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4-20mA to ohms coverter

Discussion in 'General Electronics Discussion' started by Shey Malcolm, Jul 19, 2014.

  1. Shey Malcolm

    Shey Malcolm

    6
    0
    Jul 19, 2014
    Hi everyone, im after some help with integrating a PT100 sensor into my alarm panel (its a pretty smart alarm panel, just not quite smart enough to take a 4-20mA reading and convert it to temperature)

    Basically I have a PT100 probe, and a 4-20mA converter for it, what I need to do is accurately convert the 4-20mA signal back to a known resistance change so the alarm panel can read it, preferably with the following increments (these resistances are what the panel recognises as the temperature)

    10C - .229vdc - 99ohms
    14C - .310vdc - 131.4ohms
    18C - .399vdc - 170ohms
    22C - .489vdc - 210ohms
    24C - .523vdc - 226.9ohms
    27C - .579vdc - 254ohms
    30C - .653vdc - 287.5ohm
    33C - .704vdc - 313.2ohm
    36C - .755vdc - 338ohm
    38C - .806vdc - 363.6ohm

    This is the probe, and the transmitter -
    http://www.kiatronics.com/temperature-sensor/temperature-probe-pt100-stainless-steel.html
    http://www.kiatronics.com/temperature-sensor/temperature-sensor-4-20ma-transmitter-code-70692.html


    Thank you all for any help & ideas you might have.

    Cheers

    Shey
     
  2. Arouse1973

    Arouse1973 Adam

    5,164
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    Dec 18, 2013
    I get most of that apart from the panel recognises the resistance. How is your panel measuring these resistances or does it just see the voltage as an input? Does the convertor put out 160uA per degrees centigrade rise in temperature?
    Adam
     
  3. Shey Malcolm

    Shey Malcolm

    6
    0
    Jul 19, 2014
    Thanks for your reply Adam,

    The alarm panel works purely by measuring resistance, the inputs dont measure the analog measurements like4-20mA or as far as im aware the voltage, to get the measurements I listed i literally sat there with a variable resistor and watched the temperature rise / fall. - Im not sure if this answers your question or not..

    I cant find much information on the device, however I am more than happy to measure it for you, ive found some similar products that give all sorts of specifications, but dont appear to have a amperage vs degrees measurement, heres the sheets I was looking at, is there anything helpful you can see?

    http://www.wika.co.nz/upload/US_DS_TE2401_en_us_18902.pdf

    Thanks again for your help :)

    Shey
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    So what are you doing? Connecting the resistor across the transmitter and then measuring the voltage? For something to know the resistance value of a resistor either the voltage or current to it must be fixed. And then either the voltage or current is measured to work out the resistor value. Do you have any diagrams of your setup?
    Adam
     
  5. Arouse1973

    Arouse1973 Adam

    5,164
    1,078
    Dec 18, 2013
    The easiest option for standard 4-20 loop current measurement is to use a 250R resistor which will give you a range of 1-5 Volts. So each degree rise would be an extra 50 mV. I don't understand why your voltages are so precise at whole values of temperature.
    Adam
     
  6. Shey Malcolm

    Shey Malcolm

    6
    0
    Jul 19, 2014
    Ok, so at the moment I have no way of connecting the PT100 & transmitter to the panel, if I feed the current through the two terminals, the panel doesnt understand, all it wants to see is that change of resistance (and therefore i guess a change of voltage as the resistance changes) - so what I need is something that I connect the 4-20mA loop into, then it outputs a change of resistance as per the list. I can draw up a diagram for you soon :)
     
  7. Shey Malcolm

    Shey Malcolm

    6
    0
    Jul 19, 2014
    The reason is that the alarm panel has various templates for interpreting the signal it recieves, the one im working with needs those resistances / voltages to read the temperature, to get those measurements I literally sat there with a multimeter and measured the voltage across the variable resistor, and then disconnected the vr from the panel to read its ohms value.
     
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Can you change the input levels to measure different voltages like 1 to 5 volts or are you fixed at the voltages in your first post.
     
  9. Shey Malcolm

    Shey Malcolm

    6
    0
    Jul 19, 2014
    I just tried wiring it to take a 1-5V measurement, it ended up thinking I was trying to arm the house. Go figure. Turns out I need to supply it the voltage/resistance as above to get it to read my temp :(
     
  10. Shey Malcolm

    Shey Malcolm

    6
    0
    Jul 19, 2014
    Not sure if this clarifies anything, other than the magical device i see fixing the issue
     

    Attached Files:

  11. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I am struggling to work out a simple solution for you and unless the other guys can come up with something this is going to need some more thought. The only thing I can think of is a circuit to sequentially switch in the different resistors. So are you looking for only those temps shown and those voltage values? Do you just ignore anything else.
    Adam
     
  12. Sinewave

    Sinewave

    119
    1
    Feb 15, 2013
    You can buy those converters, they're not cheap though. I may even have some spare.
     
  13. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Screenshot-23.png
    Here I extended the table of measured resistance values with the calculated current for each temperature point. It appears that the panel measures the sensor resistance by using a constant current source of 2.3 mA. Also included the standard PTC100 sensor resistance for the temperatures to be measured. Note the wide variation of the resistance expected by the panel, whereas the the PTC sensor (which operates to 450˚C has a very small variation in resistance over the temperature range of interest. It would seem that the panel expects direct connection of a particular type of thermistor and the PTC100 is the wrong sensor to use. Using the PTC100 sensor over a narrow temperature range will cause any errors to be magnified, and using a current converter which will need a current to resistance conversion will be a fertile source of signal errors. Is there any way to use the proper thermistor instead?
     
    Arouse1973 likes this.
  14. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Nice one Laplace.
    Could the OP at least supply details of the panel and any instruction he has so at least we can get a better understanding.
    Adam
     
  15. Arouse1973

    Arouse1973 Adam

    5,164
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    Dec 18, 2013
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