Spark plug wire through current transformer

[Circlotron]

I want to measure spark plug current in a running engine using a current transformer. No big deal, aside from noise issues. Then I thought - some fancier plug leads have a central fibreglass core and a wire conductor wound helically around this core. Any magnetic flux would flow axially along the length of the cable instead of around it. Would this kind of cable then not work with a current transformer???

[Richard Head]

A current transformer will work fine.
The helical twist will have no effect at all. However, to get a reasonably decent signal you may have to wind several turns of the HV wire around the core and a single secondary turn. ie a current step-up configuration.

[T3sl4co1l]

The fact that current eventually flows longitudinally, sooner or later, is the only characteristic that matters.

Indeed, if you imagine the helix bent over into a circle, you have a single turn current loop, which is a primary stray aspect of a (single layer, progressive) toroid winding (which may be characteristic of your current transformer, for instance).

Tim

[HighVoltage]

The best current transformer for this type of measurements I have found is Model 110 by Pearson / USA
It works well on the bench and in a car and completely independent of the spark plug wire type.

The sustained current of lets say 50mA peak and then a linear fall to 0 in 1 ms will not be much influenced by the type of wire.
You need to zoom in to the us or even ns range of the discharge, to see a difference in the damping of the first discharge of the high current portion of the spark, which can be in the range of 1 to 10 A easily.
 
Attached is a picture of the Pearson current probe and a current discharge, measured with such a probe.
In this case the coil peak current was over 100 mA

 

[joeqsmith]

I use a Pearson 410.  Not too bad on a test jig but would want a clamp like HighVoltage shows if I used one often.

[CaveMannDave]

Hi:

A current transformer will work fine.
The helical twist will have no effect at all. However, to get a reasonably decent signal you may have to wind several turns of the HV wire around the core and a single secondary turn. ie a current step-up configuration.

No.

Dozens of after-market accessory outfits sold tachometers and Cruise Control kits in the 60s through the 80s which used a toroidal core about 25mm O.D. x 18 mm I.D. w/a dozen or so turns of 18awg vinyl covered wire (easy, non-invasive, handy-man grade install, no Warranty violation.), through which was threaded the O.E. Coil HT lead (or, #1 cyl. plug wire), as a servo feedback for Loop Control of RPM.
Also popular in the 90s were (el Cheapo) Diagnostic aids with split-core ferrite clamps w/, perhaps, 50 turns on the secondary for a hand-held Tach/'Analyser' unit.

--------------
O.P.

Current is really not THAT important in a Kettering system, as voltage is: you can easily use current pulses to get Timing edges, useful for some measurements, but I think the Voltage Profile is more informative.  (Yes, just as wavelength and Photon Energy inter-convert, the two are Related.), because then you can see the breakdown potential curve vs. time and other variables in a whole picture (plug gap, open wires, mixture quality, cylinder pressure, fouling, arc-over, etc.), if you know what to look for.

All it really takes is an electrode of 'known' area, clamped around a wire (plug or coil), or even some Specially Shaped plates, for Coil-on-Plug systems, a high-input-impedance, high frequency cut-off, middling gain input amp, and then your choice of measurement method.

If you REALLY want Current vs. time, then a CT w/appropriate signal processing and sufficient bandwidth (different cores roll-off HF in different ways, YMMV.)
I think I would look at PRIMARY energy storage, and calculate from there. (SURELY there is literature out there concerning this.). Put a small CT on the Points lead.

But if you're not designing HEI systems for Racing engines, what is your goal?

HTH,
Cheers,

Dave

[CaveMannDave]

I use a Pearson 410.  Not too bad on a test jig but would want a clamp like HighVoltage shows if I used one often.

Wow, what IS it that you DO?
Highly specialized Spark Plug characterization for bleeding-edge race engines?

Are those Harley-Davidson after-market ignition coils?  I'll bet they would bolt-on. 

Tektronix HV probe? Does it still need Freon inside, or a newer model?

Gotta' love the DSO: LeCroy. One o' the best, of the West!

How did you induce that flash-over?   Other than no Plug boot, was it just dV/dT, or were the electrodes modified, or did you put some insane pressure in the chamber?

I love your Bench.  Wheat Berber?  The files are within easy reach, but the Metal cabinet?

Nice touch with the (Phone/DSL?) outlet so handy. Takes the worry out of being close.  (60s Mouthwash ad tag-line  ).

Seriously, though, REALLY super instrumentation, custom test fixture, nothing but the BEST in a specialized rig, all kiddin' aside, Is this all for in-crowd braggin' rights, or is there a really productive goal that I'm somehow failing to grok? (Look THAT up in your Funkin' Wagnals!)

Best wishes to you, I wish I could afford that gear.

Cheers,

Dave

[calexanian]

How funny this comes up. Yesterday I test ran an engine I just rebuilt and forgot to the the resistor in the supply of the ignition coil. Yes I am the one they print "This coil must have a resistor installed to prevent damage" for. Ran great. A few minutes after I shut it off I heard a funny boiling sound. Cooked it to a crisp! there goes $28 down the drain!

[Circlotron]

The best current transformer for this type of measurements I have found is Model 110 by Pearson / USA

That looks really cool. Is it really expensive like hundreds of dollars? Good stuff can make you catch your breath sometimes.

[HighVoltage]

Yes, that one is expensive, I think something like $ 800 new or so.
If you are lucky, you can find them used in good condition for $ 200 to $300

After testing many different current transformers for this kind of application, this model from pearson is by far the best.