Detecting an Arc/Short Circuit

Discussion in 'General Electronics Chat' started by RaoulDuke, Apr 12, 2008.

  1. RaoulDuke

    Thread Starter New Member

    Apr 11, 2008
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    I need a circuit that detects a "rate of change" in a voltage signal, rather than just a change in voltage like a comparator does.

    I need to detect Arcs in a pulsed power application, to initiate a crowbar to dump the energy before damage is done. (pulses up to 500kW)

    The output pulses range from 100V to 1kV, 50usec to 2 msec.

    The pulses also have a ripple that is necessary for the application, and the riple can be as much as +/- 500V and range from 20kHz to 50kHz.

    And arc condition will result in a voltage change of about 100V/1nsec.

    The circuit needs to respond within 1usec.

    I have a fast, clean voltage signal that is 0-10V, representing 0-1200V.

    I also have a current signal, but it is too slow to be of use.

    Anybody got any ideas of how to make a circuit that triggers on a "rate of change" in voltage rather than just a voltage level?
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Would a differentiator (small cap in series, resistor to ground) help? Can also be designed with an op-amp. Your microsecond response might pose a problem, though. I have used the former passive circuit to fire an SCR on an edge. John
     
  3. RaoulDuke

    Thread Starter New Member

    Apr 11, 2008
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    Thanks for a start......now I will go and learn what a differentiator is!;)

    I am an Engineer by title only.....never finished college. What I do know I learned from 30 years of teaching myself. (And some good, basic electronics training in the Army after Vietnam.)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Thank you for your Service, and Welcome Home. http://nwmvocf.org

    Have a look at the attached.

    The upper circuit is a differentiator; basically, it outputs the edges of the input waveform.

    The lower circuit is an integrator; it smooths the transitions between the peak input voltages.
     
  5. RaoulDuke

    Thread Starter New Member

    Apr 11, 2008
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    Thanks SgtWookie.....and thanks for your service as well!

    I will look at this later.....just got back from fishing the Cape Cod Canal today and the bats are flying all around me!:D
     
  6. RaoulDuke

    Thread Starter New Member

    Apr 11, 2008
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    Memo to: jpanhalt and SgtWookie
    Subject: After Action Report

    I want to thank you both for the excellent advice!

    I came home yesterday after fishing in Cape Cod Canal, my brain muddied by many beers and the intoxication of a warm spring day. (And a nice hunk of striped bass for dinner!)

    I sat down last night with a pot of coffee and decided to relearn just what a differentiator is, because my feeble old brain could only recall hearing that term back in 1968 when I was in electronics school at Fort Monmouth New Jersey.

    And then I had an epiphany! I saw it with a clarity that usually only happens when I am doing Peyote!:D

    DETECTING A "RATE OF CHANGE" IS THE SAME AS DETECTING FREQUENCY!

    So I went into work today.....grabbed my oscilloscope, an handfull of parts and a soldering iron and headed for the lab.

    I started with a 270pf/3kV capacitor and a 100ohm resistor. I connected them right across the output to the power supply directly into the chamber.

    I then connected a floating scope across the resistor and starting pumping down the chamber.

    I started pulsing with pulses of 600V and 600A. Everything was going good, the scope had no more than about 20mv of ripple.

    Then I started cranking up the Nitrogen till I started getting arcs!

    And lo and behold!.......every time I got an arc in the plasma, I got a spike on the scope of over 2 volts!

    I couldn't believe it was this simple.......I grabbed a ladder and put the scope up to the quartz window of the chamber so I could watch the scope and the flashes at the same time!

    It freaking works!

    Now it is just a matter of tweaking values....in a few days I should have the perfect Arc Suppression circuit!

    If you guy's ever get to Massachusetts.....I owe you a beer, at the least!
    :D:D:D
     
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    That was very nice to hear and certainly made my day a little brighter. Please let us know the final results. John
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This is good tidings, indeed. :D
    Loved the A.A.R. - also heard from R.D. offlist.
    His boss was extremely enthused about the initial results achieved, to say the least. ;)

    Success is good. :D
     
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