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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
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7. http://www.capturedlightning.com/
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