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Spark gap questions
Posted by Jim on 6/30/2004, 1:54 pm My understanding is as follows: The capacitor charges until the voltage reaches the breakdown voltage of the air gap. Then, the capacitor discharges through the resistor, the current jumps across the air gap switch, and then across the underwater gap (after bubbles are formed and an electrode avalanche occurs). I am using unfiltered tap water in this application. I am interested in knowing how the current behaves in the circuit in general, and across the spark gaps in particular. Once the capacitor voltage reaches the breakdown voltage, do the two spark gaps simply act as two more resistors, limiting the current throughout the entire circuit? Or is the current through the spark gaps different than that through the resistor? Using high-speed camerawork, I have noticed a millisecond-scale period of bubble formation and movement across the underwater gap before the underwater discharge occurs. Does the capacitor "wait" to discharge until the gap is prepared for breakdown? Or do the bubble phenomena occur after capacitor discharge, when the current has already reached the electrode surface? I wish I knew more about electricity. I only hope my questions make enough sense that someone can help me. Thanks for your time!
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Re: Spark gap questions
Posted by Bert Hickman on 7/7/2004, 11:25 pm, in reply to "Spark gap questions" Hello Jim, Interesting question! This sounds similar to the way a lithotrypsy (kidney/bladder stone buster) operates. The air gap is sometimes called a "sharpening gap", since it creates a sharply rising voltage to appear across the water gap. Before the air gap breaks down, the water gap looks like a small capacitor in parallel with a high valued resistor, and it has no voltage cross it. Once the air gap breaks down, the voltage drop across the air gap drops to 100-200 volts, and virtually all of the capacitor's voltage now rapidly appears across the water gap (since it hasn't fired yet, and thus isn't very conductive). A short time later, the water in the gap begins to break down under the high voltage stress. When this occurs, current begins to flow from the capacitor though the series resistor, and a gas bubble is formed between the water gap's electrodes as breakdown streamers bridge the gap and a high current arc channel rapidly forms. The breakdown of the water may occur in less than 10 billionths of a second (10 nanoseconds). Once the water gap fires, the voltage drop across the water gap also drops to 100-200 volts, and all of the remaining capacitor voltage now appears across the series resistor. The discharge in the water begins to occur when the capacitor voltage is near the value it was immediately prior to when the air gap gap begins to fire. As the water gap breaks down, energy is transfered from the capacitor, allowingit to form a hot, highly conductive plasma channel inside the bubble that forms between the electrodes in the water. However, once both gaps are fully firing, most of the remaining energy in the capacitor will be expended in the series resistor. Hope this helped, and best regards, -- Bert --
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Re: Spark gap questions
Posted by Jim on 7/8/2004, 12:57 pm, in reply to "Re: Spark gap questions" Thank you so much! This is exactly the type of information I was looking for. You've helped me quite a bit. I do have a couple of questions for clarification, but if you don't mind I might just email you to avoid cluttering up your board with too much off-topic stuff. Thanks again,
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