New dumb question(got a million of them)

Discussion in 'General Electronics Chat' started by shortbus, Mar 21, 2015.

  1. shortbus

    Thread Starter AAC Fanatic!

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    Wanting to measure the voltage going through a spark gap. Measuring it on the input side only tells me that, the input. Even if the spark isn't jumping the gap, it will show the voltage feeding the gap, not useful for my project.

    I did a lot of reading, and found a college thesis showing how the author did it. Will it work doing it this way? Putting a diode of the proper voltage and amperage value between ground electrode and actual ground, the measuring at junction of diode and ground electrode. Thesis claims it gives gap voltage one diode drop above ground, what do you think?

    Sorry but forum won't let me upload my schematic?
     
  2. Hypatia's Protege

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    AFAIK you should be able to upload an image -- please describe the error message you are receiving --- Speaking for myself a schematic would be very helpful!:D

    Best regards
    HP
     
  3. djsfantasi

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    Having trouble attaching files or inserting an image? What is the forum telling you? I have a blog entry for inserting images into posts. See if it helps
     
  4. shortbus

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    No error message just won't open my files on my computer, when I click on "upload a file". Computer will open locally but not from forum?
     
  5. shortbus

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    That's what I've always done. Firefox put a new download on my browser today that may have something to do with it. will try again later.
     
  6. djsfantasi

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    So what happens next when you click on "upload a file"? Nothing at all?

    What computer are you using?

    If you go to this link, do you see a bouncing cube animation?
     
  7. bwilliams60

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    Try IE or Chrome.
     
  8. shortbus

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    Finally got it to load, had to restart my computer. Firefox doesn't ask any more, they just download a new version. This is second one this week!

    edm gap.jpg
     
  9. ronv

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  10. LDC3

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    If you measure with reference to ground, I believe that you would only get the diode drop. Although it would tell you when the spark jumped the gap.
     
  11. Glenn Holland

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    Dec 26, 2014
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    You can measure the voltage across the gap with -or without- a spark being present.

    I presume the voltage would be quite high and derived from the terminals of an ignition power supply where you easily take the measurement.
     
  12. AnalogKid

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    First, semantics. You can't measure the "voltage going through" a spark gap. Voltage doesn't go through things, current does. You can measure the voltage across a spark gap or the current through it.

    Before the spark ignites, the terminal voltage will be high and easy to measure. When the spark is happening, the terminal voltage drops to almost zero because ionized air is an excellent conductor. BTW, the same is true with Tasers - they don't make 60 kV when in contact with a body, only before.

    ak
     
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  13. Hypatia's Protege

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    1) While an arc is active the signal at the indicated test point (with respect to ground) will present as noise having a (maximum) amplitude equal to the diode's bias offset.

    2) As a practical matter (i.e. ignoring effects corollary to corona, incident ionizing radiation, etc... ) there will be no signal in the absence of an arc.

    Please be advised that, from a practical standpoint, a 100v arc will be very difficult to realize under STP conditions... For a proper (i.e. quantitative) treatment of the phenomenon, I suggest a study of Paschen's (sp?) law...

    Best regards
    HP
     
    Last edited: Mar 21, 2015
  14. Brownout

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    Jan 10, 2012
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    I think you may have misunderstood what "gap" voltage the paper was referring to. The only thing you will measure with this setup is the bandgap voltage of the diode, and will have nothing at all to do with the spark gap. Maybe a more detailed explanation of your objective will help.
     
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  15. GopherT

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    Johann likes this.
  16. Hypatia's Protege

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    Re: post #13

    The echos! The echos!! ;);)
     
    Last edited: Mar 22, 2015
  17. shortbus

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  18. shortbus

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    The Paschen's law doesn't really apply in EDM. The gap is only ~0.001 inch and it is in oil. The electrodes form a basic capacitance that will break down when the electric field reaches a certain intensity, the break down is somewhat due to the microscopic carbon and metal contaminates in it. And even using Paschen's, 0.001 inch is pretty close to what that law states.

    They do this in industry with as little as 50VDC, even though no one on the forums will believe me. What I'm building is actually 95VDC @ 15Amps. After the arc starts the current and gap voltage drops to 35 to 25VDC. Depending on the electrode size.
     
  19. ronv

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    I've only seen one run a couple of times. Never realized the current was so high. How long is the pulse?
     
  20. shortbus

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    I want to use the voltage on the ground side of the gap to start a timer, to control the length of the sparking time. If I measure the circuit voltage at the other side of the electrode, That won't tell me when the sparking starts. Just the voltage level. The voltage level is measured there also, with a window comparator as feedback to the servo that controls the electrode gap size. The servo voltage is adjustable for gap working conditions from 35 to 25VDC.

    It's kind of a catch 22, I could build a circuit and measure the drop as to when the voltage jumps the gap. But to build the circuit and have it work, the voltage needs to be known. I thought I finally caught a break finding that thesis with their circuit, that they claim worked, but guess not.

    I know that it is done in industrial EDM machines. Even when I was running these machines there were no repair manuals in the plant to show the circuits. And the outside vendor techs would never let me see them, top secret stuff. I keep looking on Ebay for them but when they do come up they are in too high a price for me, and they don't come up that often.
     
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