I believe it was way back in the 90 I had an article published in Popular
Electronics about a dc coil using two ignition coils. It was also on the
magazine’s front cover. Charles Rakes
On Mon, Feb 13, 2023 at 2:43 PM Lux, Jim <jim@xxxxxxxxxxxxx> wrote:
> On 2/13/23 11:25 AM, Bert Hickman wrote:
> > Hi Jim,
> > I'm not aware of anyone trying to use ignition coils as charging
> > chokes. Their high inductance and operating voltages are very
> > attractive. However, the high DC resistance will reduce the charging
> > current, maximum voltage of the tank cap, bang size and maximum
> output
> > power. In addition, there may be concerns with current-handling and
> > heating in the ignition coil, and core saturation. For comparison,
> most
> > HV resonant charging systems use charging chokes that have DC
> > resistance of hundreds of ohms or less. In the following
> calculations,
> > I used a custom spreadsheet that uses an underdamped RLC charging
> > circuit model, a "stiff" DC supply, and a dequeing diode (a line-type
> > modulator circuit).
>
> > In all the following examples, the DC supply voltage was 10 kV, tank
> > cap was 20 nF, and the system break rate was set to 250 BPS. For
> > comparison, we can compare performance to an "ideal" (lossless) DC
> > resonant charging system with a zero-ohm charging choke. In an ideal
> > system, the tank cap would be charged to 2X the DC supply (or 20 kV),
> > the tank bang energy would be 4 J, so the average tank output power
> at
> > 250 BPS would be 1000 watts. In general, increasing inductor
> resistance
> > reduces performance, while increasing inductance improves
> performance.
>
> This is sort of where I wound up. My application is running 7 small
> coils (3" secondary, 13nf Cpri) so fairly low power (~NST class) off a
> common supply (eventually with triggered gaps or something like that)
>
>
> I figured the $18 ignition coil, while having DC resistance might not be
> so bad in this application. And it's cheap!
>
>
>
>
> > Case 1 is for a sample ignition coil with resistance and inductance
> of
> > 10 k ohm and 30 H respectively.
> > Case 1: Example ignition coil:
> > Rcoil = 10 k ohm, Lcoil = 30 H, 250 BPS
> > Max break rate: 412 BPS
> > RMS charging current: ~ 49 mA (250 BPS)
> > Ignition coil ohmic dissipation: 23.9 W
> > Tank cap max voltage: ~13.9 kV (~65% of 20 kV target voltage!)
> > Bang size: 1.71 J
> > Ave Tank power output: ~428 watts (~42% of ideal)
>
> If I cut the rms charging current in half, then the ohmic (and core)
> losses would be <10W, which is probably within the dissipation
> properties of the coil. (and it would be cheap to find out if it's not)
>
>
>
> > Suppose we instead used a "high-energy" cylindrical style non-CD
> > ignition coil such as a JEGS 555-40105
> > ([1]https://www.jegs.com/i/JEGS/555/40105/10002/-1). This coil has
> less
> > than half the series resistance (4.7 k ohm) as Case 1. We'll also
> > assume it has the same inductance as before (30 H).
> > Case 2: High-Energy JEG coil:
> > Rcoil = 4.7 k ohm, Lcoil (assumed) = 30 H, 250 BPS
> > Max break rate: 414 BPS
> > RMS charging current: ~ 75 mA
> > Ignition coil ohmic dissipation: 26.2 W
> > Tank cap max voltage: ~16.23 kV (~81% of 20 kV target voltage!)
> > Bang size: 2.63 J
> > Ave Tank power output: ~659 watts (~66% of ideal)
>
>
>
>
> yes - lower series R helps
>
> > The single JEG coil is significantly better than the sample coil.
> Let's
> > try two JEG coils in parallel to see how much more it improves the
> > results.
> > Case 3: Two JEG coils in parallel
> > Rcoil = 2.35 k ohm, Lcoil = 15 H, 250 BPS:
> > Max break rate: 586 BPS
> > RMS charging current: ~ 83 mA (250 BPS)
> > Ignition coil ohmic dissipation: 16.4 W
> > Tank cap max voltage: ~17.16 kV (~86% of 20 kV target voltage!)
> > Bang size: 2.94 J
> > Ave Tank power output: ~736 watts (~74% of ideal)
> > Finally, a comparison with a typical DC resonant charging choke is
> > shown below...
> > Case 4: Typical low-resistance custom DC resonant charging choke
> > Rcoil = 200 ohm, Lcoil = 15 H, 250 BPS:
> > Max break rate: 587 BPS
> > RMS charging current: ~ 106 mA (250 BPS)
> > Charging Choke ohmic dissipation: 2.3 W
> > Tank cap max voltage: ~19.38kV (~97% of 20 kV target voltage!)
> > Bang size: 3.76 J
> > Ave Tank power output: ~940 watts (~94% of ideal)
> > So, it looks like one or more ignition coils could indeed be used as
> > charging chokes for 1-2 kW power coils assuming no core saturation
> > issues and at reduced efficiency. However, there may be advantages to
> > sacrificing efficiency for the flexibility of a DC resonant charging
> > system. Using cylindrical form factor (i.e., open core) should help
> to
> > prevent core saturation, and the secondary windings should be able to
> > handle the RMS current at power levels shown in the above examples.
> You
> > could confirm if the secondary can handle the current by driving a
> > secondary from a HV DC source using the same DC current as the
> expected
> > RMS current in the charging system.
>
> Interesting, the "Coil in oil can" might be better from saturation
> properties.
>
> The core on those HEI coils doesn't look nearly as big as the core on
> the conventional coil I sawed open 25 years ago, but I could be
> misremembering.
>
> One could saw the core to gap it, too. A couple cuts and instantly, it's
> not a closed circuit any more.
>
>
> If you were to post your spreadsheet, that would be interesting.
>
> I'm building up a simple SPICE model but it's tedious - I'm not a "every
> day SPICE user" and getting all the diodes and switches (to simulate the
> gap) to work is, eh, tricky.
>
> There's plenty of "really nice" gap models out there (Analog Devices has
> one on their website) but I think they're sort of overkill for this.
>
>
>
>
>
> > Bert
> > Lux, Jim wrote:
> >
> > On 2/13/23 2:58 AM, Joshua Thomas wrote:
> >
> > Funny you mention this, I have a classic coil (can shape, oil
> > filled) that
> > I was thinking how to use in a coil. Interested in what others have
> > done.
> >
> > There's the GMHEICSLR (GM HEI coil spark length record) - running
> > off 110VAC with a triac - More than 30cm sparks, as I recall.
> >
> > On Mon, Feb 13, 2023 at 3:37 AM Lux, Jim [2]<jim@xxxxxxxxxxxxx>
> > wrote:
> >
> > Has anyone tried using the secondary of an auto ignition coil as a
> > charging inductor for a small coil?
> > They are 10-30 H (depending on who measured it, etc.) They've got
> > the
> > HV insulation.
> > I was wondering about the current handling. Typically, they seem
> to
> > have a DC resistance of some 10 kOhm or more, and if you're driving
> > from, say, a NST with an RMS output current of 30 mA, that works
> out
> > to
> > about 9-10 Watts dissipation (because I've not got a spice model of
> > the
> > actual charging current, etc.)
> > Or, alternately, any source for 10 H inductors that can hold off
> > 20kV?
> > Short of winding my own. 200 or so turns on a 4 cm diameter core
> > with
> > mu=5000 would do it. Have to wind in 20 turn chunks to keep the
> > voltage
> > rating, reasonable, but that's doable.
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> >
> > --
> > Bert Hickman
> > Stoneridge Engineering LLC
> > Woodridge, Illinois, USA
> > [7]http://www.capturedlightning.com
> > +1 630-964-2699
> > ***********************************************************************
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> >
> > References
> >
> > 1. https://www.jegs.com/i/JEGS/555/40105/10002/-1
> > 2. mailto:jim@xxxxxxxxxxxxx
> > 3. mailto:tcml@xxxxxxxxxxxxxxx
> > 4. mailto:tcml-leave@xxxxxxxxxxxxxxx
> > 5. mailto:tcml@xxxxxxxxxxxxxxx
> > 6. mailto:tcml-leave@xxxxxxxxxxxxxxx
> > 7. http://www.capturedlightning.com/
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