Input specs for modern ignition coils for 40kv capacitor project - Page 1
Forum section: electronics / projects

author: kkkwj
author_rank: Newbie
timestamp: August 25, 2018, 04:50:05 pm
content: I’m trying to figure out how to use a small ignition coil to generate short blips of high 40k voltage for a project of mine, which is to plot the storage and leakage performance of some homemade capacitors at various voltages. A science project, basically.Research done before posting:There are various examples of DC-24v-in-to-30kv-out power supplies and examples on the web, along with giant Tesla coils. But I’m after something much smaller. The only load on my project is the capacitor and its leakage current, so the amount of current put out by a spark plug (200ma?) at 40k for the length of a spark (200-300 usec) would be plenty for me.Design approachI like the idea of feeding my power supply into a little 555 timer circuit to fire a Darlington transistor or 2n3055 or something to drive the ignition coil (maybe up to 100 Hz or so (just guessing)). I don’t want to get into the hassle of buying or building big boxes, trying to find old flyback transformers, voltage triplers, or winding my own coils (yet), if possible.The big old ignition coils from my youth took 12V and maybe 5 amps as input (maybe 9V and 4amps after the ballast resistor), and generated 20-30kv for the spark. But I see some modern per-sparkplug coils on the net these days that generate 50kv (which would let my chart plot go another 25% past 40kv).Problem: can’t find input specsMy problem is that I can’t find any specs on what these nice little coils take for input (what my driver circuit would have to provide). Here are some sample output specs – Output: 40Kv-50kv out, peak current 220ma max for 300 usec spark duration; primary resistance 0.5 ohms, secondary 2k ohms, inductance 2.7mH, turn ratio 75:1.Just working back from 40kv divided by the turn ratio, I come up with 500 volts input. Or if there’s a built-in tripler or something, the input still has to be 100+ volts. I'm thinking there must be some kind of a “preamp” driver or controller between the 12v battery and the little ignition coils (one required for each spark plug) because I can't see how to get from a 12v car battery to 48kv with a turn ratio of 75:1. Maybe there are two triplers in there somewhere (12v * 3 * 3 = 100+v * 3 * 75 ratio would get close to 40kv-50kv).Questions:Two questions: (1) Can anyone explain or point me to a reference on how the circuits work for these 1-coil-per-cylinder systems work? (Typical voltages and currents along the way, if possible?) I love the idea of the small coil size on my bench rather than a big old oil-filled coil from a junkyard...(2) Does my design approach (12V->555 timer->driver transistor->coil->40kv) seem ok to you? If not, could you suggest better ways to generate a series of voltages up to 40-50kv, or references that I might read?(Remember that I don’t need sparks or current for the load – I only need enough current to charge a capacitor. It might take a few “sparks” to charge up the capacitor (ie, not completely discharged between pulses). If that is required, I have no idea of how to block back current from the capacitor to the coil since I haven’t seen any 30k-40kv diodes in my travels. Will the capacitor stored voltage be divided over a set of reverse-biased diodes in series? Ok, three questions...Thank you

author: viperidae
author_rank: Frequent Contributor
timestamp: August 25, 2018, 08:06:36 pm
content: I think they get such high output with a low turns ratio is because they don't Cosmo like clamp the primary to 12V. They use high voltage transistors and the collapsing field on the primary produces a high voltage which is then amplified by the 75:1 ratio in the secondary.No voltage multipliers required.You just get a single pulse exactly when you disconnect the primary current.

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2018, 08:34:20 pm
content: Quote from: kkkwj on August 25, 2018, 04:50:05 pmDoing it all in one shot is going to require a very high voltage output diode. I think you would be better off with a boost or flyback switching regulator driving a voltage multiplier because the voltage multiplier distributes the high output voltage over series connected high voltage capacitors and diodes. Much more power will be available because of the higher operating frequency. Or even better use the step-up transformer as part of an inverter like an old oscilloscope would do.

author: mzzj
author_rank: Super Contributor
timestamp: August 25, 2018, 08:49:40 pm
content: https://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=1025&context=auto_preshttp://www.msextra.com/doc/general/sparkout.htmlTypically 12 volt input, inductor charge time (dwell time) 3 to 10ms, peak current 10 to 20 amps. Inductive kickback is what makes the high voltage with lowish turn ratio, transistors used are typically 400 to 600 volt IGB's/mosfets

author: floobydust
author_rank: Super Contributor
timestamp: August 26, 2018, 12:33:42 am
content: I've built a few power supplies like this, not totally happy with any of them though.A Chevy car ignition coil HEI is 1:85 turns ratio with the module driver IGBT/BJT transistor zener-clamped to around 385V. This is to protect both the transistor and coil from overvoltage if there is no (spark plug) load.So you're limited to (385V*85) ~33kV unless you risk (the coil) using a higher voltage (1-2kV) transistor. A 2N3055 does not have high enough VCE to last.A pencil coil/coil-on plug has a similar internal setup. It might be cheaper to go to an auto wrecker and get a module+connector.Either way, you need an 555 oscillator to drive an IGBT or coil-on plug module, and pulse-skipping if you want to regulate the HV output precisely.Look at "15kV generator" for a few dollar unit.https://www.eevblog.com/forum/testgear/high-voltage-power-supply-(50kv)/Best luck I've had is a HV tripler with a car coil to get up to 100kV, which is scary enough for me.For HV diodes, you can series-connectmicrowave oven rectifiersor get small2CL-77or old colour TV triplers.Keep in mind HV capacitor leakage current is not well behaved, i.e. a few uA at 35kV and at 38kV it is breaking down and arcing internally. You are measuring corona current up to a catastrophic point, so I don't believe (dry) capacitor leakage current can predict dielectric integrity.

author: kkkwj
author_rank: Newbie
timestamp: August 26, 2018, 05:20:51 pm
content: Wow, thank you guys for all the help and answers. There's a lot here for me to process and many links for me to follow. I still don't think I understand the key step (circuit) between my power transistor and the input to the final coil. I wish I knew how to create nice circuit pictures like the ones I see posted here. Is there a piece of free software that does that? (It would be nicer for you guys if I could post some specific circuits, I think).On old cars from my youth, I think the coil primary (and ballast resistor) would just be in the load of the power transistor (eg. a TIP120 Darlington that can switch 5A). I could do that. (Don't know what kind of HV output I would get. Maybe the coil output would go into a voltage tripler for the final jump up.)But it almost sounds like if I want to work with one of the modern tiny coils (1-coil-per-sparkplug-style) that I need some kind of an additional controller box too. The controller box would be the load of my power transistor, and would step up the 12V to 300-500V or whatever the modern little coil needed. Is that what actually happens? If so, then I should abandon the idea of using a modern tiny coil -- I'm trying to reduce the size of everything, not add more black box controllers that I have no control over...Thank you again for your help.

author: chemelec
author_rank: Contributor
timestamp: August 26, 2018, 05:55:53 pm
content: Yes I believe they do step it up to the 300 to 500 volts for these new coils.

author: mzzj
author_rank: Super Contributor
timestamp: August 26, 2018, 06:07:03 pm
content: Quote from: chemelec on August 26, 2018, 05:55:53 pmNope, most of them just use the benefit of inductive kickback.There is some ignition systems that step up the primary voltage to ~400 volts storage capacitors but they are not that common (CDI or capacitive discharge system)(Saab Trionic 5 and 7, some old Bosch systems from -90's)

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 26, 2018, 07:32:32 pm
content: What about salvage the flyback transformer out of a big CRT TV? Those already have the rectifier, much to the annoyance of those who don't want that.