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  1. #1
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    Reading Plasma Voltage

    Looking for a diagram to measure voltage from the torch to the ground. I know I need to drop the voltage to a range of 0.0v-5.0v for a microprocessor.

  2. #2
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    Re: Reading Plasma Voltage

    Plasma voltage is not related to "Ground". Its the Arc Volts between the Workclamp and the Electrode. So if your table is not grounded then your voltage will not be related to ground. Since Workclamp volts are positive in relation to the electrode then your return voltage in ref to ground is usually negative. The circuit its simple just design up a resistor divider network that divides the raw arc volts by 50 and put in some filtering to filter out the PWM noise on most plasma units. It's HOW you connect and use the volts to your uProcessor that is the key. Do it wrong and you might want to use chips in a socket that can be easily replaced.

  3. #3
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    Thank you for the response!

    Should any extra consideration be given if the Plasma Cutter is a High Frequency pilot design?

  4. #4
    Registered jimcolt's Avatar
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    Re: Reading Plasma Voltage

    On most high frequency start plasma cutters the high frequency (in the range of 15,000 volts, 2 MHz on most machines) is active as soon as you trigger the torch to start, and it stops once the plasma cutter senses that a pilot arc has established between the negative electrode and the nozzle. On older technology designs...the high frequency stays on for set time period (usually between 1 and 5 seconds). This high frequency is coupled to either the nozzle or the electrode via an air core transformer....and travels the length of the torch leads to create a high instensity spark between the electrode and nozzle. The gas flow (air) passes through this spark.....which adds energy (heat) bringing the air to is ionization temperature, effectively increasing the conductivity of the air.

    Bottom line: do not attempt to measure the DC voltage between work cable and electrode until the high frequency is inactive. If you plan to use this DC voltage measurement as feedback for torch height control....there is no need to measure it until steady state cutting occurs....with the plasma arc transferred to the metal and the torch height correct, and the cut speed correct for best performance.

    Jim Colt Hypertherm




    Quote Originally Posted by j5fanclub View Post
    Thank you for the response!

    Should any extra consideration be given if the Plasma Cutter is a High Frequency pilot design?

  5. #5
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    Jim, thank you for the fantastic information.

    You mentioned not measuring voltage till the high frequency start has finished and a steady arc has been established. I'm not aware of a method for detecting the state of the arc (starting/cutting) other then sensing the drastic change in voltage. I assume my voltage reader would need to be capable of handling that initial 15,000 volts, is that correct?

    Thanks

  6. #6
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    Can I connect a multimeter to the torch lead port and clamp port on the plasma cutter to read the voltage? Is there a danger to the plasma cutter by connecting a multimeter this way?

  7. #7
    Community Moderator Al_The_Man's Avatar
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    Re: Reading Plasma Voltage

    Do you want to measure the actual voltage or detect if torch is in cutting mode? One way on most plasma tables I have worked on is a Honeywell CSDA1DC is used the Torch supply is fed through the non-invasive detector.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  8. #8
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    Ultimately, I need to continually read the voltage while the cutter is cutting so I can implement a Torch Height Control on my DIY CNC machine. I'm getting much closer to completing a circuit design that will allow this. For now, I'd like to hookup a multimeter to my plasma cutter so I can start to get an idea of the voltages I'll be working with in pilot starting mode and cutting mode.

    Here's a link to a video of a guy using a multimeter on his plasma cutter showing the different voltages during pilot start and arc ok.

    https://m.youtube.com/watch?v=4GIFnPgDezE

  9. #9
    Quote Originally Posted by jimcolt View Post
    On most high frequency start plasma cutters the high frequency (in the range of 15,000 volts, 2 MHz on most machines) is active as soon as you trigger the torch to start, and it stops once the plasma cutter senses that a pilot arc has established between the negative electrode and the nozzle. On older technology designs...the high frequency stays on for set time period (usually between 1 and 5 seconds). This high frequency is coupled to either the nozzle or the electrode via an air core transformer....and travels the length of the torch leads to create a high instensity spark between the electrode and nozzle. The gas flow (air) passes through this spark.....which adds energy (heat) bringing the air to is ionization temperature, effectively increasing the conductivity of the air.

    Bottom line: do not attempt to measure the DC voltage between work cable and electrode until the high frequency is inactive. If you plan to use this DC voltage measurement as feedback for torch height control....there is no need to measure it until steady state cutting occurs....with the plasma arc transferred to the metal and the torch height correct, and the cut speed correct for best performance.

    Jim Colt Hypertherm
    Hi to all
    I am newbie on the plasma CNC world and I know this post is old but I am really desperate.
    Las week I finally finish to integrate my first DIY plasma table (4ftx8ft) everything has been set up. I am using sheetcam and Mach3 full license both with Gecko G540.

    My problem is that I am using a Lotos Ltp5500 plasma NON HF and pilot arc, without voltage divider, so my THC is not reading the voltage when the arc start, I am using raw voltage. With a voltmeter the reading is 119 +/-5V but the THC send error and not read the voltage any suggestion??

    An apologize for my english I am from mexico

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