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current measurement using pic

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  • Yunus Emre
    Hi, I need to get the information in a 4-20 mA PLC signal using pic, I guess I can use A/D converters available in PIC MCU but I need a suggestion on
    Message 1 of 22 , May 1, 2006
      Hi,
      I need to get the information in a 4-20 mA PLC signal using pic, I guess I can use A/D converters available in PIC MCU but I need a suggestion on converting 4-20 mA to 0-5V analog signal so I can get it in PIC A/D converter , are there any ICs that I can use for this conversion or any other idea to achieve this

      thanks..


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    • Snail Instruments
      ... Yes, there is a special conversion circuit - 250 Ohm resistor :-) Josef
      Message 2 of 22 , May 1, 2006
        > I need to get the information in a 4-20 mA PLC signal using pic, I
        > guess I can use A/D converters available in PIC MCU but I need a
        > suggestion on converting 4-20 mA to 0-5V analog signal so I can get it in
        > PIC A/D converter , are there any ICs that I can use for this conversion
        > or any other idea to achieve this

        Yes, there is a special conversion circuit - 250 Ohm resistor :-)

        Josef
      • mictester
        ... It s really difficult - try a 250 ohm resistor!!  Your voltage range is actually 1 - 5 Volts corresponding to 4 - 20 mA.  You can use the loss of voltage
        Message 3 of 22 , May 1, 2006
          On Monday 01 May 2006 10:34, Yunus Emre wrote:
          > <html><body>
          >
          >
          > Hi,<BR>
          >   I need to get the information in a 4-20 mA PLC signal using pic, I
          > guess I can use A/D converters available in PIC MCU but I need a suggestion
          > on converting 4-20 mA to 0-5V analog signal so I can get it in PIC A/D
          > converter , are there any ICs that I can use for this conversion or any
          > other idea to achieve this<BR>    <BR>
          >   thanks..<BR>

          It's really difficult - try a 250 ohm resistor!!  Your voltage range is
          actually 1 - 5 Volts corresponding to 4 - 20 mA.  You can use the loss of
          voltage (to 0 V input) to detect disconnection of the current loop and to
          give an alarm.

          Chris
        • D. Kuipers
          That indeed is a simple solution, but I guess the voltage drop over the resistor might be a problem. Maybe it s an idea to use the ADC with a voltage reference
          Message 4 of 22 , May 1, 2006
            That indeed is a simple solution, but I guess the voltage drop over the
            resistor might be a problem. Maybe it's an idea to use the ADC with a
            voltage reference instead of the Vdd/Vss (5 volt reference) and use a
            resistor with a lower value for measuring current.

            Just an idea though...


            Snail Instruments schreef:
            >
            > > I need to get the information in a 4-20 mA PLC signal using pic, I
            > > guess I can use A/D converters available in PIC MCU but I need a
            > > suggestion on converting 4-20 mA to 0-5V analog signal so I can get
            > it in
            > > PIC A/D converter , are there any ICs that I can use for this conversion
            > > or any other idea to achieve this
            >
            > Yes, there is a special conversion circuit - 250 Ohm resistor :-)
            >
            > Josef
            >
            >
            >
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          • mictester
            ... No! You terminate the 4 - 20 mA current loop with a 250 ohm resistor. You measure the voltage across the terminating resistor. This voltage will be
            Message 5 of 22 , May 1, 2006
              On Monday 01 May 2006 13:51, D. Kuipers wrote:
              > That indeed is a simple solution, but I guess the voltage drop over the
              > resistor might be a problem. Maybe it's an idea to use the ADC with a
              > voltage reference instead of the Vdd/Vss (5 volt reference) and use a
              > resistor with a lower value for measuring current.
              >
              > Just an idea though...

              No! You terminate the 4 - 20 mA current loop with a 250 ohm resistor. You
              measure the voltage across the terminating resistor. This voltage will be
              within the range of 1 - 5 volts. Your voltage sensing has to have high imput
              resistance (like a modern digital voltmeter), so as not to load the 250 ohm
              resistor significantly and cause errors.

              Mike
            • Stef Mientki
              ... By coincidence I measured the input impedance of a AD-channel of a 12F675 last week. At 10 kHz the equivalent impedance was 100 kOhm. The impedance is
              Message 6 of 22 , May 1, 2006
                mictester wrote:
                > On Monday 01 May 2006 13:51, D. Kuipers wrote:
                >
                >> That indeed is a simple solution, but I guess the voltage drop over the
                >> resistor might be a problem. Maybe it's an idea to use the ADC with a
                >> voltage reference instead of the Vdd/Vss (5 volt reference) and use a
                >> resistor with a lower value for measuring current.
                >>
                >> Just an idea though...
                >>
                >
                > No! You terminate the 4 - 20 mA current loop with a 250 ohm resistor. You
                > measure the voltage across the terminating resistor. This voltage will be
                > within the range of 1 - 5 volts. Your voltage sensing has to have high imput
                > resistance (like a modern digital voltmeter), so as not to load the 250 ohm
                > resistor significantly and cause errors.
                >
                By coincidence I measured the input impedance of a AD-channel of a
                12F675 last week.
                At 10 kHz the equivalent impedance was 100 kOhm.
                The impedance is inverse proportional to the sample frequency,
                so I guess there's no problem at all.
                There should be somewhere worst case impedance graphs,
                but I didn't look for them.

                Stef
              • D. Kuipers
                ... Measuring the voltage across the resistor is indeed the obvious thing to do, that s not what I wanted to change (there must be a voltage drop across the
                Message 7 of 22 , May 1, 2006
                  mictester wrote:
                  > On Monday 01 May 2006 13:51, D. Kuipers wrote:
                  > > That indeed is a simple solution, but I guess the voltage drop over the
                  > > resistor might be a problem. Maybe it's an idea to use the ADC with a
                  > > voltage reference instead of the Vdd/Vss (5 volt reference) and use a
                  > > resistor with a lower value for measuring current.
                  > >
                  > > Just an idea though...
                  >
                  > No! You terminate the 4 - 20 mA current loop with a 250 ohm resistor. You
                  > measure the voltage across the terminating resistor. This voltage will be
                  > within the range of 1 - 5 volts. Your voltage sensing has to have high
                  > imput
                  > resistance (like a modern digital voltmeter), so as not to load the 250 ohm
                  > resistor significantly and cause errors.
                  >
                  > Mike
                  >

                  Measuring the voltage across the resistor is indeed the obvious thing to
                  do, that's not what I wanted to change (there must be a voltage drop
                  across the resistor in order to measure it). But a 5 volts voltage drop
                  might influence the current loop you are measuring. If that's no problem
                  I didn't say a word, that's up to the Topic Starter ;-)

                  Using a high resistance voltage meter is also quite obvious if you do
                  not want to influence the results too much, but as the Topic Starter
                  wants to use the ADC, the ADC is the voltage meter to use.

                  A relatively randomly picked Microchip datasheet tells me the maximum
                  recommended impedance of an analog source is 2,5k ohm.

                  So I see no objection for using a lower resistor value to measure the
                  current (although in this respect there is also no need for a lower
                  resistor value).
                  Anyway, measuring 1-5 volts over a 250 ohms resistor or measuring
                  0,1-0,5 volts over a 25 ohms resistor can both be done. The 25 ohms
                  resistor will cause a smaller voltage drop in the circuit which is
                  measured though.
                  On the downside, if you would decide to use a 25 ohms resistor (or maybe
                  even lower) instead of a 250 ohms resistor you might want to use a 0,5V
                  voltage reference or you'll have to accept a 10 times lower resolution
                  by still using the Vdd/Vss 5V reference.

                  D. Kuipers

                  BTW: mictester, is it Chris or Mike? (Or both)?
                • D. Kuipers
                  ... The 18F2420/2520/4420/4520 datasheet recommends a maximum analog source impedance of 2,5 kohms. The 12F629/675 datasheet recommends a maximum analog source
                  Message 8 of 22 , May 1, 2006
                    Stef Mientki wrote:

                    > By coincidence I measured the input impedance of a AD-channel of a
                    > 12F675 last week.
                    > At 10 kHz the equivalent impedance was 100 kOhm.
                    > The impedance is inverse proportional to the sample frequency,
                    > so I guess there's no problem at all.
                    > There should be somewhere worst case impedance graphs,
                    > but I didn't look for them.
                    >
                    > Stef
                    >
                    >
                    >

                    The 18F2420/2520/4420/4520 datasheet recommends a maximum analog source
                    impedance of 2,5 kohms. The 12F629/675 datasheet recommends a maximum
                    analog source impedance of 10 kohms.

                    You measured 100 kohm input impedance on a 12F675 (at 10kHz).

                    Hmmm, combined this means the source impedance is recommended to be at
                    least a factor 10 smaller then the input impedance of the PIC's ADC?

                    Works for me :-)

                    D. Kuipers
                  • Wouter van Ooijen
                    ... A 4-20 mA current source should mention the maximum voltage it can deliver. 10V is quite common. But the OP must verify this, and more important: is it
                    Message 9 of 22 , May 1, 2006
                      > But a 5 volts voltage drop might influence the current
                      > loop you are measuring. If that's no problem
                      > I didn't say a word, that's up to the Topic Starter ;-)

                      A 4-20 mA current source should mention the maximum voltage it can
                      deliver.
                      10V is quite common.

                      But the OP must verify this, and more important: is it realy a current
                      source (from for instancxe +24V) or is it a current *sink* (towards
                      gnd). The first is more common, but the second exists too.

                      > On the downside, if you would decide to use a 25 ohms
                      > resistor (or maybe
                      > even lower) instead of a 250 ohms resistor you might want to
                      > use a 0,5V voltage reference

                      that's out of spec for the PIC A/D

                      Wouter van Ooijen

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                    • D. Kuipers
                      ... Wouter is right (as usual :-). The PIC18C reference manual states a minimum Vrefh-Vrefl of 3,0 Volts if you want to use the full 10 bits resolution. A
                      Message 10 of 22 , May 1, 2006
                        Wouter van Ooijen schreef:

                        > > On the downside, if you would decide to use a 25 ohms
                        > > resistor (or maybe
                        > > even lower) instead of a 250 ohms resistor you might want to
                        > > use a 0,5V voltage reference
                        >
                        > that's out of spec for the PIC A/D
                        >
                        > Wouter van Ooijen

                        Wouter is right (as usual :-). The PIC18C reference manual states a
                        minimum Vrefh-Vrefl of 3,0 Volts if you want to use the full 10 bits
                        resolution. A smaller Vrefh-Vrefl is allowed, but will decrease
                        resolution. Remains the 'lower resolution' option.

                        D. Kuipers
                      • Stef Mientki
                        ... The input impedance and the minimum source impedance have no relationship. The input impedance can be determined by the average current through load the
                        Message 11 of 22 , May 1, 2006
                          D. Kuipers wrote:
                          > Stef Mientki wrote:
                          >
                          >
                          >> By coincidence I measured the input impedance of a AD-channel of a
                          >> 12F675 last week.
                          >> At 10 kHz the equivalent impedance was 100 kOhm.
                          >> The impedance is inverse proportional to the sample frequency,
                          >> so I guess there's no problem at all.
                          >> There should be somewhere worst case impedance graphs,
                          >> but I didn't look for them.
                          >>
                          >> Stef
                          >>
                          >>
                          >>
                          >>
                          >
                          > The 18F2420/2520/4420/4520 datasheet recommends a maximum analog source
                          > impedance of 2,5 kohms. The 12F629/675 datasheet recommends a maximum
                          > analog source impedance of 10 kohms.
                          >
                          > You measured 100 kohm input impedance on a 12F675 (at 10kHz).
                          >
                          > Hmmm, combined this means the source impedance is recommended to be at
                          > least a factor 10 smaller then the input impedance of the PIC's ADC?
                          >
                          > Works for me :-)
                          >
                          The input impedance and the minimum source impedance have no relationship.
                          The input impedance can be determined by the average current through
                          load the hold capacitor,
                          (so mainly by the capacitor value and the time the sample switch is on)
                          while the minimum source impedance is determined by the series resistor
                          of the switch + hold capacitor.

                          Stef
                        • D. Kuipers
                          ... Ok, but if you use a device with a lower input impedance, shouldn t you also use a lower value series resistor (in order to retain the same accuracy)? Just
                          Message 12 of 22 , May 1, 2006
                            Stef Mientki schreef:

                            > The input impedance and the minimum source impedance have no relationship.
                            > The input impedance can be determined by the average current through
                            > load the hold capacitor,
                            > (so mainly by the capacitor value and the time the sample switch is on)
                            > while the minimum source impedance is determined by the series resistor
                            > of the switch + hold capacitor.
                            >
                            > Stef
                            >

                            Ok, but if you use a device with a lower input impedance, shouldn't you
                            also use a lower value series resistor (in order to retain the same
                            accuracy)?

                            Just a question (from yet another software-person :-)

                            D. Kuipers
                          • Stef Mientki
                            ... no, Stef
                            Message 13 of 22 , May 1, 2006
                              > Ok, but if you use a device with a lower input impedance, shouldn't you
                              > also use a lower value series resistor (in order to retain the same
                              > accuracy)?
                              >
                              no,

                              Stef
                            • Chris Ellis
                              Hi If your really bothered about the impedance of the ADC can t you jsut use an Op-AMP as a buffer - unity gain. That way you will have an high impeance and
                              Message 14 of 22 , May 1, 2006
                                Hi

                                If your really bothered about the impedance of the ADC can't you jsut
                                use an Op-AMP as a buffer - unity gain. That way you will have an high
                                impeance and therefore minimal side effects on the measurement resistor.

                                Chris Ellis


                                Stef Mientki wrote:
                                >> Ok, but if you use a device with a lower input impedance, shouldn't you
                                >> also use a lower value series resistor (in order to retain the same
                                >> accuracy)?
                                >>
                                >>
                                > no,
                                >
                                > Stef
                                >
                                >
                                >
                                >
                                > Yahoo! Groups Links
                                >
                                >
                                >
                                >
                                >
                                >
                                >
                              • Dave Lag
                                Do these current loops have any defined relationship to ground? The last time I worked on current loops they were attached to teletype machines and had more
                                Message 15 of 22 , May 1, 2006
                                  Do these current loops have any defined relationship to ground?
                                  The last time I worked on current loops they were attached to teletype
                                  machines
                                  and had more than 120V on the legs...



                                  Chris Ellis wrote:

                                  >Hi
                                  >
                                  >If your really bothered about the impedance of the ADC can't you jsut
                                  >use an Op-AMP as a buffer - unity gain. That way you will have an high
                                  >impeance and therefore minimal side effects on the measurement resistor.
                                  >
                                  >Chris Ellis
                                  >
                                  >
                                  >Stef Mientki wrote:
                                  >
                                  >
                                  >>>Ok, but if you use a device with a lower input impedance, shouldn't you
                                  >>>also use a lower value series resistor (in order to retain the same
                                  >>>accuracy)?
                                  >>>
                                  >>>
                                  >>>
                                  >>>
                                  >>no,
                                  >>
                                  >>Stef
                                  >>
                                  >>
                                  >>
                                  >>
                                  >>Yahoo! Groups Links
                                  >>
                                  >>
                                  >>
                                  >>
                                  >>
                                  >>
                                  >>
                                  >>
                                  >>
                                  >
                                  >
                                  >
                                  >
                                  >Yahoo! Groups Links
                                  >
                                  >
                                  >
                                  >
                                  >
                                  >
                                  >
                                  >
                                  >
                                  >
                                • Snail Instruments
                                  ... You software guys should get familiar with the Ohm s law. At least :-) The input impedance of the A/D converter is parallel to 250Ohm resistor. It is three
                                  Message 16 of 22 , May 2, 2006
                                    >Ok, but if you use a device with a lower input impedance, shouldn't you
                                    >also use a lower value series resistor (in order to retain the same
                                    >accuracy)?
                                    >Just a question (from yet another software-person :-)

                                    You software guys should get familiar with the Ohm's law. At least :-)

                                    The input impedance of the A/D converter is parallel to 250Ohm resistor. It
                                    is three orders of magnitude higher so we need not to take it into account
                                    in preliminary calculation. Parallel combination of 250Ohm and 100kOhm
                                    results in 249.4Ohm equivalent. Since the device will undergo calibration,
                                    this would be accounted for. Only temperature and possibly long-time
                                    changes of the A/D input impedance will cause errors, but probably on
                                    sub-1LSB scale.

                                    Josef
                                  • Yunus Emre
                                    actually the current sources are just pressure transmitters operating on 24 V supply, the current values are output in 4-20 mA range. I am going to try
                                    Message 17 of 22 , May 2, 2006
                                      actually the current sources are just pressure transmitters operating on 24 V supply, the current values are output in 4-20 mA range. I am going to try measuring the current using a resistor, and if works this will be a little loss of resolution because of the 10 bits ADC where as the transmitter resolution is in the range 0 - 16000 :(

                                      Dave Lag <daveismissing@...> wrote:
                                      Do these current loops have any defined relationship to ground?
                                      The last time I worked on current loops they were attached to teletype
                                      machines
                                      and had more than 120V on the legs...



                                      Chris Ellis wrote:

                                      >Hi
                                      >
                                      >If your really bothered about the impedance of the ADC can't you jsut
                                      >use an Op-AMP as a buffer - unity gain. That way you will have an high
                                      >impeance and therefore minimal side effects on the measurement resistor.
                                      >
                                      >Chris Ellis
                                      >
                                      >
                                      >Stef Mientki wrote:
                                      >
                                      >
                                      >>>Ok, but if you use a device with a lower input impedance, shouldn't you
                                      >>>also use a lower value series resistor (in order to retain the same
                                      >>>accuracy)?
                                      >>>
                                      >>>
                                      >>>
                                      >>>
                                      >>no,
                                      >>
                                      >>Stef
                                      >>
                                      >>
                                      >>
                                      >>
                                      >>Yahoo! Groups Links
                                      >>
                                      >>
                                      >>
                                      >>
                                      >>
                                      >>
                                      >>
                                      >>
                                      >>
                                      >
                                      >
                                      >
                                      >
                                      >Yahoo! Groups Links
                                      >
                                      >
                                      >
                                      >
                                      >
                                      >
                                      >
                                      >
                                      >
                                      >



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                                    • Dharmasiri Mohottige
                                      Hi, normally all 4-20ma output type devices mentioned in its data sheet, what will be the maximum loop resistance it can handle. some types its less than or
                                      Message 18 of 22 , May 2, 2006
                                        Hi,

                                        normally all 4-20ma output type devices mentioned in its data sheet, what
                                        will be the maximum loop resistance it can handle. some types its less than
                                        or equal 180 ohms. some models can go upto 300 ohms max. check it before use
                                        it.
                                        2nd case was: some sensors use "heart" protocol super imposed on current
                                        loops. in that case ADC will give wrong values. if it is, diable digital
                                        comminucations over analog current loop for accurate reading( that can done
                                        from sensor setup).
                                        FYI. there is standard units available in market for 4-20ma to 0-10v or
                                        0-5v, ect,,,, ect. refer some of there data sheets also to get more info
                                        and factors to take into account.

                                        Dha.
                                        -----Original Message-----
                                        From: jallist@yahoogroups.com [mailto:jallist@yahoogroups.com]On Behalf Of
                                        Wouter van Ooijen
                                        Sent: Tuesday, May 02, 2006 6:14 PM
                                        To: jallist@yahoogroups.com
                                        Subject: RE: [jallist] current measurement using pic


                                        > a little loss of
                                        > resolution because of the 10 bits ADC where as the
                                        > transmitter resolution is in the range 0 - 16000 :(

                                        Resolution != accuracy

                                        if you are interested in the real value, it makes no sense to A/D more
                                        finely that the *accuracy*. The resolution is interesting only if you
                                        are interested in small variations.

                                        Wouter van Ooijen

                                        -- -------------------------------------------
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                                        consultancy, development, PICmicro products
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                                        [Non-text portions of this message have been removed]
                                      • Wouter van Ooijen
                                        ... Resolution != accuracy if you are interested in the real value, it makes no sense to A/D more finely that the *accuracy*. The resolution is interesting
                                        Message 19 of 22 , May 2, 2006
                                          > a little loss of
                                          > resolution because of the 10 bits ADC where as the
                                          > transmitter resolution is in the range 0 - 16000 :(

                                          Resolution != accuracy

                                          if you are interested in the real value, it makes no sense to A/D more
                                          finely that the *accuracy*. The resolution is interesting only if you
                                          are interested in small variations.

                                          Wouter van Ooijen

                                          -- -------------------------------------------
                                          Van Ooijen Technische Informatica: www.voti.nl
                                          consultancy, development, PICmicro products
                                          docent Hogeschool van Utrecht: www.voti.nl/hvu
                                        • Javier Martinez
                                          Hi all, Wouter, you re absolutely right. Take a multimeter, range 2V, 1999 counts . Specs (taken from manufacturer) 2%+5mV: - You can measure from 0V to
                                          Message 20 of 22 , May 2, 2006
                                            Hi all,

                                            Wouter, you're absolutely right.

                                            Take a multimeter, range 2V, 1999 "counts". Specs (taken from
                                            manufacturer) 2%+5mV:

                                            - You can measure from 0V to 1.999V (ignoring negative values)
                                            - Resolution 1mV
                                            - Accuracy (at 1.999V): 1.999*0.02+5mV = +-0.04498V (44.98mV)

                                            Accuracy is ~45 times worst than resolution!!


                                            [PS] ... I suppose that the 4-20mA signal will come with a lot of
                                            noise.


                                            Regards,
                                            Javi.


                                            On mar, 2006-05-02 at 14:14 +0200, Wouter van Ooijen wrote:

                                            > Resolution != accuracy
                                            >
                                            > if you are interested in the real value, it makes no sense to A/D more
                                            > finely that the *accuracy*. The resolution is interesting only if you
                                            > are interested in small variations.
                                            >
                                            > Wouter van Ooijen
                                            >
                                            > -- -------------------------------------------
                                            > Van Ooijen Technische Informatica: www.voti.nl
                                            > consultancy, development, PICmicro products
                                            > docent Hogeschool van Utrecht: www.voti.nl/hvu
                                            >
                                            >
                                            >
                                            >
                                            >
                                            > Yahoo! Groups Links
                                            >
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                                            >
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                                          • Ian (Yahoo)
                                            A 4 -20mA loop is normally powered by a 24V DC supply. The limitation is the total resistance which can be up to 1200 ohms, therefore your 250 ohm sense
                                            Message 21 of 22 , May 2, 2006
                                              A 4 -20mA loop is normally powered by a 24V DC supply. The limitation
                                              is the total resistance which can be up to 1200 ohms, therefore your 250
                                              ohm sense resistor is fine as long as the rest of the circuit impedance
                                              i.e. 4-20mA transmitter and cable resistance is no higher than 950 ohms
                                              (1200 - 250). You should use an isolated power supply if your sense
                                              resistor can be placed anywhere in the loop.

                                              Ian
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                                            • Wouter van Ooijen
                                              ... 1200 (=24/0.02) is the maximum *total* resistance in the loop, including the driver, so it is not a very relevant figure (1200 load resistance would leave
                                              Message 22 of 22 , May 2, 2006
                                                > A 4 -20mA loop is normally powered by a 24V DC supply. The
                                                > limitation
                                                > is the total resistance which can be up to 1200 ohms

                                                1200 (=24/0.02) is the maximum *total* resistance in the loop, including
                                                the driver, so it is not a very relevant figure (1200 load resistance
                                                would leave 0V for the driver to work with).

                                                Wouter van Ooijen

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