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HART, Loop-powered 2-wire instruments

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android
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2009/06/02 01:11:36 (permalink)
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HART, Loop-powered 2-wire instruments

Hi all,

This question goes especially to the Microchip techies.

Do anyone have any idea as to which Microcontroller would be the right choice for loop powered 2-wire applications having HART Protocol Compatibility.
The criteria being a very low runtime/working current (<1 mA approx). As far as I know, microchip microcontrollers do not stand in the list where one can make HART Instrument working with just 2-wires. I agree, the MCP microcontrollers can be used with 3-wire, 4-wire instruments but as of now, microchip does not have any microcontroller which can be utilised for 2-wire loop powered instruments.

Anyone please suggest the same, in case I am wrong.

Texas Microcontroller which fits the 2-wire application very well is as mentioned below for reference.
MSP430 Microcontroller

But I would be extemely pleased to work on MCP products for my application.

So techies help is much appreciated.

Best regards.
#1

19 Replies Related Threads

    android
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    RE: HART, Loop-powered 2-wire instruments 2009/06/02 23:02:15 (permalink)
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    Sorry guys,

    I had made some wrong conlusion.

    I went through the nanoWatt Tech. page and it mentions all the details I ever needed on the following link:
    Extreme Low Power Microcontrollers

    That's really a morale boosting for me to continue my projects with MICROCHIP.

    Thanks Microchip.
    #2
    xiaofan
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    RE: HART, Loop-powered 2-wire instruments 2009/06/03 00:24:56 (permalink)
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    It all depends on the application.

    I have not done any HART instrument. But I did a project back in 2002 in my previous job for a 2-wire NAMUR Level sensor powered up by a source (8.2V with 1k internal resistance), So you can call it loop powered as well. The normal current consumption of the whole device need to be below 1.2mA. I use a PIC16LF872 running at 3.3V and 1MHz (always active, not allowed to sleep because of the slow recovery time).  I have a push-pull isolated converter and some other analog circuits inside the sensor.

    http://www.am.pepperl-fuchs.com/products/product.jsp?product_id=18111 (it is a family device with three option, relay, transistor and the NAMUR version).

    For your particular application, if you need the PIC24 to be active all the time and runs at higher speed, then that can be a problem.You know, PIC24 @ 1MHz is pretty slow.
    post edited by xiaofan - 2009/06/03 00:29:33

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    android
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    RE: HART, Loop-powered 2-wire instruments 2009/06/03 13:11:45 (permalink)
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    Thanks xiaofan,

    PIC16LF has the option I require but incase I run out of space I would try the higher version as my application demands 32x32-bit multiplication.

    And regarding your comment:
    I have a push-pull isolated converter.....


    I am new to push-pull isolated converter so I may to have to do a bit of r&d for the same. My current instruments are without any isolators as they are purely analog but we are about to convert them to uC based so maybe I have to use it.

    Is there any Microchip part available to do the same or any link concerning push-pull isolated converter.

    Thank you once again for your help in advance.

    Regards
    #4
    xiaofan
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    RE: HART, Loop-powered 2-wire instruments 2009/06/03 21:45:51 (permalink)
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    For the push-pull part, you can use any cheap oscillator (I used a CMOS 4000 series multi-vibrator) to drive a transformer. The key is the transformer here. Luckily we had in-house transformer which was usable at that time. It is unregulated. So I use an LDO to get 3.3V to power up the PIC16LF872.

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    xiaofan
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    RE: HART, Loop-powered 2-wire instruments 2009/06/03 21:46:29 (permalink)
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    Do you need isolation? If not, you do not need the push-pull converter.

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    xiaofan
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    RE: HART, Loop-powered 2-wire instruments 2009/06/03 21:50:51 (permalink)
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    By the way, I believe you can typically go for up to 4mA for your loop powered 4-20mA transmitter.

    Here is an application notes from Maxim.
    http://pdfserv.maxim-ic.com/en/an/AN1873.pdf

    It seems to me that the compliance voltage range can be an important factor for your design.

    Typically our 4-20mA output can drive up to 550Ohm load and even 750 Ohm load so the compliance voltage supplied can be as high as 15V (internal is about 18V).

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    android
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    RE: HART, Loop-powered 2-wire instruments 2009/06/04 07:49:02 (permalink)
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    Hi xiaofan,

    As you quoted:

    Do you need isolation?

    Isolation is not a must but if provided would make my circuit more immune to external disturbances and would also provide galvanic isolation. Hence I would prefer the same but I have never used one.

    Regarding your comment:

    For the push-pull part, you can use any cheap oscillator (I used a CMOS 4000 series multi-vibrator) to drive a transformer. The key is the transformer here. Luckily we had in-house transformer which was usable at that time. It is unregulated. So I use an LDO to get 3.3V to power up the PIC16LF872.


    I think when we include transformer in the circuit, it is definitely going to suck up reactive power and would increase the overall mA consumption. The isolators we are currently using for 4-wire instruments takes almost 29mA at no-load with SG3525 as the oscillator driver for SMPS.

    So I am clueless about how to make isolation and that too with transformer in the middle, while maintaining the overall current below 3.5 mA.

    http://pdfserv.maxim-ic.com/en/an/AN1873.pdf

    Though I had many links from maxim for 4-20mA, but this is new for me. Thanks for the same.

    Best regards.
    #8
    xiaofan
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    RE: HART, Loop-powered 2-wire instruments 2009/06/05 02:35:20 (permalink)
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    I am pretty good at DC/DC converters (got my Master degree in Power Electronics) but I do not know much about transformer design. It is not easy to design such a low power isolated converter, especially the transformer part.

    As for SG3525, that is pretty old. The new generation PWM controllers have better characteristics, like low quiescent current.
    post edited by xiaofan - 2009/06/05 02:36:26

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    android
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 05:36:31 (permalink)
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    Ok, thanks xiaofan.

    Since I have got the solution I will be a having a cheerful month to program and design.

    Thank you once again.

    Best regards.
    #10
    isa.guru
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 06:32:59 (permalink)
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    Hi android,

    You can do 4-20 mAloop power on almost any small microcontroller. The point is to have very stable and accurate 4 mA consumption. I do this with a current source for about 3.9 mA, so I have a little for adjustment and tollerances. So the current source takes care about 4 mA and your circutry has to take care for 16 more mA to add to it. You have 2 current sources - one fixed and one comtrollable by your circuit.

    I am missing a little the point with the isolation: if it is a loop power 4-20 mA - the receiving device provides the power to the loop over those 2 wires. You need isolation only on the signal. So what I have been doing is: I have a main circuit which is powered by some power supply or battery. It measures some signal and has to control the 4-20 mA loop power output. The output takes power from the receiving device - that is the meaning of "loop power". So you only need to isolated the control signals to your controllable current source. I have been doing this with ADuM1300 and other from ADuM series. Have not found anything better till now :)

    If you want to power your whole circuit from the loop you can do that but it is not easy at all :)
    You will have to transfer the control signals from the main circuit but trnsfer power to it. You can do this in many ways. You can even combine both power and signal.
    I have used many kinds of circuits (my own design). For an instance an oscillator on LMC555, or MCP6541 or TLC3701 and a small transformer 6505 from Tamura or like 31204 from Midcom, or 78602 from www.dc-dc.com, or Rhombus etc. etc. You can also do your own transformers or use many other (pulse, ADSL etc.). I have done many - the best of them has 83 % efficiency with L-45053 of Rhombus.
    When the current you provide for the main circuit is not enough you just use higher voltage at the loop power side and have transformer reducing the voltage (but increasing the current). For an instance if the loop power is fixed 24 VDC you can use say 18-20 V @ 2-3 mA for the loop power side windings but reducing the voltage say to 5-6V at the main side and have a couple of times more current.

    For the HART you will need generally again small tranformers and galvanic isolation (decoupling) from the loop power. Of course the HART signal has to be added and subtracted from the 4-20 mA signal, so you need again current source or efficient way to mix currents :). In your case you can omit the galvanic isolation and just connect in a proper way the HART communication signal to your controllable current source. Reading is atrickier than writing :)

    Regards
    post edited by isa.guru - 2009/06/06 06:56:39

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    android
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 07:20:21 (permalink)
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    Hi Guru,

    Its you again. I am really glad to hear from you after such a long time. I felt you may be on a holiday.

    Thanks Guru for that enlightening reply. Let me have a deep study of what you have written.

    I will surely look into the changes that you have suggested. (I am sure you are not from "americas finest city" but rather from "Heaven"grin).

    Best regards.

    Android.
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    isa.guru
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 07:47:14 (permalink)
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    No, definitelly not from heaven :). Jsut ask my wife :)
    I am always very busy and do not visit frquently this forum. So I missed your post.

    HART has some advantages but also many disadvantages. It is mostly good if you want to set something remotelly over the loop on your device without goint directly to it. And ...... that is it. I do not see any other advantages in HART. If you want to communicate frequently and read/write to your device, HART is a bad choice.

    7th dan instrumentation and controls
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    xiaofan
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 08:03:54 (permalink)
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    We do not do transmitters but rather the I/O modules interfacing to the 4-20mA transmitters (or actuators). And I do not have real HART related experiences.

    But in my opinion, typical 2-wire loop-powered transmitter will get the power from the 4-20mA loop.  If you have other power supply, then it becomes a 4-wire transmitter. If you have a battery, that is kind of cheating. grin

    Example here (not a transmitter but a isolated repeater by my previous employer)
    http://www.am.pepperl-fuchs.com/products/product.jsp?product_id=17422

    As for HART, it is still very popular in the process automation market. I've seen HART transformers in the previous job as well.

    Isolated HART Transmitter Power Supply, which provides the compliance voltage and pass through the HART signal. The design is really smart (done by some UK engineers).
    http://www.am.pepperl-fuchs.com/products/product.jsp?product_id=17395
    post edited by xiaofan - 2009/06/06 08:05:04

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    xiaofan
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 08:06:09 (permalink)
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    The main advantage of HART is the huge install base and the associated software packages like Emerson AMS asset management suite.

    New development like Wireless HART will probably bring new technology to HART.
    post edited by xiaofan - 2009/06/06 08:07:19

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    xiaofan
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 08:10:18 (permalink)
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    By the way, do you know of any commercial off the shelf HART transformer? In my previous job, some gurus designed the HART transformers (for Intrinsic Safety application so they are relatively big).

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    isa.guru
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 08:20:29 (permalink)
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    Hi xiaofan,

    Yes of course the power comes from the loop. That is exactly what I wrote above. I have been doing loop power devices since early 1970s.

    I was talking about an analog output - isolated loop power 4-20 mA. For an example the magmeter that android designs :)
    =the main circuitry needs a lot more power like at least a couple of watts. So it has a DC or AC power supply.
    =but this magmeter has an isolated loop power 4-20 mA output that follows the flow rate
    =so the main circuit sends periodically the SPI signals to MCP4921 over ADum1300 to control the current in the loop
    =the isoalted circuit is powered only from the loop but receices isolated signals to control the 4-20 mA signal

    I was also talking for another case:
    =the whole power comes from the loop but we measure some parameter lie pH ot water conductivity etc.. so we really want isolation.
    =then the loop side procides isolated power to the sensor side but sensor side sends back (isolated) signal(s) to control the 4-20 mA
    =this way for an instance are done some of the devices of DataForth

    About HART: everything has its own purpose, advantages an disadvantages and fans and users. So HART has its (big) plase in the industry. But it does not mean it is really good for all applications, just for some of them.

    Try building a reliable network of many HART devices over a few square miles and have a lot of traffic on that network :) and be carefull with ground loops :)

    But again anything has its own best applications, so does HART. I would not use it but many guys love it :)

    7th dan instrumentation and controls
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    isa.guru
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 08:26:04 (permalink)
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    ORIGINAL: xiaofan

    By the way, do you know of any commercial off the shelf HART transformer? In my previous job, some gurus designed the HART transformers (for Intrinsic Safety application so they are relatively big).


    No, I do not.

    I have done only 1-2 HART loop programmers long ago and never after that. Used some HART devices but at least because of my applications :) I do not like HART and never really had HART on my devices. So I can't help with that, sorry.

    7th dan instrumentation and controls
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    xiaofan
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    RE: HART, Loop-powered 2-wire instruments 2009/06/06 08:38:06 (permalink)
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    No problem. I also do not develop for HART but recently my colleague asked me about this issue. Typically we will let the HART experts do the HART (HARD grin) job for us in my current job. grin

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    RE: HART, Loop-powered 2-wire instruments 2009/06/08 06:34:10 (permalink)
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    Hi Guru,

    Thanks for elaborating the issue. Also I would like to explain you that I am currently working on 2 projects.

    For magmeter I am working on the firmware but the next thing that I need to take on is a 2-wire Metal Tube Rotameter (MTR) transmitter for which I queried so that by the time my magmeter is ready, I would have the necessary ICs, as you suggested, in hand, to proceed quickly. For Magmeter, I am working on AC supply only and without any HART implementation (luckily grin). I have atleast 3-instruments to work.

    I know HART is not a very good choice but the thing is that in the industry 4-20 mA loop already exists and to make use of it without increasing the cost of the installation, we just need to implement HART for digital data transfer. Also, most of the PLC inputs are HART compatible these days, so you know we are running out of choice.

    Otherwise we have RS-485 as a better option, which is also available in the dsPICDEM board (though I have not yet gone through its usage).

    Thanks Xiaofan and Guru for your classy remarks.

    Best regards, android.
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