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Registered Member #70
Joined: Thu Feb 09 2006, 08:19AM
Location:
Posts: 43
Hey everyone, last night while playing around with a piezoelectric ignitor and a small camera flash tube I saw something that seemed strange. When I attached both wires to either end of the flash tube a arc through it was generated as expected, fair enough, but when I only connected one wire and left a small gap of a few millimeters between the flash tube and the other wire the arc generated was much brighter. This struck me as odd, since I would have expected it to be dimmer. Does anyone have any ideas on why it does that?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
My guess would be that it has to due with the independence of the tube. It breaks down at like 500v, and the piezo thing is putting out like 10kv, so very little of the energy can be used. When you put the spark gap across the tube it allows a greater voltage to rise up on the tube, and then once the tube is up to a few kv it breaks down. Because the igniter/wires/tube/etc all have a little bit of capacitance and thus store a little energy when charged up to several kv, the discharge across the tube is brighter (as it dissipates that energy)
Registered Member #70
Joined: Thu Feb 09 2006, 08:19AM
Location:
Posts: 43
But if it breaks down at 500v the same amount of energy would still go into the tube, so wouldn't it be, higher voltage, shorter pluse, brighter arc sorta thing?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
That would depend on what kind of source the piezo is. My guess is that it is a constant current (set by how big the crystal is) source, so by (essentially) charging a capacitor up to a few kv then discharging that into the tube you are getting a higher current than if you just discharged the piezo strait into the tube...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, I agree with ... The piezo crystal behaves like a capacitor of several thousand pF that is charged by squeezing it. If you leave an air gap, the capacitor is allowed to charge to a higher voltage before the system fires, so you get a brighter spark.
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Piezoelectric ignitor arc question
