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half bridge ignition coil driver - power your SGTC cheaply.

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LAN_403

Marko

Sat Oct 01 2011, 11:36PM

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

Helloes everyone

Very often beginner tesla coilers tend to use ignition coils as their power supply.
Yet for some reason I've never seen anything more advanced than a NE555 and a 2N3055 or a mosfet driving it in flyback mode. This works to a point and tends to produce very high voltage pulses, but overall power throughput and efficiency remains quite low.
It's also normally not possible to run these from mains without a step down transformer.

So I started wondering whether something could be done to make iggies more useful as power supplies without overly complicated electronics.
I wanted to see if it could be used more as a normal transformer rather than in flyback mode, in hope of seeing some higher output current and lower voltage.

I wondered whether it would work to use a half bridge of IRFP450's with line voltage supply (ok, with a variac in between for start).
I also wanted to avoid the use of troublesome and expensive high side driver chips. Instead I used small iron audio transformers, one from a telephone and another from a modem in my case. Small ones from dial up modems would also be great.

I used TL494 with some bjt totem pole output amplification - something that has been used so many times over and over here that I'm sure everyone knows how to use it. The only important difference from tipycal flyback and whatever drivers is that the DC blocking cap used before the gdt's has to have a lot more capacitance than usual, to support the magnetizing current at such low frequencies. I used a 33uF bipolar lityc here, and 100 ohm gate resistors on the mosfets.

Results - didn't go to full voltage yet, but runs at 150V or so pegged the 5 amp meter and produced some fat, MOT-like arcs, and ignition coil heating up fast.

I would like to see if someone could dare to use one like this to power a sgtc. Ideally, the output should be fullwave rectified before being fed into the coil.

Video first, more after interest shows up.

Patrick

Sun Oct 02 2011, 12:08AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

It has a wickedly cool sound!!!

Dr. Dark Current

Sun Oct 02 2011, 08:05AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Yes the coil will heat up quickly, as you found out. For this reason they are better for pulsed power. A good project for a beginner is a "dimmer circuit" (triac phase angle controller) with a capacitor in series, as it produces several cm long sparks from the coil and contains just a few components.

Marko

Sun Oct 02 2011, 03:30PM

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

Hi guys

Well, yeah, the coil does heat up somewhat faster than I expected. Plenty of forced air cooling would be in order for continuous operation, and no more than 500W or so per coil. I think it should be fairly easy for most people to get a plenty of iggies for cheap.

One thing that makes me worried is the aluminum case of the coil. To me it looks like it might be suffering a lot of induction heating with the coil ran this way.

Unpotting the coils and running them with circulating oil supply would be a next step probably if anyone wants to push them to real extreme.

Marko

Patrick

Sun Oct 02 2011, 04:17PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Do you now how much real power your getting to the arc?

Marko

Sun Oct 02 2011, 09:24PM

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

Patrick wrote ...

Do you now how much real power your getting to the arc?

Hi,

I don't really have a clue since my wattmeter and most other required instruments are fuxored. From the size of the arc (compared to my OBIT's) I estimate it must be well over 500W.

The arcs do not start at great length but carry a hefty amount of current, as I hoped would be more suitable for use as a power supply.

Marko

Marko

Proud Mary

Sun Oct 02 2011, 11:15PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Here is one of the best analyses of ignition coil performance - and how it can be enhanced - that I know of:

G. J. Rohwein L.S. Camilli Automotive Ignition Transfer Efficiency Society of Automotive Engineers 2002

Marko

Tue Oct 04 2011, 10:43PM

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

Hey guys...

Hm, that is a nice paper, though I'm not sure how much does the ignition system efficiency have to do with efficiency of punishing the hell out of the coil with 150V square wave.

I was wondering, of all of you who experimented with ignition coils, how much power could I expect to push continuously and for short durations? 500W to 1kW per coil would be nice, but it heats up rather fast.

I'm wondering whether it would make sense to have a fan blow over the case or better just un-pot the coil alltogether. That would be messy though and I only have vegetable oil to replace the (likely PCB) internal oil. It might be easier to run a water supply around the coils for cooling!

Marko

Dr. Dark Current

Wed Oct 05 2011, 01:02PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Maybe you could try driving the coil at resonance, this could increase the efficiency but you must not let the output unloaded.

Marko

Wed Oct 12 2011, 10:17PM

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

Hi kilovolt,

Well I've upped the frequency to about 1.5kHz and the efficiency improved a lot, I went up to full line voltage and got some fat 8-9 cm arcs, and one fan blowing against the coil kept it well from overheating. It looks like I might have been saturating the iron core at the low frequencies I used before as the coil drew like 2 amps with no load and now it draws hardly any current at full line voltage in.

I thought about adding resonant caps in series with output of the coil, much like resonant MOT's - this way I wouldn't have trouble with current running off the scale with primary resonance.

I'm not sure if I'll have time for it myself, but one thing I'd like to see done is ignition coil driven by a full bridge from mains - this should push the IC's balls to new heights, along with some monstrous arcs.

With some ambition one could create a multi-phase power supply with 3 or 6 IC's, perhaps with a 3 phase IGBT module.

Anyone willing to paticipate in a renewed "ignition coil arc contest"? Like in good old days :)

Cheers,

Marko

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