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Measuring voltage (and frequency) of spark

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petert:
Hello,

I experimented with a piezo crystal from a lighter to generate a high voltage spark. A coherer was able to detect the generated EM wave (from the spark/spark gap) in close proximity.

Now I am trying to generate a spark with a coil that has the same effect. I charge a coil/inductor with a 4.5V battery and when I break the contact or make it, a spark is generated as expected.

How could I measure the spark voltage (and frequency) between the two leads? I am worried it might damage my scope.

Rigolon:
It would probably damage the oscilloscope, a spark (if I remember correctly) of 1mm has 3kV (in air).

Here is a link that I found useful when learning about this effect, it's called Inductor Kickback by the way.


I don't know of a method to actually measured it, but you could at least do some math to calculate approximately.

Zero999:

--- Quote from: petert on December 20, 2019, 01:27:11 pm ---Hello,

I experimented with a piezo crystal from a lighter to generate a high voltage spark. A coherer was able to detect the generated EM wave (from the spark/spark gap) in close proximity.

Now I am trying to generate a spark with a coil that has the same effect. I charge a coil/inductor with a 4.5V battery and when I break the contact or make it, a spark is generated as expected.

How could I measure the spark voltage (and frequency) between the two leads? I am worried it might damage my scope.

--- End quote ---
The voltage depends on the current and the length of arc. Spark gaps have an inverse current/voltage relationship. A kV may be required to initiate the arc, but once it's struck, the current will increase and the electrode voltage will decrease. If nothing limits the current, then there will be magic smoke as the cable and power supply start to overheat.
https://en.wikipedia.org/wiki/Electric_arc#Overview

Sparks produce broadband electromagnetic radiation. The lowest frequency will be dependant on the length of the pulse, with harmonics extending beyond the microwave band towards infrared and visible.

iMo:
The first transmitters/receivers were made of spark gaps and coherers.
The frequency is set by the standard LC tank, or an tuned/resonant antenna (what is the same basically).
So the spark creates a broadband emission, the tuned antenna or LC tank will select the proper frequency out of it and radiate towards the receiver. The receiver should get the same tuned antenna or LC, of course, and the coherer should detect something at that frequency then..

https://en.wikipedia.org/wiki/Spark-gap_transmitter
https://en.wikipedia.org/wiki/Coherer

T3sl4co1l:
You could measure it with an oscilloscope and an inductive probe (say by clipping the probe ground to the tip and holding that in proximity to the coil), or a capacitive probe (holding the probe tip near the coil, without directly contacting it, and with the ground clipped to the battery).

You won't get a calibrated measurement this way -- the vertical scale will be arbitrary, and the inductive probe will sense change in current (which is to say, effectively the voltage) rather than current directly.  But it's linear, so the reading will be proportional to what's actually on the coil, and you'll be able to pick out the frequency from that.

Tim

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