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question re: stepping up ignition coils

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ragnar

Sun Sept 23 2007, 03:02PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Thrival, I think the theory being preached here are pearls being cast... and probably misunderstood as criticism.

By all means give it a try, but to "step up" the voltage of an ignition coil is a counterintuitive strategy (according to all experience) if what you want is long arcs.

Have you tried two ignition coils out of phase? How about driving the coils harder with a proper transistor like an MJ10012 at 48V.

A voltage doubler is something you might seriously consider -- that's about the only solution I've seen proposed here which would result in more (not less) volts appearing anywhere in the circuit.

uzzors2k

Sun Sept 23 2007, 03:22PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I'm not sure if you've understood what Sulaiman said. A 3 turn primary being run with many thousand volts at audio frequencies would saturate immediately. It wouldn't matter how many secondary turns you have because all the power would be lost. Basically transformers don't work when saturated that violently. There's a reason HV transformers have many primary windings, and even more secondary windings. If we didn't have to worry about saturation, every HV transformer would have a 1 turn primary and a million secondary windings.

To do what you want would take a huge core, many primary windings, and even more secondary windings. It's not worth it.

thrival

Sun Sept 23 2007, 04:11PM

Registered Member #1019 Joined: Sat Sept 22 2007, 02:39AM
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Posts: 29

Matt:

I hope you're not calling me swine, because I didn't take most of the posts so far as critical,
just a bit distracting

...anyway, please help me understand why stepping up HV
to even higher HV is "counterintuitive," given that higher HV arcs further than lower HV?
I'm trying to use existing hardware, 12V car battery. Stressing ignition coils beyond their
ratings just destroys them. I need an add-on. CW multipliers use AC and higher frequency
cycles just to stay charged, again stressing the coil.

Uzzors:

Thank you for getting to the heart of the matter and my ignorance. I found the relevant
formula in the 2007 ARRL Handbook and see what you say is true, but confused why in
this case, higher frequencies actually help lower the flux density; --weird.

OK, so what about air cores? or dielectric core? Could I achieve my goal using a resin-mica
core? The Radio Handbook air-core formulas don't use V as an operator.

uzzors2k

Sun Sept 23 2007, 04:56PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Higher frequencies decrease the time voltage is applied in each direction, which keeps the average flux density down, preventing saturation. So high frequencies and air cores are what you need, and this is where SSTCs come in. If you don't want to make a SSTC, try flyback transformers, double ignition coils like Matt suggested or various voltage multipliers. However for 250 - 300 kV like you mentioned a Tesla coil would be necessary.

EDIT: for long arcs try the Mazzilli flyback driver, it can run from 12V and give nice firey arcs. It's very simple too.

thrival

Sun Sept 23 2007, 09:54PM

Registered Member #1019 Joined: Sat Sept 22 2007, 02:39AM
Location:
Posts: 29

What about using a MOT @ 12V for a flyback to drive an ignition coil; the heater coil looks
like it could work for feedback. Has anyone tried this?

Steve Conner

Sun Sept 23 2007, 11:03PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Not that I know of. Why don't you try it? It sounds like it might work.

thrival

Wed Oct 03 2007, 06:01AM

Registered Member #1019 Joined: Sat Sept 22 2007, 02:39AM
Location:
Posts: 29

OK, I've spent the last week studying oscillators. I've no doubt I can build a flyback or
RC relaxation oscillator to drive an ignition coil, above the rated input. But that will still
stress the coil even assuming it can hold up to the higher HV.

I understand why the toroid was a bad idea, but can someone please help me get a handle
on why I can't step the ignition coil output up with an add-on autotransformer of relatively
few turns? (0 - 10 primary, 20 - 50 secondary) and within a relatively confined space.
Assume I can space the turns sufficiently and pot in mica-resin, to prevent internal arcing,
air-core and not too high a frequency (0-20khz.) I've found evidence it should at least be
possible. See below links. This company has not responded to my requests for more product
info.

and

Dr. Slack

Wed Oct 03 2007, 07:12AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

but can someone please help me get a handle
on why I can't step the ignition coil output up with an add-on autotransformer of relatively
few turns?

The output of the ignition coil has got lots of volts on it.
That means your following transformer has to support lots of volts (or it will saturate)
As volts is proportional to frequency * turns * flux, this means it needs either
a) lots (and lots) of turns (like an ignition coil)
b) lots of volts per turn (which means very large core area like a substation tranny)
c) or a very high frequency of operation (like a Tesla coil)
d) or better still all three!

As you have already specified relatively few turns, which trashes a above, and compact, which does for b, and a low frequency, which rules out c, you don't really have anywhere to turn. As the engineer Scotty was fond of pointing out to Captain Kirk in Startrek 1, "ye canna change the laws of Physics, cap'n". Once you have at least one of those, you can tradeoff the others, but you need to start with at least one.

If your following transformer does not have an adequate the freuqency*area*turns product to withstand the input voltage, it will saturate. This means it stops acting as a text-book transformer stepping up by N, and starts behaving as a voltage source in its own right, using its stored energy to generate a few volts in its output turns, according to flyback-like equations. With 50 output turns and a compact core without an airgap (you did design it as a transformer, so not much stored energy), it won't be producing 10s of kV. An ignition coil is a lot better and more convenient flyback than anything compact you might build with 50 turns.

Your only hope for a compact, low frequency, few turns transformer is the discovery of a new core material Unobtanium, which has a permeability of 10^7, and can operate with low loss at 20kHz. The entire electricity industry will beat a path to the door of anyone who finds that.

thrival

Wed Oct 03 2007, 01:55PM

Registered Member #1019 Joined: Sat Sept 22 2007, 02:39AM
Location:
Posts: 29

Neil:

Thank you. But to avoid missunderstanding, would all
you said apply to air-core / potted auto-transformers?
Reason I ask is that chinese company links do make
it appear that 'unusual' parameters are still possible.

Dr. Slack

Wed Oct 03 2007, 03:27PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Apologies, I was a little sloppy in my last post.

Volts is proportional to turns * frequency * core area * flux density, that's the physics that you canna get away from.

Air core does several good things for you, it doesn't saturate, and has no losses even at very high frequencies, keeps interwinding capactice low, and is a fairly good insulator. On the down side, it has a relative permeability of only 1 (comapred to ~100 of ferrite and ~1000 of iron), so you get very little flux for your current, and have to use it in resonance (to circulate a decent primary current), or at very high frequencies (so you don't need many turns), to be useful.

Chinese physics uses the same laws are here, so they need to be doing something that complies. perhaps you could post the link of the site?

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