Analog Reading from a voltage divider thats is part of a -350V supply

Discussion in 'Analog & Mixed-Signal Design' started by Turdnugget, Oct 29, 2015.

  1. Turdnugget

    Thread Starter New Member

    Mar 3, 2012
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    Attached is a schematic of the the basic configuration for a hypertherm powermax45 plasma cutter Output and its analog voltage signal provided to the interface port for CNC operation.
    In the service manual schematic there is not a indication that the 36khz power transformer secondary is going to ground. The positive voltage of the rectifier is referenced to ground via work clamp to work piece which grounds through the table to earth.

    Im lost on how i should build this part of the circuit with regards to the micro controller ground ref point for the analog reading ... As well as the best approach to filtering it

    CPC-5(-) and CPC-6(+) are the CPC ports pins for the CNC interface.. So these are the signals which i must filter and apply to a micro controller as a analog reading.

    10.JPG
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Where did you get this design? Isn't there a whole community of people doing this type of thing?
     
  3. Turdnugget

    Thread Starter New Member

    Mar 3, 2012
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    I drew this to help simplify the basic idea of what i need help with.. This is from the service manual for a Hypertherm Powermax 45 it is the circuit inside the machine.. If there is a whole community working on plasma CNC electronics please tell me where.. I have seen other open source designs but i would like to make one as well that is a little different.
     
  4. Turdnugget

    Thread Starter New Member

    Mar 3, 2012
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    The way I see it is the micro controller needs to have its own isolated supply and the CPC5(-) connection which is the center node
    of the voltage divider needs to be tied to the micro controllers isolated ground plane. Then CPC-6(+) connection would go to a filter and then a analog pin to be read

    11.JPG
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    That will be just peachy until the first time you forget to unplug the programming cable. What are the "outputs" of the microcontroller tied to?

    You need galvanic isolation between the plasma cutter and the micro...
     
  6. Turdnugget

    Thread Starter New Member

    Mar 3, 2012
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    So then something like,
    ACPL-790B-000E
    Precision Isolation Amplifier
    Applications :

    Current/Voltage Sensing in AC and Servo Motor Drives
    Solar Inverters, Wind Turbine Inverters
    Industrial Process Control
    Data Acquisition Systems
    Switching Power Supply Signal Isolation
    General Purpose Analog Signal Isolation
    Traditional Current Transducer Replacements
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What are the voltage range, min and max frequencies, and wave shape of the signal across the 2K resistor? Analog Devices and others make galvanically isolated signal amplifiers and buffers for this type of application.

    ak
     
  8. PeterCoxSmith

    Member

    Feb 23, 2015
    148
    38
    I've been working on an isolation amplifier that uses the ACPL-C79B to galvanically disconnect the source from the controller. That circuit will be available as a DIN rail module in a couple of months. If you want to design your own circuit the ACPL chip is a good place to start as it has a bandwidth of 200kHz.

    As an alternative you can design a non-isolated circuit based on a differential amplifier like AD8278ARZ. That is just an example, you need to select a component that has the bandwidth you require and can accept the negative common mode voltage.

    You also might like to read this...
    http://forum.allaboutcircuits.com/b...eed-an-amplifier-difference-or-isolation.693/
     
  9. Turdnugget

    Thread Starter New Member

    Mar 3, 2012
    23
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    Thanks MIKEML for pointing that out. That is a good point!

    AnalogKid,
    I hooked the o-scope up to the plasma today and read the output while cutting with it.as well as open circuit.Attached are some of the scope shots showing exactly what I'm dealing with as a signal... The O-scope is a tektronix that has 4 channels with isolted grounds.. and when i was using it it was running on a battery.. So the There is no conflict of scopes ground with the plasma cutters.

    PeterCoxSmith,
    I believe that will be the chip i use!
    When i was doing my scope readings, 1 of the tests I set the scope to AC mode for the channel. Then i turned it to 500mV/DIV. There is about a 18Mhz Ring with a delta 2.5v happening in 6.8uS periods (+- 148khz)

    So This ACPL0C790B chip will actually help act as the filter to the 18mhz ring since it has 200khz bandwidth. If anything the signal should just need to have a RC lowpass filter on the output side of the isolation amplifier.

    The ACPL-C790B
    21.JPG

    I drew up the circuit real quick that seems right to me. 23.JPG

    This scope shot shows the scope Channel in AC mode @500mV /DIV with 100ns/DIV. t shows the Frequency of the ringing from the power supply. The ringing is close to 18MHz. with a deltaV of 2.5v.

    24.jpg

    This Next Scope shot shows the Channel in DC mode.. So the initial higher analog voltage state is the time that the plasma is establishing a conductive path to the work piece being cut.. The ringing is the 18MHz. Take note that the Time Domain is 50mS in this shot
    17.jpg

    This Final Scope shot Shows the frequency of the reoccurring supply ringing which is around 150khz
    26.jpg




    19.jpg 18.jpg I 20.jpg

    Any thoughts or opinions which may help to improve the performance/ stability/ accuracy are welcome :)
     
  10. PeterCoxSmith

    Member

    Feb 23, 2015
    148
    38
    Some thoughts:
    The ACPL-C79B has a max input of 200mV so your 7V needs to be divided down some more.
    And yes I've used a filter to limit my bandwidth to 80kHz.
    Peter.
     
  11. Turdnugget

    Thread Starter New Member

    Mar 3, 2012
    23
    3
    updated and open for suggestions/ constructive criticism/ positive vibes

    30.JPG
     
  12. PeterCoxSmith

    Member

    Feb 23, 2015
    148
    38
    I'd be interested to hear how you get on with this project.
     
  13. Turdnugget

    Thread Starter New Member

    Mar 3, 2012
    23
    3
  14. Turdnugget

    Thread Starter New Member

    Mar 3, 2012
    23
    3
    Here is another interesting chip for analog reading isolation..
    http://www.mouser.com/ds/2/368/Si8900-1-2-9491.pdf


    Its isolation is built into the internal communication port of the circuit. and allows ADC circuit to be on the High voltage side of the circuit

    31.JPG
     
    PeterCoxSmith likes this.
  15. DNA Robotics

    Member

    Jun 13, 2014
    123
    26
    I am also interested. I don't think my Chinese Cut 50 plasma torch has the voltage divider signal. It looks easy enough to add and even trim it to less than 5 volts.

    You might check http://www.cnczone.com for the whole community of people doing this type of thing that GopherT referred to.

    If you start a topic there, please keep us posted or leave a link to it here.
     
  16. Turdnugget

    Thread Starter New Member

    Mar 3, 2012
    23
    3
  17. DNA Robotics

    Member

    Jun 13, 2014
    123
    26
    I am not very happy with my capacitive torch height controller and would like to make one voltage controlled. Did you make any progress?
     
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