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MSD Ignition Coil #8251

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jpsmith123

Tue Jun 23 2009, 05:03PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

I'm wondering, has anyone here experimented with one of these MSD ignition coils?

,_Use_w/MSD_7_Series.aspx

With such low winding resistances (0.04 and 86 ohms) and an "E" type core, it seems to me this coil is potentially capable of some relatively high power through-put.

If the coil was not so expensive ($122.00 is the cheapest price I see on ebay) I would buy one right now to fool around with, but at that price, I'd like to find out some more information first, e.g., what is the material and cross sectional area of the core, the number of primary turns, and the secondary self-resonant frequency.

(I sent an email to the company asking for more information but so far I haven't received a reply).

jpsmith123

Sun Jun 28 2009, 08:34PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

Well I wasn't able to find out very much about the #8251 coil, other than that the core is comprised of "common transformer laminations", and would not be good at "high frequency". I'm told that the core cross section is 0.675" x 1.275" and it is gapped.

Apparently the laminated core could be replaced (somewhat sloppily) with a Ferroxcube ferrite UI core (U93/76/16 and I93/28/16) although this would likely also increase the inductances of the windings.

Anyway, I'm told that a different msd coil, the #8201, actually uses a ferrite U core (also gapped). With its primary resistance of 0.03 ohms, and 1 mH inductance, this coil might be a good off-the-shelf solution for a HV power supply.

It seems to me that two of these in a bipolar configuration could potentially handle 1 to 2 kw or so. The unknowns are the self resonant frequency of the secondary and the duty factor of the coil (the spec says its intended for "short duration" applications).

Dr. Dark Current

Sun Jun 28 2009, 08:59PM

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

jpsmith123 wrote ...

The unknowns are the self resonant frequency of the secondary and the duty factor of the coil (the spec says its intended for "short duration" applications).

The secondary has no inherent resonant frequency, you could think of its self resonant frequency only with primary unconnected, between the core's magnetising inductance and secondary self-capacitance, but no power transfer can happen this way (this resonant frequency is usually very low).

The resonant frequency we are interested in is when we connect the primary to a low-impedance drive circuit such as a half-bridge. When connected directly, the resonant frequency is determined by the pri-sec leakage inductance and secondary self-capacitance. When you add an inductor in series with the primary (for the purpose of lowering output short-circuit current), the system's resonant frequency also changes.

Also, you do NOT want to drive it at the resonant frequency (or its sub-harmonics when useng square wave drive), because this can lead to destructive voltages on the secondary winding and possibly damage to the driver from high currents flowing through the primary.

Proud Mary

Sun Jun 28 2009, 10:03PM

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

Establishing the time constant of the primary will demystify the optimum working PRF of an ignition coil.

In the case of your coil where R = 0R03 L = 0H001, then if we decide that 77% magnetization of the core represents the most time we want to invest in the charging phase, we find this to be a very convenient 50ms.

Moreover, I have determined by observation that the secondaries are indeed resonant in a flattish low Q way, such that as dwell time decreases with increasing engine RPM there is some compensation provided by higher OP voltage.

(fo was determined using a negative resistance oscillator to excite the secondary) Three new, unused mass market coils were investigated, and fo in each case was discovered to wander around 1kHz - an adequate match for the primary time constant.

In the three coils examined, there was a clear relationship between the time constant of the primary, and the resonant frequency of the secondary.

It is possible that my sample may be too small to mean very much, or that I have bungled something, but it makes good sense to compensate for falling spark power with increased RPM by winding a weakly self-resonant secondary.

By the way, I am conducting this little investigation because it seems to me that ignition coils are too quickly passed over as beginner's toys, and that they might provide useful amounts of rectified, smoothed, EHT if their particular properties were better understood (by me! who else would waste their time on it?

)

speakerguy79

Sun Jun 28 2009, 10:52PM

Registered Member #1460 Joined: Thu May 01 2008, 12:45AM
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Posts: 19

I have used this:

,_7_Series_Ignitions.aspx

Its performance fell off drastically at 10kHz. I was running a little more than a hundred watts through it. The EMI actually caused a reset of the programmable power supply I was using, and it ended up getting 60V instead of 13.8V and made a really, really fat spark before I shut it down.

jpsmith123

Mon Jun 29 2009, 05:04AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

Dr. Kilovolt wrote ...

The secondary has no inherent resonant frequency,

Well of course it does. The "secondary" is an inductor, and, as with all inductors, it has a parasitic stray capacitance; thus it has a resonant frequency. Of course the purpose of measuring the SRF is to get an idea of the distributed capacitance, Cd, for modeling and design purposes.

wrote ...

you could think of its self resonant frequency only with primary unconnected,

Well what did you think I was referring to?

Wow that's an expensive coil! Anyway, given the low winding resistances, it seems that coil might handle some power if driven properly.

Can I ask, what kind of drive circuit were you using?

speakerguy79 wrote ...

I have used this:

Dr. Dark Current

Mon Jun 29 2009, 10:28AM

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

jpsmith123 wrote ...

Well of course it does. The "secondary" is an inductor, and, as with all inductors, it has a parasitic stray capacitance; thus it has a resonant frequency. Of course the purpose of measuring the SRF is to get an idea of the distributed capacitance, Cd, for modeling and design purposes.

OK, you are right, I didn't think of measuring the capacitance this way.

I just wanted to say that the resonant frequency is not a constant and depends on how you drive the transformer, that's all.

Sulaiman

Tue Jun 30 2009, 11:54AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Just a quick note, I've been using normal ignition coils for eht for many years,
ignition coils are rugged and can give quite a lot of power BUT
they operate best around 1 kHz, just where hearing is sensitive.
So ignition coils are great when you want to hear the inverter - very bad if not.

Mates

Tue Jun 30 2009, 01:01PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

I played a lot with ignition coils and they can be pushed to transfer many hundreds of watts at 50KV. My experience is strictly empirical, but it seems there are kind of resonant peaks within 1 KHz-3 KHz where the efficiency seems to be the best. On the pictures below is my single iggy (made in 1974) running at 3 Khz powering SGTC. It is definitely not toy Harry ;)

Dr. Dark Current

Tue Jun 30 2009, 01:14PM

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

Mates wrote ...

I played a lot with ignition coils and they can be pushed to transfer many hundreds of watts at 50KV.

I would be surprised if they handled that continuously, for short runs - all right. I can also push a kilowatt through a television transformer under a minute.
If you want to test the reliability, you should leave it on at least an hour at full power. If it doesn't burn up, you can conclude the smoke test OK :)

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