    4-20ma loop is still around but not serial, it is an analogue transmission.
    Max.
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    AFAIK: RS232 evolved out of 4-20mA.
     
  13. MaxHeadRoom

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    Jul 18, 2013
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  14. ian field

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  15. MaxHeadRoom

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    Jul 18, 2013
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    I just implemented one for a Hydraulic (4-20ma) pressure sensor!
    Max.
     
  16. ian field

    Distinguished Member

    Oct 27, 2012
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    I suppose there's nothing to stop you using it for analogue level transmission, if you don't mind the possibility of drift.

    My first introduction to 4-20mA loops was from computer books - I did start reading up on it all those many years ago, but it quickly became apparent that by the time I'd learned enough, it would be entirely ousted by RS232.

    Desperate as I was all those years ago for a printer, I refused a number of offers of 4-20mA loop printers because by then RS232 was the only game in town.
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    But we are not talking about digital transmission, the PDF gives an excellent description of the 4-20ma loop which is still popular 60yrs hence for all kinds of transducers, Omega (et-al) , one of the biggest manuf of transducers still offer their products in this medium, and if there were reliability issues due to drift etc, I doubt they would be continuing with this type for critical measuring, weigh scales etc.;)
    Max,
     
  18. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Guys:

    1) For serial transmission, there is no 4 in Current loop. A 20 maybe. See, http://www.google.com/url?sa=t&rct=...=8ZCKiNW7OBbtr-u2J9fERQ&bvm=bv.96041959,d.b2w

    DECwriters and ASr33 Teletypes were inherently current loop with cards to change to RS232.

    2) 4-20 mA is an analog loop where 4 mA is used to power the sensor through the signal wires. Voltage drops due to wiring resistance is therefore not an issue.
    It's very common in process control.
     
    MaxHeadRoom likes this.
  19. ian field

    Distinguished Member

    Oct 27, 2012
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    I was vaguely aware of analogue applications for current loop communication, but my own interest in it was purely to get a printer on my IBM XT. By that point in time, RS232 was rapidly taking over and any interest in current loops rapidly evaporated.
     
  20. Tarmizi Hamid

    Thread Starter New Member

    Jun 7, 2015
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    Just got back from client's office, what I need basically something from off the shelf, 15vac convert to 0-5vdc, the gate will be control by 4-20ma current. So the output will proportionate with the control current. Can come body help me what product that I can buy.
     
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