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Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
I always ment to saw apart a doorknob to see how it is constructed, now you save me the trouble! Could you smash the remaining bigger piece a bit further to see how it is constructed internally? I would not be surprised if there were no actual plates, but the metal was sputtered directly onto the ceramic substrate.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
and so I smashed it. The construction is pretty simple actually. Just a ceramic shell, picture it as two coffee mug glued back to back. the both "inner surface of the mugs" are coated with some conductive material(plates). Looks exactly like the surface of those piezo speaker. then a spring loaded electrode clip is press fitted. but before that, a thick piece of wax paper is put in to protect the plates.
The first pic is one electrode with the waxed paper removed, the bottom of the HexNut is pretty sharp. thats why I am guessing the waxed paper is for protection/ or corona?
It arced thru the dielectric, as you can see in the pic.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
I wonder if there are any fun things one could do with the strontium titanate or wahtever it is, now that you have got it out of there. I suppose it could be piezoelectric or even pyroelectric. Be careful grinding it up though, strontium is possibliy toxic I think. I am surprised how low the dielectic strength is, 1kV / mm is worse than air, I would have guessed it is at least 10 times that! I think I need to do some stress testing with some of my doorknobs.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
It was my bad, lol, and according to Wiki, they have a dielectric strength of 8Mv/M, so 5mm is 40kV.
I was running them way over their spec, continuously in a mini SGTC. My old SG was badly designed, and this cap actually sparked over the surface a few times. you can probebly see the trace on the first pic.
I sanded it down a little bit, put ~5 layers scotch tape around it, and it works perfectly again.
Until I decided to use my ZVS driven flyback instead of a wimpy 12W 5kv SS NST. They get HOT within minutes. I was thinking about putting them under oil, but I guess its too late now.
One thing I don't understand is that, when i try to calculate the capacitance of it, using the dimentions. I don't know how to do the cylindrical part, so I only calculated the area of the parallel circular plates. If C was indeed 1000pF, I would need ~4000 dielectric constant, which is nothin close to 1200 as stated on Wiki. Can anybody Help me out? My Phy and Calc are not that good.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Yeah, I think these ceramics are tough little critters, capable of standing much more voltage that what's on the label. Regarding the capacitance, the parallel plate formula ignore any part of the electric field that is "buldging out" between the plates, which might be a very significant amount for these small plates. So I think you are within a realistic range.
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Ceramic doorknob caps construction
It was my bad, lol, and according to Wiki, they have a dielectric strength of 8Mv/M,