Wouldn't adding a flyback diode to a flyback or ignition coil reduce the v out? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: April 30, 2023, 10:40:35 pm
content: In older car ignition systems the coil is switched with a mechanically actuated contact, there's a 0.5uF cap across the primary of the coil (I don't know the inductance), the primary is connected to the positive rail +14V, the capacitor is connected from the other primary terminal to ground, and the contacts are shorting the capacitor+primary terminal connection to ground to charge the primary coil and then release it, resulting in a high voltage generated in the secondary from the cap+coil LC resonant circuit.In later ignition systems the contacts where changed to a magnetic Hal-sensor and usually a triac is used to switch the primary of the ignition coil, with a flyback diode usually across the primary.So the question now, wouldn't the flyback diode greatly limit the output voltage? and also shorten the output pulse length?If so, is there some way to not do this but still have fast electronic switching?Maybe instead putting a TVS diode across the triac to limit the max voltage switch below the max rating of the triac?

author: Circlotron
author_rank: Super Contributor
timestamp: April 30, 2023, 11:26:35 pm
content: Having a diode across the coil primary most definitely will not work. You don’t want to clamp the primary voltage spike like this. If you do, the secondary will produce nothing useful. You can beneficially put a capacitor across the switch, be it contact points, transistor, IGBT, mosfet (triac will latch on permanently and not work) and size the cap according to the primary current and the maximum voltage the switch will tolerate. The primary voltage across the switch and cap will rise and then fall in a half sine as the primary leakage inductance resonates with the cap. The higher the switched current the higher the voltage spike so the larger the cap needs to keep the spike under control. For those that say this cap is unnecessary with a solid state switch, the benefit of having it is it captures the leakage inductance energy and transfers it to the secondary, other wise it would simply be lost as heat in a clamp zener. Adds about 10% to the spark energy. Have a look at the secondary current with a scope and see how it adds to the initial current spike. Current, not voltage.Another useful thing is to have a reverse diode across the switch. Always done with a solid state setup, but is also beneficial with contact points. Reduces arcing a little and lengthens spark duration just a little. One last thing, if you have a points plus ballast resistor, put a 10,000uF electrolytic cap from coil positive to ground. Smartens things up quite a bit at high rpm.Edit -> Yes, TVS diode across the switching device for sure.

author: Circlotron
author_rank: Super Contributor
timestamp: May 01, 2023, 01:14:53 pm
content: Quote from: ELS122 on April 30, 2023, 11:51:39 pm2mm plug gap is way cool! I've done the same in the past.I was meaning a diode that is across the switch (points, transistor, whatever) that stops the coil neg voltage from swinging below ground. A mosfet body diode works great for this. I meant that if the diode is across contact points is slightly reduces the unwanted arcing on them. It actually adds slightly to the secondary arc duration.

author: langwadt
author_rank: Super Contributor
timestamp: May 01, 2023, 04:02:45 pm
content: ignition coils usually have a ratio of ~1:100 so it relies on the primary flyback to a few hundred volts to produce tens of kilovolt on the secondaryIGBTs/mosfets made for driving ignition coils are usually made to clamp the voltage at ~350-400V, limiting the secondary voltage to protect the coil in case of a bad ignition wire or similar