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
5,692
901
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
18
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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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1,728

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.

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5. RaoulDuke Thread Starter New Member

Apr 11, 2008
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I will look at this later.....just got back from fishing the Cape Cod Canal today and the bats are flying all around me!

6. RaoulDuke Thread Starter New Member

Apr 11, 2008
18
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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!

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!

7. jpanhalt AAC Fanatic!

Jan 18, 2008
5,692
901
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.
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.