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Experiments with piezos

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Madgyver

Tue Apr 04 2006, 07:16PM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

Oh I see... I was wondering myself.

DrZoidberg

Wed Apr 05 2006, 08:10PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

The piezos I ordered finally arrived. I measured the capacitance and got only 30 to 40pF which means the data given on the website is probably wrong and the piezos are not multilayered. Thats good.
I then tried to charge one of them with a 10kV supply and then discharged it again. It worked and the discharge spark also looked like I would expect it from a 30pF cap charged to 10kV.
Next I will try to build some kind of transformer out of them.

They are very fragile by the way. How do you hit one hard enough to produce a spark without breaking them?

Madgyver

Wed Apr 05 2006, 08:21PM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

You want something like aluminium or copper on top of them. That will evenly distribute the shock. Hit them fast but not hard, with something like a Spring or a Softairgun. Coilgun maybe?

DrZoidberg

Fri Apr 07 2006, 08:24PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
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Posts: 106

I doubt that speed is the key.
I did another experiment. I put one crystal in a vice and used some aluminum foil as electrodes. I got small sparks out of it that way. Around 1kV. I had difficulties measuring the applied force but I guess it was at least 100kg.
btw. I wouldn't recommend soldering these piezos. If they get heated too much they could loose their piezo properties. Also of course you can't measure the voltage with a multimeter since the charge is so small, it will have discharged through the multimeter before it even displays a result.

Madgyver

Fri Apr 07 2006, 09:45PM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

Mhh..so if not by speed, how to they get such nasty sparks out of those grill igniters? They don't have much force applied to them I think.

Fri Apr 07 2006, 09:52PM

Registered Member #256 Joined:
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Posts: 124

oo yes bbq sparkers, my favorite easy to get HV toy!! someone should cut one in half lengthwise and see what it looks like inside :|?

vasil

Fri Apr 07 2006, 10:22PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Inside the grill ignitor is an iron frame, which contains a piezo cilinder. The cilinder has metal plates collectors at both ends and is pre- tensionated by applying a force with a screw, so just with a lower force applyed on the button, the spark is obtained. The big igniters have two piezo cilinder in parallel/series configuration for brighter spark.

Sat Apr 08 2006, 04:48AM

Registered Member #256 Joined:
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Posts: 124

i wonder if it would be possible to parallel these little piezo ignitors to make a more intense spark

DrZoidberg

Sat Apr 08 2006, 09:56AM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

I now tried to build a piezo transformer.
I glued 2 piezos together with some aluminum foil in between both and some at each end and put it in a vice.
Then I connected a 555 oscillator to one of the crystals. The other crystal was connected to a bridge rectifier made from uf4007 diodes and a 1.5 nF capacitor. To find the resonance frequency I kept changing the frequency until I got the maximum output voltage at around 58 kHz.
I got around 2 V out. The output current was 15 uA.
Input voltage was 8V and the input current was 40uA.

I think the main reason why I got much less out then I put in was because of the glue between the end of the crystals and the aluminum foil. It's only a thin layer of glue but it's dielectric constant is much smaller then that of the piezo so a big portion of the voltage drops across the layer of glue instead of the piezo.
I guess to step up the voltage I will need to use 2 different piezo crystals. A piezo speaker for the low voltage side and one of the big crystals for the high voltage side.

edit:
tried it again without using glue.
Input current: 60 uA
Output current: 40 uA
Output Voltage 3.5 V

Bjørn

Sat Apr 08 2006, 10:51AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If driven at resonance a piezoelectric transformer can be very efficient. I saw someone selling some very expensive ones that was made up of bending plates. They also had generators.

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