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[TCML] Re: inductors for DC charging



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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