Troubleshooting a Piezo Lighter: An Unreliable Task

[wushumaster]

So I have the piezo lighter hooked up to a rheostat with one mega ohm resistance, then a 30 kilo ohm resistor and then another resistor of 1 mega ohm. I have an oscilloscope- it's really old though. It's max voltage if you set the axis on the bottom of the screen is about 80 Volts. It's hard to discern the erratic and unreliable peaks I'm getting, and I'm not even sure their right? The other way I tried was to just see how far I could hold the wires apart and see a spark shoot through one to other and apparently around 33 kilo ohms per cm is accurate. But I keep getting varying results here too. It ranges from 1 to 2 cm... I even tried hooking the lighter to a 2-1 transformer to see if I put wires from the transformer half way apart than without the transformers, I would have been able to see a spark, but I did not.
Ideas, anything, any help- would all me most wonderful. I'm pretty stuck and I need a quantitative reading...
Thanks so much!

 

[jsgruszynski]

The output of a piezo element probably really looks as erratic as you are seeing. Strictly the way to be sure (but you may not have the equipment) is to use a single-shot digital scope instead. Then you can capture a single event entirely and then compare success events.

Also if you were measuring with laboratory equipment, you'd first do a slow voltage/mechanic force ramp to measure the transfer characteristic in displacement/voltage. A typical piezo device's the transfer function is pretty much linear up to high compression and voltages.

This is also how you should measure dielectric breakdown. In semiconductors you typically get breakdown voltage using a "Vramp" test - basically does a ramp at varying rates until a breakdown criteria is met. There's a JEDEC 35 standard that probably could be applied to a piezo dielectric breakdown. If you are doing this commercially, my company sells measurement systems for this kind of test.

Once you confirm the transfer characteristic is linear (which is likely), then you know for certain the problem is mechanical - probably the striker is squirrelly and unreliable in the force or displacement it's applying.

Mechanical features are always the reliability demon of electromechanical parts.

 

[wushumaster]

Is there any way your company can loan one to me or rent it out somewhere in the Denver Metro area for perhaps one time use? I'm in high school and I'm doing a science fair project. It would be extremely valuable- thank you for your help!

 

[vk6kro]

An airgap will arc if the voltage across the gap exceeds 30000 volts per centimeter of gap.

So, it sounds like you have about 50000 volts.

So, whatever else you do, do not connect this to an oscilloscope. There is a switch in there with some very hard-to-replace resistors and you don't need to damage these with high voltages.
And if that voltage gets to the vertical amplifier, you will have some serious problems.

You really need a high voltage probe for this. It would have to drop 50000 volts down to about 50 volts so that you can observe the waveform.

These are expensive but you may find something on Ebay.