Creating an electronically controlled spark gap - Page 1
Forum section: electronics / projects

author: spacedementia87
author_rank: Contributor
timestamp: February 13, 2017, 10:38:53 am
content: Hello everyone, I am a hobbyist when it comes to EE and I am trying to design a project that has a spark controlled by a microcontroller.I have worked out that for a 1mm gap I will need about 1kV and so I have designed this circuit in LTSpice:I seem to be getting the required 1kV spikes across my inductor and the current is much less than I expected...Would I need something protecting my power supply? I am getting significant (~600mA) of current flowing back into the 5V supply...

author: Zero999
author_rank: Super Contributor
timestamp: February 13, 2017, 12:14:30 pm
content: I doubt this would work that well in real life.The parasitic capacitance in the coil will make 1kV unlikely.You should add another winding to make a flyback transformer.

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2017, 12:28:59 pm
content: You aren't going to get reliable sparks with a 1mm gap and only 1KV, even with needlepoint electrodes. Seehttp://onlinelibrary.wiley.com/doi/10.1002/9780470495070.app3/pdfA better approach would be to use a car or motorbike ignition coil, and copy the primary drive circuit from one of the many DIY electronic ignition projects that can easily be googled.You could also get there with a line output transformer recovered from a 12V CRT monitor or TV, but you'd have to carefully limit the energy applied to the primary to prevent transformer and/or drive transistor breakdown as it was never intended to drive a spark gap.Stay Safe, and please screen the HV wiring and spark gap to avoid annoying Hams and AM radio listerners.

author: spacedementia87
author_rank: Contributor
timestamp: February 13, 2017, 01:33:04 pm
content: Thanks for the replies!I did think that sounded too good to be true!I have looked at auto ignition coils, would these be appropriate? Hard to tell as I know nothing about cars/bikes and they come with very few specs:http://www.ebay.co.uk/itm/252533090901http://www.ebay.co.uk/itm/361902382388Then I guess I would attach the coil in the same place as the inductor in my diagram, probably also have a capacity to ground too?

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2017, 02:38:37 pm
content: You must get a coil suitable for a traditional 12V Kettering (contact breaker points) ignition system, (or transistor aided points system) not solely for a magneto system or an electronic CDI system. CDI systems are designed to pulse the primary with hundreds of volts, and Magneto systems are designed to have the energy stored in the coil by induction from the flywheel magnets, not by applying DC to a primary winding. While a Magneto coil could theoretically be used if you completed the magnetic circuit of the core, the primary is not intended for DC drive and finding an effective voltage and duty cycle , without burning it out would require cautious experimentation.A car coil (coil in a can) will almost always be for Kettering or CDI but a motorbike or other small engine coil may be for any of the possible systems. If the core is a straight bar through the coil with a bolt hole either end you *dont* want it. The type you want has a single HT lead and two LT terminals for the coil, though you may be able to use a single LT terminal coil that uses the core as the common connection if you dont mind making up an isolated mount so you can put 12V on the core.Don't use a capacitor directly on the coil primary, it will seriously stress the switching transistor. Instead, use a fast HV diode to route the back EMF spike into a small cap with a bleeder resistor to discharge it to the +12V rail. The capacitor should be chosen to keep the primary voltage under the max rating of the switching transistor, and the resistor to discharge the cap sufficiently between pulses.

author: spacedementia87
author_rank: Contributor
timestamp: February 13, 2017, 02:44:42 pm
content: ok, designing this is getting much more complicated than I expected!I am guessing something like this might be simpler for me to use?https://www.sparkfun.com/products/retired/11218

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2017, 02:55:14 pm
content: Yes. Still available from other stockists. You'll need a transistor that can switch at least 5A to control it and you MUST ALWAYS load its output with a spark gap of no more than 1/2", and MUST NOT short its output or it will die.Seehttps://www.eevblog.com/forum/reviews/spark-gap-igniter/

author: spacedementia87
author_rank: Contributor
timestamp: February 13, 2017, 02:58:51 pm
content: Great thanks,I only need the spark to work over 1mm or so, so that should be fine.I will look into getting some high current 5V Vds mosfets

author: spacedementia87
author_rank: Contributor
timestamp: February 13, 2017, 04:24:28 pm
content: I'll look into using a photoflash as I have a few lying around.I found this...http://www.spudfiles.com/spud_wiki/index.php?title=Camera_flashStill requires a beefy coil it seems. If I am just going for a gap of 1mm or so, would using a mains wall wart transformer be suitable as the output of the flash unit is much higher already?

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2017, 04:49:26 pm
content: No. If you use a wallwart transformer in reverse, its former primary wont have good enough insulation to provide the couple of KV you need. It may work for a short while but is likely to break down sooner rather than later.

author: mikerj
author_rank: Super Contributor
timestamp: February 13, 2017, 05:59:52 pm
content: Quote from: Ian.M on February 13, 2017, 02:38:37 pmPlenty of motorcycles use this type of coil with a traditional inductive discharge type ignition system, in fact pretty much all larger displacement bikes and the vast majority of cars still use inductive ignition systems. CDi tends to be used on modern small engines such as mopeds, chainsaws, lawnmowers etc.

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2017, 06:16:27 pm
content: Yes, but if it has a closed loop U+I core you *KNOW* it has to be for Kettering or CDI applications and isn't a magneto coil. Otherwise you have to be familiar with the model(s) bike the coil is used for to eliminate Magneto coils.