Re: spark plugs

What a terribly unscientific explanation. I'm aghast, quite frankly.

Indeed. Could easily be done with a Raspberry Pi or an Arduino even that matter I would imagine.

Cursitor Doom doesn't want science, and doesn't understand it even when his nose gets rubbed in it. You can expose a horses arse to education but you can't make it think.

That's strange, an arc lamp is low voltage, high current. The ones I have seen needs the points to touch for an instant, then separate, either automatically or manually. During use, the points can be at a distance of about one centimetre.

Using a a high voltage spark to initiate the thing? I don't know if that is possible, I have not seen it.

In the older style transformer-only arc welders, the arc has to be struck manually by momentarily shorting out a low voltage/high current secondary winding; typically 50 to 80V. This can be a messy and frustrating process at times, even for a skilled tradesman. Newer style inverter welders use HF AC for a 'soft start' which is much cleaner and faster. I've not heard of a high voltage start method either.

It's how Xenon flash lamps are triggered. The big capacitor is charged less than 1 KVolt, which isn't enough for such a long gap (the length of the fused quartz flash tube). The trigger wire is wrapped around the flash tube and when pulsed with 10 or 20 KV, causes a conductive path to form in the Xenon gas (about 0.1 Bar) within, and boom!

Joe Gwinn

The arcs I handled were the lantern of cinema projectors, and they used DC, of about 30 volts when running. With DC, the positive electrode wears significantly faster (electrons hitting it, was the explanation we got), so it is thicker and longer than the negative to compensate. The positive also develops a cavity on the tip, while the negative sharpens. The most light goes back in the direction of the negative, where an hyperbolic mirror projected it forward towards the film and the lenses. A tick metal plate with a lever was used instead of a switch, to allow the light to pass or not.

Unless I got positive and negative reversed, this was a bunch of years ago :-)

They used arc lights because not only they were very intense and white, but the light came from a tiny spot, which is ideal for focusing. But they were temperamental. Ours were adjusted with an electric motor with a lot of speed reduction, plus manual adjustments. The projectionist had to be watching the thing almost constantly; a bit of arc light was projected with a tiny periscope and mirror on a piece of cardboard, so we could see the shape of the electrodes, distance, position.

The modern counterpart is a xenon arc lamp. It doesn't wear out, I understand, but has to be replaced after a number of hours (10000?).

The high pressure xenon arc lamps I built my power supply to drive were being used as UV light sources in the Photochemistry group at Southampton University.

It took about 20kV to drive a spark between the electrodes, which dumped enough xenon ions and electrons into the gap to sustain a glow discharge for the few microsceconds it took for the tungsten electrode surface to warm up enough to sustain an 24A arc at about 20v or so.

They ran hot and were filled with a couple of atmospheres of xenon gas, and relied on silica glass to keep the gas inside and the tungsten electrodes spaced apart. Moving the electrodes wasn't an option. Providing a third - trigger - electrode to allow easier starting was possible, but not popular.

I set up a stable constant current so the lamp provided a more or less stable output - turbulent convection currents in the xenon gas meant that it wasn't all that stable in the short term.

Ah, ok, yes.

But there is no conductivity between the high voltage and low voltage parts.

Gas lasers and CFLs are started by a HV transient on the terminals.

Cheers

Phil Hobbs

As can the flash lamp - it was sometimes done that way.

The advantage of the capacitive coupling (through the fused quartz lamp tube wall) is that the 20 KVolt pulse does not appear on the 1 KV terminal or circuit, a simplification.

Joe Gwinn

Series trigger transformer?

Krytrons have a seperate trig terminal and a keep-alive electrode.

It certainly simplifies things. The high voltage spike can destroy the lower voltage parts. The design would have to cope with that.

Nah, just a high (inductive) impedance, like a magneto.

Cheers

Phil Hobbs

Interesting.

The tungsten wears out?

Big flashlamps are usually run in simmer mode. A small, constant anode current greatly reduces both electrode wear and trigger jitter.

Cheers

Phil Hobbs

He uses as much of it as he needs.

The process of initiating an arc starts with a single charge carrier - usually an electron - showing up in the gap to start the process. Cosmic rays will do it, or potassium-40 nearby. That happens at random.

It gets a lot simpler in practice. The questions tend to be - does what we have work at all? And if the answer is yes, how do we change it to make it work better?