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Greetings and a small starter project (ignition coil / Jacobs Ladder)

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Fulmen

Tue May 17 2011, 08:29PM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

Grenadier wrote ...

Also if you have 30kV, 5 watts means there is only .1mA --way too little to make a nice jacob's ladder. I'd say 20W would be the lower limit, and also you won't get that "beeerrrawww" sound with an iggy coil since there is no 60Hz hum: instead you'll likely get a 1-2khz squeal.

The sound won't be that bad, I'm feeding the coil at appr. 150Hz (should allow the coil to charge completely). And while I know it won't be that impressing, it should work. Maybe I can squeeze a few extra watts from it, and perhaps use twin coils to increase output (a 12A FET should handle the extra load).

Ash Small

Wed May 18 2011, 04:16AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

It's good to see someone else building an astable from discretes.

This has to be more robust than an IC. Your dad does have a point.

I was advised to use a TL494 instead, but I'd like to get things going with an astable first, if only from the educational standpoint.

My design involves using power transistors in the astable, and replacing R1 and R4 with the two halves of a centre-tapped push-pull primary.

I'm currently looking into utilizing a ferro-resonant GDT, and hope to post something soon.

I'm following this thread with interest.

Xray

Sat May 21 2011, 01:17AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Fulmen -- People (especially engineers) tend to complicate things. The best design is usually the simplest one (Occam's Razor). If all you want is a high voltage arc to climb up a Jacob's Ladder only one time when your door bell rings (AND to use an electrically safe circuit) then you are on the right track. I'm not so sure that an ignition coil would provide a hot enough arc to provide an impressive looking ladder, but it's worth a try. Keep in mind that it's the HEAT that makes the arc climb the ladder. As the arc heats the surrounding air, the air naturally flows up because it's less dense than the surrounding air, and the arc simply follows it. If your arc is too wimpy, then it will sit at the smallest gap and not move.

Also, (as someone else noted) switching C3 or a combination of caps is a bad idea. Select C3 for the hottest arc, and leave it alone! The best way to switch the arc on and off is simply by cycling the DC power. You can do this with a 555 as a one-shot monostable powering a relay. Set the timing so that the relay stays energized during the time that the arc is running up the ladder.

Also, I would put a cap (about 0.1 uF) across the battery. That should reduce the signal loss across the power rails due to the battery impedance.

Remember.... "KISS" (Keep It Significantly Simple)

Edit: I forgot to mention that the frequency and duty cycle of your multivibrator are very important. That's because ignition coils like to operate at some maximum frequency, and the duty cycle can greatly affect its efficiency. I don't know why you chose those particular values for C1 and C2, but if you want a 50% duty cycle (a symetrical square wave) then both sides of the multivibrator must be balanced with equal resistor and capacitor values. In order to find the ideal frequency for the particular coil, make the frequency variable with a pot, and adjust it for the hottest and longest arc.

Hope this helps.

Ash Small

Sat May 21 2011, 01:48AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Xray wrote ...

The best way to switch the arc on and off is simply by cycling the DC power. You can do this with a 555 as a one-shot monostable powering a relay. Set the timing so that the relay stays energized during the time that the arc is running up the ladder.

Or you could build a monostable from discretes. It all depends on what you feel more comfortable with.

Fulmen

Sat May 21 2011, 08:15AM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

The reason for the asymmetrical signal is to increase duty cycle. If you look at the plot earlier in the thread you'll see that the coil needs about 5ms to attain full current, but the kickback should discharge in far less than 1 ms. I didn't have a suitable pot to tune this, but tests I ran seemed to confirm the simulation. I could probably increase charge time a bit more, but it's hard to fine tune a circuit without a scope.

If the output is too low for a reliable ladder I can simply add another coil ("twin coil" has a nice ring to it anyway). Or double the voltage, although I'm not so sure the coil will like this. I'm planning to order some 600V/12A FET's that should be able to handle such loads.

As for switching C3 being a bad idea, maybe you're right. But I don't understand WHY, and I really like to (learning is a big part of this project, besides I'm not good at just taking others word for something). It's also the only non-mechanical solution to the one-shot function I can think of. I don't want the ladder to just run for a while, I want a single run that shuts down as the arc reaches the top. This might go against the KISS-rule, but "clever" is cooler than "simple"

Dr. ISOTOP

Sat May 21 2011, 06:04PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

magnet18 wrote ...

or if you go really big, an x-ray transformer.

No.
A coil with a 80KV primary voltage is a terrible idea, especially since it is power, not voltage, that largely determines your streamer length.

Fulmen

Mon Dec 12 2011, 04:32PM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

Greetings. I got sidetracked this summer, but now I'm back again. Just finished the coil driver, although the discrete circuit worked and was quite educational, I ended up with a 555 on my finished circuit. Works like a charm, 100V on the primary (I have a scope now), 7mm sparks (appr. 8KV) out. It doesn't provide enough power to run a reliable ladder, so I guess I'll have to find some other use for it. Guess a small Tesla would be fitting.

1323707570 3883 FT115450 555 Coil Driver

Marko

Mon Dec 12 2011, 04:40PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

It doesn't provide enough power to run a reliable ladder, so I guess I'll have to find some other use for it. Guess a small Tesla would be fitting.

Then MOAR power I say!

Inducktion

Mon Dec 12 2011, 04:59PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Fulmen wrote ...

Greetings. I got sidetracked this summer, but now I'm back again. Just finished the coil driver, although the discrete circuit worked and was quite educational, I ended up with a 555 on my finished circuit. Works like a charm, 100V on the primary (I have a scope now), 7mm sparks (appr. 8KV) out. It doesn't provide enough power to run a reliable ladder, so I guess I'll have to find some other use for it. Guess a small Tesla would be fitting.

You could possibly try making a half bridge driver for your ignition coil. They're a little more complicated, though, and would require a decent PWM chip, like a SG3525 or a TL494/KA7500, and a Gate drive transformer. (which, is just a ferrite core transformer with 3 wires wound on it!)

With a half bridge, you can get insane arcs like what Marko/Dr. Pork did with his.

Fulmen

Mon Dec 12 2011, 05:17PM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

Dr. Pork wrote ...

Then MOAR power I say!

WTF are you feeding that thing? Plutonium?

Can you actually get that kind of performance without seriously reducing the life span of the coil? I don't mind stressing the components a bit, but I don't like to design tings that are destined to fail prematurely.

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