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Idea: peizo triggering relay

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Idea: peizo triggering relay

Unread postAuthor: linuxexorcist » Sat Sep 19, 2009 11:55 pm

i had an idea today, in a camera flash-based ignition system, couldn't the relay used to trigger the caps be fired by a peizo? the caps could easily burn out many switches, making a high-voltage relay ideal.
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Re: Idea: peizo triggering relay

Unread postAuthor: Technician1002 » Sun Sep 20, 2009 12:01 am

linuxexorcist wrote:i had an idea today, in a camera flash-based ignition system, couldn't the relay used to trigger the caps be fired by a peizo? the caps could easily burn out many switches, making a high-voltage relay ideal.


I'm a little confused by this post. I understand camera flash ignition. I understand burnt switches from very high inrush. Are you suggesting using a piezo to initiate an arc in a gap the same way Xenon lamps and high pressure sodium lamps are ignited? If so, I think you may have an excellent idea. The initial current may be too low from a lighter piezo to do the job and an inductor will need to be used to decouple the current from the flashcap which would shunt the current.

Let me know if this is to do a high current discharge in the chamber or if this is a high current spark gap switch to power an ignition coil. If the latter a proper SCR will work fine.
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Unread postAuthor: jimmy101 » Sun Sep 20, 2009 10:19 am

Like Tech said. Unclear what you mean. If you mean using the piezo to trigger the gap then the piezo part sounds new but using a small HV spark to trigger a much more powerful spark is pretty common. Indeed that's how the flash tube works. There are two voltage sources. One is 300V and the other "10KV". The 10KV is used to trigger the 300V. Search for "trigatron" for how to replace the flashtube with a spark gap. Could a piezo replace the HV part of the circuit? Perhaps, certainly worth a try. Of course, you are just adding a part to the system that is already there, the board has a HV source on it. The piezo will cycle much faster than the board's HV circuit so that might be useful.

It might be possible to use the piezo and omit the third electrode used in a trigatron setup. Hmmm. Wire all three, piezo, cap and gap in parallel. The piezo's internal resistance is probably high enough (it needs to be at least many megaohms). The problem would be that the energy storage cap would tend to absorb the voltage spike from the piezo. A single HV diode might be enough to isolate the piezo + gap from the big cap.

If you mean use a piezo to power a relay ... I doubt it. Typical BBQ piezos put out somthing like 1mJ. Not nearly enough energy to operate a relay.
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Unread postAuthor: Technician1002 » Sun Sep 20, 2009 11:16 am

jimmy101 wrote: The problem would be that the energy storage cap would tend to absorb the voltage spike from the piezo. A single HV diode might be enough to isolate the piezo + gap from the big cap.


The common way to do this is to add a small hf choke in the DC line from the cap and couple the AC ignition spike on top of the DC to ignite the gap, and the DC surge sustains it. The ignition spike can be either capacitively coupled or inductively coupled into the output DC line. This is how the igniter works for DC high pressure short arc xenon lamps in movie projectors, spotlights, and other high power Xenon fixtures. Paralleling it with the main flash cap will shunt the HV. High voltage high current diodes are expensive and are seldom used for this.
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Unread postAuthor: linuxexorcist » Sun Sep 20, 2009 11:48 pm

it was just a quick question, a thought i had while looking for alternate ignition sources, i have no plans to really go ahead with this,just so you know, don't pull any hair out for me.
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