Measuring the inductance of an automotive coil - Page 1
Forum section: electronics / beginners

author: injb
author_rank: Regular Contributor
timestamp: October 09, 2025, 02:14:20 am
content: Hi, I'm trying to measure the inductance of the ignition coil from a car. I've been told it's around 5.85mH by someone who measured one from the same car but I want to check myself.I tried using the ubiquitous "component tester" that you can buy everywhere. This measures the tiny inductors that look like resistors with reasonable accuracy. But when I try using the ignition coil, it consistently comes out with 0.4 - 0.5mh. I really don't think that's accurate.I have to use fairly long leads to connect the coil to the tester, but even with those same leads it still measures the small ones decently. The only other thing I can think of is the ignition coil has a resistance of 0.6ohms, and the little ones are much higher, 30-40ohms. Is that why it won't work? Is there anything I can do to get a reasonable measurement, or is there some reason this just isn't possible?I will try and do it using a tuned circuit and an oscilloscope, but I just wanted to see if there's anything obvious I'm doing wrong that would get me such weird results.Thanks!

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2025, 02:20:21 am
content: The component tester is probably not designed to measure such a high inductance. A real LCR meter or an impedance bridge could probably do it, although ignition coils are kind of weird.

author: mawyatt
author_rank: Super Contributor
timestamp: October 09, 2025, 03:27:19 am
content: If you have an AWG and DSO, then attach DSO across coil and connect coil to AWG with a series resistor R. Set the AWG for a Pulse (Step Function) of 1V peak across coil and capture the waveform. The waveform should follow a step with a decay following exp(-t/tau), where tau is the L/R Time constant. Measure time of 63% decay (0.63V) and evaluate L as T*R. Note be sure to include AWG source resistance (usually 50Ω).Best

author: inse
author_rank: Super Contributor
timestamp: October 09, 2025, 05:31:23 am
content: The inductance will be quite nonlinear so as the component tester only applies a small amplitude it will not be able to measure the effective inductivity.Also the function generator with it’s 50Ohms impedance will not drive much.Btw, be careful, I once damaged a (cheap) function gen when playing around with inductors - add a freewheeling diode.At work we have a special Power Choke Tester, a cool instrument that can drive pulses up to 100A through the DUT.It can graph the inductance vs. current which gives interesting insights, sometimes.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 09, 2025, 09:15:29 am
content: My first attempt would be to connect coil to 12V supply and observe current (using 0.1Ω resistor or Hall-effect I->U converter placed on the cable).

author: HighVoltage
author_rank: Super Contributor
timestamp: October 09, 2025, 09:22:13 am
content: The Keysight U1733C Handheld LCR Meter works well for ignition coil primary inductance measurements.Probably other handheld LCR meters will also work.Make sure to use 1kHz frequency, which is the standard to measure these coils.

author: xavier60
author_rank: Super Contributor
timestamp: October 09, 2025, 11:33:15 am
content: What I do is connect a 0.1Ω resistor in series on the ground side. Then repeatedly apply 12V while using my DSO to capture the ramp across the resistor which indicates the current ramp up, until I get a clean waveform. Definitely put a diode across the winding to avoid equipment damage.Some coils have magnetically biased cores, so wrong polarity can cause premature core saturation.Then I calculate the inductance using the 12V and the time taken to ramp to some current like 1A.And, a current limiting PSU is needed.

author: u666sa
author_rank: Frequent Contributor
timestamp: October 09, 2025, 12:37:15 pm
content: aa1car.com/library/ignition_coils.htm

author: injb
author_rank: Regular Contributor
timestamp: October 09, 2025, 01:05:21 pm
content: Quote from: xavier60 on October 09, 2025, 11:33:15 amOK a couple of people have mentioned using a 0.1ohm resistor this way. Sorry if this is a stupid question but what is this for? The coil primary has a resistance of 0.6ohm. I'm assuming adding 0.1 to that isn't going to make it any safer for the coil right? I know that in operation, it charges to 9amps so if I set my PSU to 12v/1amp, and measure how long it takes to get to 1 amp, would that be good enough to calculate the inductance?Thanks for all the replies BTW!

author: joeqsmith
author_rank: Super Contributor
timestamp: October 09, 2025, 01:13:05 pm
content: Quote from: injb on October 09, 2025, 01:05:21 pmYou want to measure the current through the part, not the voltage across it. The voltage would be a step, but the current will ramp. The knee is where the core saturates, and you are left with the E^2 R loss.I assume from your original post of mH, you are only interested in the primary side.

author: injb
author_rank: Regular Contributor
timestamp: October 09, 2025, 01:33:45 pm
content: Quote from: joeqsmith on October 09, 2025, 01:13:05 pmOh I get it now, thanks. I actually have a current clamp probe for my scope so I was going to just use that for current measurement over time.

author: xavier60
author_rank: Super Contributor
timestamp: October 09, 2025, 01:42:06 pm
content: Quote from: injb on October 09, 2025, 01:05:21 pmThe 0.1Ω resistor acts a current sense resistor. When the ramp is captured by the DSO, the time it takes for the ramp to cross 100mV from its start is used for the inductance calculation. The 12V supply can be increased slightly to compensate for the combined resistive loss.

author: xavier60
author_rank: Super Contributor
timestamp: October 09, 2025, 02:09:15 pm
content: This is a quick test I did on a 1.2mH inductor that I make using gapped ferrite pot cores. I used a 0.22Ω sense resistor with the PSU set to 12.3V. The ramp crosses 220mV at 100us.

author: inse
author_rank: Super Contributor
timestamp: October 09, 2025, 02:11:53 pm
content: Nice scope, btw

author: injb
author_rank: Regular Contributor
timestamp: October 09, 2025, 02:32:20 pm
content: Does the resistance of the power supply itself matter, and is there any way to account for that?

author: xavier60
author_rank: Super Contributor
timestamp: October 09, 2025, 02:43:57 pm
content: Quote from: injb on October 09, 2025, 02:32:20 pmThe PSU voltage can be monitored on another DSO channel to see how it holds up during the current ramp. If there is too much drop, the voltage can be increased or a capacitor can be put across the supply, 1000uF or higher.

author: xavier60
author_rank: Super Contributor
timestamp: October 09, 2025, 03:00:30 pm
content: The voltage need not be a particular value so long as whatever it happens to be is used in the calculation.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 09, 2025, 03:39:55 pm
content: Quote from: xavier60 on October 09, 2025, 03:00:30 pmI wonder if you have an actual ignition coil that you could use to work through the whole measurement. Primary & secondary inductance verified with the turns ratio. Work the math. Not a big deal if you had a coil on-hand. I am a bit curious on measuring the secondary.For OP, in case you did not follow, they measured 220mV in 100us with a 12.3V source. The resistor was 0.22 ohms. Calculating current V/R = 0.22V/0.22 ohms = 1A. V = L (di/dt). Vl = 12.3 (source) - 0.22 (drop across sense) = 12.08V. di/dt = 1A/100us or 10k. L = 12.08/10,000 = 1.208mH. Also, you could see the knee I mentioned in their scope capture where the current transitions from a nice linear increase to almost vertical. If you were designing a power supply, I normally de-rate the part by 50%. So if that knee is say 10A, I would use it on a 5A supply....***OP, also may be helpful to understand your ultimate goal, if someone were to go though and characterize an actual coil. Do you want to know the resonance, DC resistance, leakage inductance, capacitance.... Or, just simply measure the primary inductance?

author: injb
author_rank: Regular Contributor
timestamp: October 09, 2025, 04:48:42 pm
content: Quote from: joeqsmith on October 09, 2025, 03:39:55 pmI do have a coil removed from the car that I can test with. But I can't disassemble it to count turns or anything like that.Here's what I'm planning to do now:* I'll use my PSU with a low voltage for safety, say 1 volt, limited to 1 amp.* I'll use a diode across the coil to protect the PSU and scope.* I'll use a spare scope channel to monitor the voltage from the PSU* I'll use my current clamp probe to measure the current from when I turn the PSU on, and that way I don't need the series resistor.Then I can use the formula to work out the inductance. Does that sound right?

author: inse
author_rank: Super Contributor
timestamp: October 09, 2025, 05:31:26 pm
content: What are you planning with those values?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 09, 2025, 06:05:43 pm
content: Same. Are you just wanting to measure the inductance for the fun of it, or are you actually trying to do something with the coil that you need to characterize it?

author: floobydust
author_rank: Super Contributor
timestamp: October 09, 2025, 06:13:25 pm
content: You need to know the inductance but also saturation current.When testing these at say >5A, it's common for a power supply to sag and check for that as well.Old familiar graph for GM HEI IC MC3334, note they include the transistor losses.Primary leakage inductance is around 0.6mH, that might be what the tester is measuring.

author: injb
author_rank: Regular Contributor
timestamp: October 09, 2025, 07:00:13 pm
content: Quote from: joeqsmith on October 09, 2025, 06:05:43 pmThe short answer is I'm not building anything with it exactly. It's mainly educational. But I want decently reliable info to help with tuning. I'm trying to get to the bottom of a misunderstanding/debate. That's how it started...but now my curiosity is piqued and I just want to learn how to measure it, regardless!I could go into more detail but I don't want to bore everyone...let's just say it's the classic "argument with a bloke in the pub about the inductance of an ignition coil". You've all been there right?

author: inse
author_rank: Super Contributor
timestamp: October 09, 2025, 07:15:19 pm
content: Sure, all the time 😄P.S. Stay curious 👍

author: joeqsmith
author_rank: Super Contributor
timestamp: October 09, 2025, 07:29:25 pm
content: No problem then. So for fun, took and automotive coil I had and measure it with my BK RLC meter @ 1kHz.Primary with secondary open, 3.049mH 0.411 ohmsPrimary with secondary short, 234.4uH (primary leakage inductance)Secondary with primary open, 20.15H 6.32 kohmsSecondary with primary short, 1.615H (secondary leakage inductance)Injecting a 100.07mV @ 1kHz sine into primary, measure 8.0515V at secondary. Or a turns ratio of 80.46 : 1Sqrt of 20.15H/3.049mH = 81.29 (close enough to the turns ratio)Measuring the resonance with scope, I get 31kHz.Using a 23.995V step, current in primary reaches 4A in 549.6us. di/dt = 7278V=L(di/dt) 23.995/7278 = 3.297mH (close to what BK measured)I tried using the 24V step on the secondary but need to change the setup. I am using a driver I built for this that has a 0.01ohm shunt. The output is amplified to give me 4V = 40A. It is not nearly sensitive enough to make this measurement. I need to stick a much larger resistor in there....Anyway, that's the jist...***In floobydust's image, note how the current is limited so you never see the saturation knee shown in xavier60's graph. If you have a 12V car coil, at you apply a 100V step with unlimited current, you will see the knee.

author: MrAl
author_rank: Super Contributor
timestamp: October 09, 2025, 07:40:37 pm
content: Quote from: injb on October 09, 2025, 02:14:20 amHi,I have to ask the reason you want to test the inductance. It's not a more common test maybe because it is not normally a requirement to test for this.Anyway, testing for inductance is strongly application specific. It depends on what you are going to use it for. Testing with a low-end inductance meter would usually give you a reading that has nothing to do with how well it will work in a power supply, and testing with a pulse has little to do with how well it will work in a low-level oscillator or filter. If it is to test to how well it does in its intended application, then there should be a set test setup and procedure.If you specify why you want to test it you might get some even better replies.

author: Circlotron
author_rank: Super Contributor
timestamp: October 09, 2025, 08:57:01 pm
content: Quote from: injb on October 09, 2025, 07:00:13 pmI design and build ignition systems a living, having done a little over 20,000 of them in the last 30 years. So I have something of a half assed idea of what they are about. What exactly is the misunderstanding or debate you are having with them?

author: injb
author_rank: Regular Contributor
timestamp: October 09, 2025, 09:38:27 pm
content: Quote from: Circlotron on October 09, 2025, 08:57:01 pmOK, me and this other guy are involved in an online group helping people to tune/maintain/upgrade their cars (specifically the Porsche 944).The other guy recommended that people upgrade the stock ignition coil on the basis that his tests indicate that it doesn't have enough time to charge fully. He measured the OEM coil at 5.85mH. He says it takes ~6.5ms to get to 9 amps, where it's then limited by the ECU. He's very knowledgeable about this stuff, much more than me (his tests can be seen here:https://roguetuning.com/944951_ignition)But I just noticed that on my car (identical make/model) the coil was charging to 9amps much faster, in around ~3.5ms. I checked this with a DSO and a current clamp probe.I noticed that:* the ECU map uses 3.6ms as the dwell time for most of the rpm range at 13-14v, even at rpms where there's much more time available.* when I put the numbers (5.85mH, 13.8v, 9 amps) into this calculator, it comes out very close to what I saw on my scope (~3.8ms)So I concluded that the OEM system is fine and there's no advantage to using a different coil, all other things being equal.He says I'm wrong, and says I shouldn't use the formula that calculator uses because there are resistances in the ECU etc that I'm not taking into account. He modeled the circuit in LTSpice and says there's no way it would charge in the time I'm getting, and therefore my coil must have much lower inductance than the ones he measured.He dismissed the ECU dwell map thing as "they probably didn't have the thermal budget to charge it fully". Whereas I think it's not a coincidence that the ECU dwell times are a very close match for what I see. I mean, they chose the coil, the current limit and the dwell times - why wouldn't they make them match? My gut feeling is they would have chosen these numbers very carefully.Anyway me and others have now tested this with multiple coils on multiple cars and they all come out the same as my original result for charge time.So now I just want to know why this discrepancy is there so that the community ends up with good info. Did the other guy have an abnormal coil or did he make a mistake somewhere?https://www.daycounter.com/Calculators/Inductor-Current-Power-Calculator.phtml

author: joeqsmith
author_rank: Super Contributor
timestamp: October 09, 2025, 10:30:33 pm
content: Same 3.2mH ignition coil at 13.8V (lots of droop). Takes 2.35ms to reach 10A. Your calculator show 2.32ms to reach the same current.The link you provided did not work for me.

author: xavier60
author_rank: Super Contributor
timestamp: October 09, 2025, 10:53:00 pm
content: Also, it is mentioned in the 1st post that the coil has 0.6Ω resistance which now becomes significant at 9A and no longer can be ignored.LR time constant calculations will be needed to relate the measured inductance and battery voltage to peak current.https://www.electronics-tutorials.ws/inductor/lr-circuits.html

author: injb
author_rank: Regular Contributor
timestamp: October 10, 2025, 01:50:15 am
content: OK so next silly question, for this simple test, how do I turn the voltage on quickly? If I turn the PSU on it ramps up slowly. If I have it already on and try to make contact quickly, there's always a lot of noise. So I don't see how I can measure the charge time with a constant voltage this way.

author: xavier60
author_rank: Super Contributor
timestamp: October 10, 2025, 02:26:27 am
content: Although Im able to use a MOSFET driven with my function generator, for relatively large inductors I manually make the connection. I physically touch wires together. Set the DSO to Normal or single shot trigger and repeat the test until a clean ramp is captured.

author: xavier60
author_rank: Super Contributor
timestamp: October 10, 2025, 02:44:12 am
content: Or use a switch, although contact bounce might be a problem. My first choice would be a press type like a horn switch.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 10, 2025, 03:07:19 am
content: I'm using an IRFZ44N for the low side driver and an IRF2113 for the gate drive. Basic pulse comes from my Arb. Something like this, maybe just a 555 timer and FET.***I did check some of the other ignition coils I have on-hand but I don't have anything close to what you are using.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 10, 2025, 01:22:10 pm
content: Again, link not working. I wonder also if your friend compared the engines on a dyno with the coil swap? What improvements did they see? Same for emissions?Looks like some of the 944's were turbocharged. Did they compare the coil upgrade on both normally aspirated and turbocharged motors?Hard to believe changing a coil out on a stock car would offer any advantage. Then again, there are people who believe in all sorts of strange things when it comes to ICE.https://www.eevblog.com/forum/rf-microwave/ferrite-bead-on-spark-plug-wire/

author: injb
author_rank: Regular Contributor
timestamp: October 10, 2025, 01:44:15 pm
content: I don't know what's going on with the link, it works for me sorry. I just checked it again.He really did a huge project, re-writing the ECU code to improve all kinds of things, and sold chips with his new system. By all accounts it worked really well, though I never tried it in my car.The ignition article was intended for for turbo cars - I think mainly for turbos running higher boost, where the spark might need more energy. A lot of DIY tinkerers don't understand the risks to the ECU of using a different coil that might hit peak current much faster, so he wrote this article explaining how it all works and advising people on what coil upgrades make sense, are safe etc.But part of it was that he said the stock coil doesn't charge fully and so it's worth having an aftermarket one regardless of other upgrades. I doubt he dyno tested that claim and to be fair he wasn't telling people they're going to see a huge improvement. I think it was more "you might as well do this".But when my measurements contradicted that, he says I'm wrong. So I just wanted to get to the bottom of that. From what I'm seeing it has enough time to fully charge all the way through the rpm range and so it probably shouldn't be high on the list of changes to make even with mild upgrades.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 10, 2025, 03:26:56 pm
content: I suspect my hacked router is blocking it but was able to view the article. The following quote is from the article:QuoteI would expect something closer to the 13.8 you mention when running and 9 during cranking. Obviously, higher voltage will cause the current to ramp up faster. Maybe your friend was testing at 9V.QuoteSo you drive a transformer deep into saturation and it somehow creates a more powerful spark?QuoteSeems right. Because we don't know the voltage, we can't calculate the inductance. They do state:QuoteSo easy enough to calculate but again, we don't know what voltage they actually have at the coil. Loss in the harness..... Not sure how tight the cars regulator is over the RPM range..... Seems like some missing data.QuoteLOL. Been there, even with aftermarket ignitions.Again, doubt the coil makes any difference. I assume the goal isn't to just add parts to do some chair racing at the pub. They should show how it effects output power/emissions, something.... anything.***Quick search shows that same thread copied over and over across several car sites.The last post in this thread has inductance listed for some other Bosch coils.https://rennlist.com/forums/928-forum/1310329-calculated-coil-dwell-times-and-measured-inductance-resistance.html

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2025, 12:06:23 am
content: There are practical differences in ignition coils intended for different applications. Higher compression produces a higher breakdown voltage across the spark plugs, so requires a higher ignition voltage or a smaller spark plug gap. Transistorized ignitions require a higher turns ratio.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 11, 2025, 01:00:11 am
content: It appears roguetuning.com are using Microsoft, which I block ALL known traffic. This is actually the first time it has caused me any trouble. But again, that post was copied all over the internet, so not a problem..Quote from: David Hess on October 11, 2025, 12:06:23 amYou would need to expand on it. Maybe site some sources. Isn't every ignition in the last 40 years solid state. Why would using SS require a higher turn ratio?

author: David Hess
author_rank: Super Contributor
timestamp: October 11, 2025, 08:26:47 am
content: Quote from: joeqsmith on October 11, 2025, 01:00:11 amIgnition systems have been transistorized for decades now, but I remember when that was not the case, and Motorola was sending out kits to designers for their new transistors intended for ignition applications which included a suitable ignition coil in the standard form factor used at the time; it had 2.5 times as many turns on the secondary. The transistors were based on horizontal output transistors of course.Points in old style ignitions can tolerate breakdown much better than transistors even now can, which explains the difference.I still have the ignition coil somewhere; I used it to trigger a flashtube for a stroboscope.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 11, 2025, 08:55:25 am
content: Quote from: David Hess on October 11, 2025, 08:26:47 amThe earliest one I have allows you to run pointless, or works with the OEM points. The idea was to improve the life of the points. Worked with the stock coil. One thing it did allow was higher switching currents. I think it had drivers from TRW. Let me know if you would like to see it. But we are not really talking about vintage ignitions.Let me provide you with a data point. I have two bikes based on engines made in the early 80s. Not much of these are stock outside of the engine cases and head castings. Using spec fuel (VP C16), I can run about 30PSIG intake pressure. Both are a draw through system, and the charge is cooled with a small amount of N2O. Both are using an MSD ignition and coils (multi-spark CDI). Fast forward 20+ years. My street bike is also tubrocharged. It makes about the same power using a stock motor. It's injected and uses two ECMs to run a second bank of injectors. This bike has a bone stock ignition system. They have more than enough headroom to light the charge, at speed, under load with boost.I don't think swapping coils would do anything for performance. Like different spark plugs, wires, stickers on your windows...***Had the camera out anyway so took a couple of pictures for you showing the vintage ignition I mentioned. High res for the ignition buffs.

author: floobydust
author_rank: Super Contributor
timestamp: October 11, 2025, 07:01:52 pm
content: With any (ign. coil) transistor switch you are losing at least 10-15% of the system voltage. Points rule! lol.ExamplesBU931Thigh voltage ignition coil driver NPN power Darlington transistor VCE(sat) at 8A is about 1.6-1.8V and worse at low temps.An IGBTNGD8201is about the same.Then add the voltage drop of the sense-resistor, at peak current. This all might be what is compensated for by increasing the turns ratio.I find ignition system design is full of religion and voodoo beliefs, psychology and emotional design as well. I know the (combustion) science behind it is complicated.People conflate ignition energy with firing voltage, and burn time, use terms such as "heat" or "stronger"; it makes for a huge mess.OP needs to track down what semiconductors Bosch uses in that ECU for ignition coil driving. IGBT, IPM etc. then there is the current-limit setpoint.To know if a different coil is actually going to behave as the mind wishes it to, or be a placebo against more horsepower lol.

author: floobydust
author_rank: Super Contributor
timestamp: October 11, 2025, 07:08:34 pm
content: Quote from: joeqsmith on October 11, 2025, 08:55:25 amNice pics of some very good hardware, for the day. Although that screw head is a little close to the TO-3I dislike MSD offerings, their multi-spark module is a few discretes - no 555, no IC's. I looked at an old MSD-5 and tried running it and surprised how high it pushed the coil current and the multi-spark crapped out at quite low RPM's, it was only good for the low end and a bit better idle.

author: injb
author_rank: Regular Contributor
timestamp: October 11, 2025, 07:22:42 pm
content: Quote from: floobydust on October 11, 2025, 07:01:52 pmI've read that it's a custom Bosch part 1012 and is an NPN Darlington transistor. We're talking early 80s if that sheds any light on it. There's a modern aftermarket drop-in ECU replacement that I think uses IGBTs. But I have the original. It limits current at 9amps. I suppose I can measure the voltage drop with the scope with the car running. I'll take a look at that.I'm not getting into horsepower differences though - as people here have noted it's like trying to talk people out of their religion and it's beyond my knowledge anyway. I'm limiting myself to things I can measure and prove. The current info circulating out there is that the stock system doesn't charge fully. I have disproved that now with multiple coils and cars and I just want to round it all off by figuring out where the other guy went wrong, hence why I want to know the inductance.Where I left it off was that I couldn't get a clean switch-on by hand so I need to see what parts I have and make something.

author: langwadt
author_rank: Super Contributor
timestamp: October 11, 2025, 07:42:05 pm
content: Quote from: floobydust on October 11, 2025, 07:01:52 pmbut why would the voltage make much of a difference except requiring a slightly larger dwell time? the primary is ~1 OhmThe turns ratio is only about 1:100

author: floobydust
author_rank: Super Contributor
timestamp: October 11, 2025, 07:54:46 pm
content: Firing voltage is very important. The need increases with combustion chamber intake pressure and turbulence. So at high/full throttle, and when any turbo/supercharger is roaring is when the firing voltage requirement goes up. Higher compression ratios, or shitty combustion chamber design, larger plug gaps etc. it goes up as well.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 11, 2025, 08:21:53 pm
content: QuoteSaw that with the whole ferrite bead on a plug wire causing the timing to change. You see the same thing with hams and their Bird Watt meters.Quote from: floobydust on October 11, 2025, 07:08:34 pmMy MSD ignition has a micro. Boost supply is an IC. I used their patents to repair one of mine.Yes, it's not going to provide multi-spark at high revs. Part of my problem was that fuel would hit the compressor wheel and separate from the air, pooling inside the turbo (again draw through). Once you start to open the throttle, there would be enough slug of fuel that it would cause plugs to sometimes foul. That MSD basically solved it. I had an older one that used the resistors to set it, but the new one is all digital.Quote from: injb on October 11, 2025, 07:22:42 pmDoes it really matter? Even if you watch them take the measurement and see where they went wrong, I know a lot of people in similar conditions will just double down.Flat earth, free energy.....

author: joeqsmith
author_rank: Super Contributor
timestamp: October 11, 2025, 08:52:22 pm
content: Quote from: floobydust on October 11, 2025, 07:01:52 pmI remember those old ignition systems having that ballast resistor stuck on the firewall. These were in series with the coil's primary which limited the current to about 4A. I wouldn't be surprised that the companies selling these early solid-state ignitions didn't instruct you to remove the ballast as part of the install, or maybe swap it out.My MSD is a CDI. Even without a coil, you grab those primary wires your in for a surprise...***Like this:https://www.ebay.com/itm/306521684384So 0.8 ohms, plus the harness, at 4A is about 3.2V drop. Almost 13W. Those points kept the circuit active as long at the ignition was turned on. Motor not turning, 100% duty cycle is the points are closed. Times have changed.

author: floobydust
author_rank: Super Contributor
timestamp: October 11, 2025, 10:08:15 pm
content: In old Chevelles, the ballast resistor is a run of nichrome wire in the wiring harness, from the firewall. I don't think too many noticed this, aside from when the wire breaks and the car won't run lol. That was a bitch to figure out, and then you can't solder to it.I say it's a myth you can simply plop in a "hot rod" ignition coil and get more ponies.Looking at aftermarket ign coils, they drop/halve the inductance and then make really big claims "65kV!". Of course it depends on what the transistor and ECU can put out. Turns ratios seem the same.I can't see Porsche/Bosch being cheap with all that in the first place.MSDcoil datasheetsare garbage, full of typos so I could not figure out what they are doing aside from adding marketing hype, aggressive looks etc. to play on people's psychology.The MSD-5 I tried kept the coil current flowing all the time! It was 6-7A with key on and points open. It defaults to the coil being energized all the time, and pulses it off, to get the extra dwell time you need for multi-spark.Sad to see MSD outsourcing some of their products to china and then dealing with the crappy knockoffs, 6AL included.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 12, 2025, 12:25:34 am
content: I had an MSD MC-1 (motorcycle 1) that used the same package as your MSD-5. This is going back to 1992. It was old enough it actually supported using points, similar to that other vintage ignition I showed. It supported kill, but that was about it. The 2-step was violent. I think both the MC-1 and MC-4 drive the primary at 300V or so. They don't have the problem you describe with the coils always being active. Imagine the power supply required to put 300V across a 12V coil at 100% DC. The MC-4 has a lot of features that were not available in the old models. It has a boost timing curve, and launch control for example. I like it a lot, but again my newer bike with a stock ignition makes about the same power. Just more modern tech.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 12, 2025, 12:32:30 am
content: I have an earlier version of this ignition:https://store.schnitzracing.com/schnitz-pro-series-2-ignition/Yes, you program it with all those switches, buttons and LED readout!! The MSD MC-4 connects to the PC and they have their own software you use to program it.

author: xavier60
author_rank: Super Contributor
timestamp: October 12, 2025, 01:08:13 am
content: An easy-to-follow lecture on LR time constant with good audio.

author: injb
author_rank: Regular Contributor
timestamp: October 12, 2025, 04:19:54 am
content: So I tried using this method since I had the parts:It's a fast pulse generator using a 74HC14, with an LC circuit using a known capacitor. I should then be able to measure the ringing frequency and use the formulaL = 1/((2*pi*f)^2)*CBut it's pretty confusing. I must be doing something wrong. My oscillator is ~1Khz and I started out with a 1nf capacitor and tested that with a few pcb mount inductors I have. I got results very close to what the component tester gave me, and within the same ballpark as the marked values.For instance, one was marked as 4.7mH and rang at ~80Khz, for which the formula gave me around 3.9mH (see images). The component tester came up with 4.0mH for that one.I got similarly promising results with another value so I was confident enough to try my ignition coil, which I expect to be in the same ballpark of 3-6mH or so. I can see the ringing, although it's very weak. But it's in the order of 300Khz, which comes out to way less than 1mH using the formula - around 0.25mH.This is very similar to the results I got with the ignition coil in the component tester (it said it was something like 0.3 - 0.4mH iirc)This isn't a plausible value so there's obviously something wrong with the test. But what? The coil should have a similar inductance to the pcb mount ones I tested, so this circuit seems to be ok for inductances in that range. But those pcb mount inductors have resistances of ~30ohms or so - my ignition coil is around 0.6 ohms. So I'm guessing it's just loading the circuit too much to get the resonance I need, and the higher frequency ringing I'm seeing is from some other source?

author: xavier60
author_rank: Super Contributor
timestamp: October 12, 2025, 08:15:17 am
content: This online calculator can be used,https://www.omnicalculator.com/physics/resonant-frequency-lc#google_vignetteI tested one of my ignition coils using a 1uF capacitor which actually measures 0.93uF and got 1.7KHz resonance, indicating 9.4mH inductance.Using the current ramp method gave 8mH. I set the PSU to 12.8V to compensate for the 1.6Ω of total series resistance. Im not even certain if this is a valid way to compensate for the resistance.I didn't ring the LC circuit, I drove it continuously via a 1K resistor, adjusting the frequency until a resonance voltage peak was found.

author: Circlotron
author_rank: Super Contributor
timestamp: October 12, 2025, 10:43:25 am
content: Quote from: xavier60 on October 12, 2025, 08:15:17 amI'd say no because the resistance causes a voltage drop that starts at zero and increases as the inductor current ramps up.

author: Circlotron
author_rank: Super Contributor
timestamp: October 12, 2025, 11:02:11 am
content: Quote from: floobydust on October 11, 2025, 10:08:15 pmMSD advertising simply does not tell the truth. For example, have a look at their graph of coil output voltage and current while multi sparking. Theblacktrace on their drawing at the left is spark current. The duty cycle is about 50% in their fantasy land graph, but in reality it is about 12.5% as shown on the right. 400mA peak, 120uS duration, 960uS repetition rate.Also, the spark energy of their 6AL for example is quoted as 100mJ, but when you take coil losses into account, depending on the actual coil, it ranges from 40mJ down to as low as 25mJ. You can prove this yourself. With a CDI, as you decrease the spark gap neither the spark current or the spark duration increase despite the voltage across the gap decreasing. Where does the extra energy go? Coil losses.

author: xavier60
author_rank: Super Contributor
timestamp: October 12, 2025, 11:38:56 am
content: Resonance is 970Hz with 3.77uF giving 7.14mH. I suspect that secondary self-resonance is messing with measurements.I did another ramp test with the PSU set to 21V but calculating with 20V. This time, 7.5mH.The secondary self-resonance can be seen imposed on the current ramp.

author: injb
author_rank: Regular Contributor
timestamp: October 12, 2025, 03:58:36 pm
content: Quote from: xavier60 on October 12, 2025, 11:38:56 amDoes "self-resonance" mean it is resonating with its own capacitance instead of the one in my circuit?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 12, 2025, 05:20:35 pm
content: Quote from: injb on October 12, 2025, 03:58:36 pmMaybe something here will help.https://www.eevblog.com/forum/beginners/measuring-the-self-resonant-frequency-of-an-inductor/msg5122983/#msg5122983

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2025, 05:41:54 pm
content: Quote from: injb on October 12, 2025, 03:58:36 pmYes, large coils have considerable capacitance between the windings leading to low self resonance frequencies, and there can be more than one self resonance. Ignition coils have a lot of windings on their secondary and because of space constraints they are wound closely.

author: injb
author_rank: Regular Contributor
timestamp: October 13, 2025, 12:43:42 am
content: I tried using a function generator with an LC circuit. First I tried the same small pcb inductors I previously tested with, and used a 1nf capacitor. Then by sweeping the frequency by hand and watching the strength of the output, I was able to find peaks at the resonant frequencies and calculate the same result I got before, very closely matching the component tester. But I noticed that they have multiple peaks, and the strongest one doesn't always correspond to the true inductance!I tried the ignition coil in this same circuit, and at first I was disappointed to find exactly the same result as with the previous test - the strongest response by far was up in the 300Khz range, corresponding to an implausibly low inductance. But then I tried more slowly and with a smaller voltage range, and sure enough I could see a subtle but repeatable spike in the signal strength around 66Khz on one coil I have, and around 59Khz on another one. Those numbers give me 5.8mH and ~7mH respectively. So these must be the real values of the 2 examples I have.One thing I am not sure about: I coupled the signal into my LC circuit with a 100pf capacitor because that's I was using to couple the pulse generator for the ringing test from the video I linked. But someone here said they use the frequency sweep with a 1k resistor. I couldn't spot the "real" spike that way (though the higher one around 300Khz was still very clear). I'm not sure why. I tried bigger capacitor values and the overall signal is stronger but the spike is not any clearer.What's the best way to do this?

author: xavier60
author_rank: Super Contributor
timestamp: October 13, 2025, 01:22:01 am
content: Like I mentioned before, I got different calculated inductance values at different resonant frequencies.The closest to what I think it should be was when the resonance was close to 1KHz with 3.77uF of measured capacitance. I have been using combinations of mains X rated capacitors which I then measure with a Fluke 87V.I also switched to a 2.7K resistor for a higher Q and more distinct peak.If using a square wave, it's possible to get resonance when the function generator is set to certain subharmonics. The actual frequency displayed on the DSO should be of the real resonance frequency.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2025, 01:55:03 pm
content: Quote from: Circlotron on October 12, 2025, 11:02:11 amThose graphs look like something they put together to explain to the car people how it works. I'm not sure if there was much of a spec for my ignition.I would assume if they spec the energy, they define how it was measured. Maybe not. From my experience most car guys won't care or have any way to measure it.How are you measuring that 400mA peak current?

author: injb
author_rank: Regular Contributor
timestamp: October 13, 2025, 04:26:59 pm
content: OK so it looks like I'm able to get different inductance values with the signal generator approach depending on the capacitor value I choose for the tank circuit, so that's not much help.I tried a different approach - I took a closer look on the scope to try and capture the voltage drop.Here's what I came up with. The blue trace is the coil ground side, with that probe's ground on the ECU's ground. So this should be the actual charging voltage right? The red trace is the coil current.It looks like the average voltage during charging is about 12.5 - 12.6v. It's pretty symmetrical so I'm assuming I can take this middle value as the voltage, 3.2ms as the charge time to 9A, and work out the inductance from that right?With this I get about 4.48mH. Please let me know if I'm interpreting anything wrong here. Thanks again everyone!

author: al777
author_rank: Contributor
timestamp: October 13, 2025, 04:33:05 pm
content: Hi folks, thanks for the fascinating reading about ignition coils! Just in case the original poster's question still stands - my 2 cents on measuring inductance across a broad range of inductors. Some time ago I came across Elliott's Project 250:https://sound-au.com/project250.htm. I was primarily interested in testing inductors for their core saturation current (e.g. Class D amplifier modules from AliEx, sometime inductors are undersized, etc.), but it also serves well to measure/calculate the inductance. My implementation is almost exact copy of Project 250, I only added a small Schottky at the output to suppress the negative spur and also added 27Ω at the power input for slow-charging, so that it can work from a small 12VDC PSU. Also added a green LED, parallel to capacitors, in series with 10K and a 9V1 Zener - it indicates that the device is "ready to pulse" :-). See the photos. After finding that it's convenient to use for inductance measurement as well - I added a label on its side to remind me the formula when I need it :-). I use 44000uF total and when shorted - the max current pulse is 60A, IRF540Z handles it well (when pulse length is kept <100μS :-) ). The "comb" of contacts at the top panel is such that I can solder various size of inductors under test (any point on the left "wing" - any point on the right "wing"). I'll keep reading about the ignition coils now :-).

author: xavier60
author_rank: Super Contributor
timestamp: October 13, 2025, 06:07:37 pm
content: The voltage at the + coil terminal could be dropping while the current is ramping so needs to be measure also.The downward spike when the transistor turns on is unusual. Has the probe had its frequency compensation adjusted?

author: floobydust
author_rank: Super Contributor
timestamp: October 13, 2025, 06:41:01 pm
content: From the Picoscope traces, the blue coil(-) trace shows the "switch" voltage drop increase a lot above 8A. At 9A it is around 4V drop! OUCH.Not sure what is crapping out - the power supply, or the ECU ignition driver. Or test equipment, probing technique etc.I would say it's not good for the ECU lifetime to be current-limiting like that, it's a lot of peak heat generated in some silicon. The dwell table seems to be for a higher inductance coil?What coil is this? Stock or aftermarket etc. The vehicle is not setting any OBD trouble codes?

author: floobydust
author_rank: Super Contributor
timestamp: October 13, 2025, 06:57:37 pm
content: "400mA" seems way up there, but where, when? Including the coil, plug wire, spark plug resistance I'm not a believer, seems like MSD specmanship in a theoretical situation.MSD: "... when used with an MSD 6 Series Ignition, the Blaster SS produces 300 milliamps of current and maximum voltage output of 40,000 volts!""The CDI spark burn time has a much shorter duration than MDI at about 200us versus 1ms or more for the MDI system. However the peak currents are much higher at around -140mA for the first negative peak." Dr. HoldenI use the SAE J973 jig. They have an Ignition Standards Committee. I was designing igniters for big burners and furnaces, not vehicles.For measuring spark current/energy - it's a 1kV string of zeners and a 150Ω sense resistor to GND. For a car, distributor air gap is another 500V, so around 1,500V burn plus resistive losses.Silicon Chip Magazine 201502, Pt.203did an good project: Spark Energy Meter by Dr. Hugo Holden and John Clarke. "... it measures the spark burn time energy".Included the spark plug's 5kΩ resistor plus series string of 30 100V 5W zeners 1N5378B for +/-1,500V, and 150Ω 5W sense resistor. Does not use an MCU so a discontinuous integrator with CMOS switches.I first got pissed off at MSD because tearing it apart, their big red box is mostly air. Enough with the hollow, oversized monster please.Then bench testing it, the multispark was nothing special, crapped out above mid-RPM. I gave it away to a charity thrift shop.Today, some MSD ignition coils, they want $350- which is absurd. Even if they are made in El Paso TX, not Mexico or china. HVC-2 8261.MSD Group - Mr. Gasket, Accel, Superchips, Edge Products, Racepak, Mallory Ignition, Hays, QuickTime, and Lakewood. Then Holley acquired MSD."Face it, anything having to do with electronics is a tough business to be in with hundreds of raw material components, and it’s nearly impossible to build a product out of 100% USA components. You can be assured that if you do have an MSD coil that is made in China or in the United States, we will stand by its performance and provide you with the same support as any other MSD products."We’ll see you at the races -"Russell Stephens{former} PresidentMSD IgnitionEl Paso, TXWell, that President is no longer at MSD, he's at some cheap chinese ignition parts companypertronixbrandsI look forward to using an Alibaba coil, straight off the junket. Like "Patriot" aluminum cylinder heads out of china. Sigh.

author: injb
author_rank: Regular Contributor
timestamp: October 13, 2025, 07:08:30 pm
content: Quote from: xavier60 on October 13, 2025, 06:07:37 pmI assumed a drop on the + side was very unlikely because that side of the coil is wired to the battery + via the ignition switch. I'll have to double check the probe though, it's a 100x probe I don't use very often and it might not be compensated properly.Quote from: floobydust on October 13, 2025, 06:41:01 pmNothing is crapping out, that's the ECU's 9A current limit kicking in. The whole system is stock and this car doesn't have OBD.

author: floobydust
author_rank: Super Contributor
timestamp: October 13, 2025, 07:56:09 pm
content: Quote from: injb on October 13, 2025, 07:08:30 pmCan you tell the year and engine details. It's a 4 or 6 cyl wasted-spark (2 or 3 coils) turbo, what redline, it has MAF and MAP sensors?I know an ECU expert and will ask but it would be later next week to hear back.I would say you're not going to get more spark energy 10A or 11A out of the ECU without triggering an over-current fault or smoking the driver.There is a limitation there and I think we are already seeing it play out at 9A. That leaves increased primary inductance- but needs greater dwell time at high RPM that is likely not there.I don't see dropping in a high performance ignition coil is going to work, beyond human psychology thinking it's better, faster.For measuring the coil's inductance, using the current waveform slope (before the knee point) I think works. The part where the driver is going into constant-current mode, leave it out, the coil is starting to saturate there. You can zoom in even a few amps is a place for the datapoints, not end-end.

author: injb
author_rank: Regular Contributor
timestamp: October 13, 2025, 08:42:10 pm
content: Quote from: xavier60 on October 13, 2025, 06:07:37 pmYou were dead right about the probe compensation, thanks! I fixed that and the negative spike is gone. But it doesn't change the calculation, I'm still coming out with roughly the same value.Also I did measure the drop between the ECU ground and the battery ground separately. When the coil current is flowing, the ECU ground is pushed up around 100mv above battery ground. I still need to investigate whether that's an issue for the ECU or not (maybe I should clean my ECU ground connection) but it doesn't make an appreciable difference to the coil charge time.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2025, 09:36:37 pm
content: Quote from: floobydust on October 13, 2025, 06:57:37 pmI'll have to see if I can find that SAE spec. When looking through the plug, I would expect several Amps peak. They must not really be measuring the peak but integrate over time?

author: Circlotron
author_rank: Super Contributor
timestamp: October 13, 2025, 10:44:35 pm
content: Quote from: joeqsmith on October 13, 2025, 01:55:03 pmTheir energy spec is from the 1uF dump capacitor being charged to 450VDC. Trouble is, among other things, the MSD6 boxes have a diode going from coil pos to battery pos to demag the coil after the dump cap hits zero volts and the coil rings upward. A certain amount of energy is then returned to battery instead of being transferred through the coil.Lets assume the coil has a secondary resistance of 5000 ohms. 400mA secondary current means a 2000V drop internally in the secondary winding, and seeing that 2000V is in series with the typical 1500V across the plug gap while "arcing" (actually a glow discharge) we can see right away that efficiency is down the toilet.The 400mA spark current from a MSD6 CDI was measured using a 100R resistor to ground in parallel with 33nF to tame really high frequencies. Spark sent to top of this RC combination. I have a scope screen shot of 40A primary and 400mA secondary using a typical 1:100 coil that I will post later.

author: injb
author_rank: Regular Contributor
timestamp: October 13, 2025, 11:36:57 pm
content: Quote from: xavier60 on October 13, 2025, 06:07:37 pmI measured the + side drop separately (it was too awkward to do it all at once). During the charge time it seems to be averaging around 400mv or so. That drops my inductance calculation down to 4.26mH.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 13, 2025, 11:39:44 pm
content: Quote from: Circlotron on October 13, 2025, 10:44:35 pmThat's not peak current then. Everything in the peak will happen in <1ns. Fast scope.I don't have any details about the MC4 beyond what is in the patents. I do have a test jig along with their coils if there is anything you would like to see measured.

author: xavier60
author_rank: Super Contributor
timestamp: October 13, 2025, 11:40:04 pm
content: Quote from: injb on October 13, 2025, 08:42:10 pmTo minimize the effect of the resistance, base the inductance calculation on a smaller part at the start of the ramp, say 0 to 2A. Assuming that accurate cursor measurements can be made.

author: injb
author_rank: Regular Contributor
timestamp: October 14, 2025, 12:21:54 am
content: Quote from: xavier60 on October 13, 2025, 11:40:04 pmThis was a great idea - you made me realize I had made a mistake earlier. I overlooked the fact that the current limiting seems to start well before 9A, and so the inductance value I calculated must be high. So I tried using shorter intervals, well before the non-linear part of the voltage drop and I'm coming up with 3.2-3.3mH consistently.Interestingly this is actually around what I was getting this morning with the LC circuit and signal generator, using a 4.7uF capacitor and a 2K resistor. I was getting a peak around 1.3Khz which comes out very close to this...but I didn't trust it because it didn't agree with the measurements of known PCB inductors I have.Anyway I think I have now I have accounted for all voltage drops so this has to be the close to the right number.

author: Circlotron
author_rank: Super Contributor
timestamp: October 14, 2025, 12:29:04 am
content: Quote from: joeqsmith on October 13, 2025, 11:39:44 pmYeah, I know. I'm disregarding that part because not very much energy is contained there AFAICT.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 14, 2025, 12:32:11 am
content: Quote from: Circlotron on October 14, 2025, 12:29:04 amThink of an ESD event. Everything is over in that first few ns.

author: Circlotron
author_rank: Super Contributor
timestamp: October 14, 2025, 12:33:13 am
content: Quote from: Circlotron on October 13, 2025, 10:44:35 pmYellow is MSD6 CDI 40 amp coil primary, cyan is 400mA coil secondary.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 14, 2025, 02:15:01 am
content: So the output of your coil is directly driving the 100ohm in parallel with the 33nF, no gap? You then place the probe across that RC network, right into the scope? And, the scope survived?If this is all correct, I wonder what voltage you are seeing?

author: floobydust
author_rank: Super Contributor
timestamp: October 14, 2025, 02:23:19 am
content: That's such a short duration maybe 100usec event but on-par with CDI.If this (MSD-6) is a typical SCR CDI, I have seen the SCR retriggers multiple times on the ringing from the coil primary. The diode to B+ might be to stop that.

author: Circlotron
author_rank: Super Contributor
timestamp: October 14, 2025, 04:11:39 am
content: Quote from: joeqsmith on October 14, 2025, 02:15:01 amSlight mistake on my part. Had a look at the test jig just now. There is ~1700V of zeners to simulate a spark gap, then the parallel RC. R=100 ohms so 40V max.

author: Circlotron
author_rank: Super Contributor
timestamp: October 14, 2025, 04:14:38 am
content: Quote from: floobydust on October 14, 2025, 02:23:19 amDiode is D7 on page 3 of the schematic.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 14, 2025, 07:08:18 pm
content: Quote from: Circlotron on October 14, 2025, 04:11:39 amAt <1ns, seems like you are balancing the parasitics of the fixture. A little higher capacitance in the diodes and a little more inductance in the RC.... bang... Do you have something else in the chain to prevent damage to the scope? Is the circuit you are using from that IEC standard that was mentioned? Can you show a photo of your fixture?