CDI capacitor - Page 1
Forum section: electronics / beginners

author: NMNeil
author_rank: Contributor
timestamp: February 03, 2017, 04:43:12 am
content: I've been looking at a variety of circuits for a capacitor discharge ignition system. They are all similar in that a DC-DC converter is used to raise 12 volts to 250-300 volts by inductor flyback from a transformer. The high voltage is then stored in a 1uF capacitor and at the appropriate moment discharged through the coil. The only problem is that none of the circuits are very specific on the type of high voltage capacitor, electrolytic, film etc.. Any ideas which would be best and why?Learning all the time.

author: helius
author_rank: Super Contributor
timestamp: February 03, 2017, 05:21:59 am
content: The ESR of the final capacitor will affect the damping of the output pulse. The C of the cap and L of the coil form a LC tank circuit that oscillates through several cycles each time a pulse gets triggered: the ESR of the capacitor damps that oscillation.

author: Gyro
author_rank: Super Contributor
timestamp: February 03, 2017, 09:28:17 am
content: Certainly not an Electrolytic. Not even a photoflash one would withstand thousands of discharges per minute! You need a film type.You need to pick Polypropylene one that is designed for pulse application (decent thickness metalization), iirc Epcos do some nice ones. The ultimate would be film and foil, but that would be very large at 1uF.

author: NMNeil
author_rank: Contributor
timestamp: February 03, 2017, 02:50:15 pm
content: Quote from: Gyro on February 03, 2017, 09:28:17 amThanks for the advice.I have a fair selection of tantalum, ceramic and electrolytic in 1uF plus sizes but they just had the 'look' that they would be too small to hold whatever the joule equivalent of 50 plus watts is.

author: jpanhalt
author_rank: Super Contributor
timestamp: February 03, 2017, 03:09:04 pm
content: There are several threads on the modelers' forums for CDI ignitions. They (and I) usually use a Mylar (polyester) capacitor for that. The other major problem is unwanted and destructive feedback to the microcontroller that is used to provide spark advance/retard.Here is a link to a thread on RC Groups:https://www.rcgroups.com/forums/showthread.php?1781959-Programmable-Open-Source-CD-Ignition-PIC1840&highlight=diy%20ignitUnfortunately, the threads become quite lengthy and typically do not a conclusion.What sort of engine and RPM will you be aiming to get? Getting up to 10,000 rpm is a common goal. But those involved in boat racing often need more.John

author: Seekonk
author_rank: Super Contributor
timestamp: February 03, 2017, 03:12:49 pm
content: I would use a snubber capacitor made with polypropylene rated for current. This has to pump a lot of current and the welds of many capacitors fail in this mode.

author: Gyro
author_rank: Super Contributor
timestamp: February 03, 2017, 07:33:43 pm
content: Actually an old CDI kit I had on one of my early cars used a Rifa metalised paperIt lasted several years but did eventually expire (using the catch fire method), taking most of the PCB with it. Luckily the CDI had a revert switch which got me home. A terrible mess when I got round to taking the lid off!

author: NMNeil
author_rank: Contributor
timestamp: February 04, 2017, 02:34:25 pm
content: Quote from: jpanhalt on February 03, 2017, 03:09:04 pmIt's going to be fitted to a 4 cylinder side valve 1949 Ford 8N tractor. 0 to 20MPH in about a month and the revs limited to 2000, so it's not exactly a fire breathing monster. I'm keeping the points and mechanical bob weight advance for the moment because as a newcomer to electronic experimentation I want to go one step at a time and learn while I'm doing it. With all the equipment and components I have bought over the past year or so I could have bought an off the shelf complete ignition system for a tenth of the price, but that defeats the object of the hobby.Once I understand the capacitor requirements for the circuit I'll be moving on to learning about step up transformers and the needed calculations for winding my own.Learning all the time.

author: jpanhalt
author_rank: Super Contributor
timestamp: February 04, 2017, 02:54:01 pm
content: Sounds like a fun project -- at least if the CDI fails, you won't be far from home.There are a fair number of those Ford tractors where I live. New Holland came out with a "retro" 8N model a few years ago that looks a lot like it (http://www.tractor.com/manufacturers/new-holland/new-holland-boomer-8n-review-1455.html). Apparently, it is diesel, so that won't help. I suspect you have seen it.Don't worry about winding the step-up/oscillator coil (not the HV ignition coil). I have wound a few. All you need is a shuttle that will fit the hole in the toroid to put your wire on. My experience, however, is limited to single cylinder and boxer opposed twins (2 stroke).John

author: Gyro
author_rank: Super Contributor
timestamp: February 04, 2017, 03:21:04 pm
content: QuoteThat's where keeping the points helps, you can revert in an emergency, even if you only do it by pulling and swapping a couple of wires. I wouldn't fancy pushing a tractor home over even a small ploughed field.

author: NMNeil
author_rank: Contributor
timestamp: February 04, 2017, 03:32:30 pm
content: Quote from: Gyro on February 04, 2017, 03:21:04 pmThanks for the link to the retro 8N, but the pension won't stretch to that I'm afraid.Part of the learning is how to choose components, like the capacitors and the transformer and how to match each component to each other. It's still a struggle to learn but sometimes the mist clears and I grasp a concept either completely or completely enough for my purpose.

author: Seekonk
author_rank: Super Contributor
timestamp: February 04, 2017, 04:17:33 pm
content: I have old boats and the best ignition system I have found is MSD. That multiple spark discharge works great on those old flat heads, It stops making multiple firings at about 1,000 rpm. I can run that engine in gear at 250 rpm for hours without fouling. Have one of the Mark 10 CDI on a lawn tractor. The coil went bad and they drive off the front of the engine. Fortunately the points are on the back side of the engine. Best starting engine.

author: SeanB
author_rank: Super Contributor
timestamp: February 04, 2017, 05:23:16 pm
content: An alternative to a single 1uF capacitor is a few smaller size units in parallel. 3 470n units will be roughly twice the size of the 1uf unit, and will have slightly better pulse capacity as well, just from the reduced current per capacitor. 10 100n caps will work as well, but will be a lot bigger, but at least there you can use class X capacitors which are cheap, and have quite reasonable pulse capability plus will mostly fail open circuit.

author: NMNeil
author_rank: Contributor
timestamp: February 04, 2017, 05:59:44 pm
content: Quote from: SeanB on February 04, 2017, 05:23:16 pmI'll have to read up on doing that. As I said my knowledge is improving albeit slowly.I'm not sure about referring to a component supplier because it may be construed as advertising, but a well know supplier has 1uF Polypropylene film AC and pulse capacitors with a voltage rating of 250AC and 520DC for just under $1 each.If the moderators give their OK I'll post the datasheet and ask opinions if these would be suitable, just to verify my calculations.

author: SeanB
author_rank: Super Contributor
timestamp: February 04, 2017, 06:48:10 pm
content: Post it, it will be helpful.

author: NMNeil
author_rank: Contributor
timestamp: February 05, 2017, 04:33:17 am
content: Quote from: SeanB on February 04, 2017, 06:48:10 pmThankshttp://www.mouser.com/ds/2/212/KEM_F3030_R71-540901.pdf

author: Seekonk
author_rank: Super Contributor
timestamp: February 05, 2017, 11:35:24 am
content: That looks good and it only takes $15 to ship it.

author: NMNeil
author_rank: Contributor
timestamp: February 05, 2017, 01:28:00 pm
content: Quote from: Seekonk on February 05, 2017, 11:35:24 amI missed that.The bay of all things here I come.

author: senso
author_rank: Frequent Contributor
timestamp: February 05, 2017, 02:45:55 pm
content: I have designed a couple CDI ignitions that use ceramic 1 to 4uF caps, rated to 500V, they cost around 2-3€ from Mouser, and the low ESR helps with the spark strength,the best results where achieved with 2 1uF caps in parallel.

author: NMNeil
author_rank: Contributor
timestamp: February 05, 2017, 03:48:45 pm
content: Quote from: senso on February 05, 2017, 02:45:55 pmThanks for that.Many of the circuits I looked at specify 1uF but is there any reason why a larger uF value with the same voltage range can't be used?