half bridge ignition coil driver - power your SGTC cheaply.

Marko, Sat Oct 01 2011, 11:36PM

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.

Link2
Re: half bridge ignition coil driver - power your SGTC cheaply.
Patrick, Sun Oct 02 2011, 12:08AM

It has a wickedly cool sound!!!

Re: half bridge ignition coil driver - power your SGTC cheaply.
Dr. Dark Current, Sun Oct 02 2011, 08:05AM

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.
Re: half bridge ignition coil driver - power your SGTC cheaply.
Marko, Sun Oct 02 2011, 03:30PM

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
Re: half bridge ignition coil driver - power your SGTC cheaply.
Patrick, Sun Oct 02 2011, 04:17PM

Do you now how much real power your getting to the arc?
Re: half bridge ignition coil driver - power your SGTC cheaply.
Marko, Sun Oct 02 2011, 09:24PM

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
Re: half bridge ignition coil driver - power your SGTC cheaply.
Proud Mary, Sun Oct 02 2011, 11:15PM

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

Link2
Re: half bridge ignition coil driver - power your SGTC cheaply.
Marko, Tue Oct 04 2011, 10:43PM

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

Re: half bridge ignition coil driver - power your SGTC cheaply.
Dr. Dark Current, Wed Oct 05 2011, 01:02PM

Maybe you could try driving the coil at resonance, this could increase the efficiency but you must not let the output unloaded.
Re: half bridge ignition coil driver - power your SGTC cheaply.
Marko, Wed Oct 12 2011, 10:17PM

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
Re: half bridge ignition coil driver - power your SGTC cheaply.
Sulaiman, Thu Oct 13 2011, 03:11PM

from memory, there's quite a bit on this topic in the old Pupman archives, (previous millennium) search for HEI.
EDIT: I just had a quick look,
a good start may be here
Terry's GM HEI Coil Update

Ignition coils have an open magnetic system
- good for energy storage / poor as a transformer.
.... lots of leakage inductance.

Also, the laminations aren't very thin
so eddy current losses become a problem at higher frequencies.

For RELIABLE operation in flyback mode 50W is easy.

For a newbSGTC 50W can be quite exciting
using exactly the components/values as if for an NSTsgtc etc
(c10nF etc.) the spark rate is low
... but the arcs can be longer than a 450VA NST sgtc
IF the primary capacitor can take the higher voltage.
e.g. 30 kV from a flyback instead of 20 kV using NST
Re: half bridge ignition coil driver - power your SGTC cheaply.
Marko, Sat Oct 15 2011, 11:22PM

Hei guys,

Some more fat (10cm) arc drawing from this thing. Now I want to drive it by a full bridge to truly test it's balls, perhaps until destruction.

Link2
Re: half bridge ignition coil driver - power your SGTC cheaply.
kiat, Sun Oct 30 2011, 11:45AM

From an ignition coil I disassembled, it had the core and windings potted in some kind of tar and on top of that there's a few layers of sheet steel, followed by the aluminium case.

I think the most eddy current loss is in the steel sheeting, but to avoid internal arcing I think getting the tar out for oil immersion may be troublesome, although there maybe oil filled ignition coils that I haven't come across.
Re: half bridge ignition coil driver - power your SGTC cheaply.
Marko, Tue Nov 01 2011, 05:28PM

Hi kiat,

well judging from sounds my ignition coils make when shaken I'm pretty convinced there's some kind of oil inside them - I'm not sure what kind it is nor how toxic it might be.

For the fun's sake I tried running a smaller IC (the one visible in background of my last video) from a H-bridge, but I didn't even reach half the input voltage and it died internally. I suspect it's internal construction might have been faulty and perhaps it arced over along the output connections which might not be completely submerged in oil, as output is practically 0V now.

I know many people push their ignition coils to arc over externally very easily, and easily get arcs that start at 5cm or more... mine only started at like 2 at most, so I probably shouldn't take this coil as a reference. I didn't dare to run my other coils to external arcover without oil (which would ruin them instantly) as significant corona was already showing on the terminals.

After I get some more coils I might pick out some for special H bridge punishment and see what can I get out of them...

Marko







Re: half bridge ignition coil driver - power your SGTC cheaply.
radhoo, Wed Nov 02 2011, 07:41AM

I had some very good results with a Bosch coil , from a VW car but using it for a completely different purpose, my electric fence HV generator: Link2
Not the same performance with a Dacia (local car manufacturer) coil , so when you say people get better results it might be due to better coils.
The Bosch put out a 2 cm spark with my pulse driver, while the Dacia coil would work up to 1cm the most, for the same input parameters. I know that the Dacia uses oil for insulation, not sure about the Bosch one.

I don't think pushing coils to failure levels is a good idea, instead a very reliable driver might be a useful project.
Re: half bridge ignition coil driver - power your SGTC cheaply.
Marko, Wed Nov 02 2011, 03:16PM

Hi radhoo,

150V drive from a half-bridge seems like a fairly sweet spot for driving an ignition coil for power supply use. Full bridge would be rather pushing it but would surely produce some fun looking sparks but I'm not sure how long would it last this way.

What about those HEI ignition coils, what are their voltage standoff characteristics compared to usual can style coils?

Marko

Re: half bridge ignition coil driver - power your SGTC cheaply.
radhoo, Thu Nov 03 2011, 12:32PM

Marko wrote ...

What about those HEI ignition coils, what are their voltage standoff characteristics compared to usual can style coils?
I was wondering the same thing, but I didn't have the chance to test one yet.