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custom ignition system voltage booster circuit problem

Tim Iacobucci

Jun 30, 2012
Jun 30, 2012
Hello my name is Tim Iacobucci. I am trying to build a simple voltage booster circuit and I cannot get it to work the way I need it to.

Before I explain the circuit I am using let me explain the ignition problem I am trying to solve in case I am just going about this all wrong in the first place when there may be a more appropriate or simple solution to my problem.

I am uprgading the ignition system on a turbo engine as I have exceeded the capacity of the stock distributor based system. I already have an aftermarket engine management system on the car called a 4424 stinger made by an Australian company called EMS. I can easily set this to run fully sequential ignition coils to replace the single coil and distributor system. I have some pretty hot coils from a gm ls truck engine. These are considered "smart coils". They feature built in ignitors which only require logic level input from the engine managment system to charge and fire, they also feature overdwell protection.

The 4424 system has a feature where it automatically sets cranking dwell to 50% duty cycle. On a limited single coil distributor or wasted spark setup this is not very long, but on a full sequential system the coil only fires once every 2 revolutions. At slow cranking speeds this is quite a long time relative to the several milliseconds the coils need to fully charge. This ems "feature" interferes with the smart coil overdwell protection "feature" because the coils charge way too soon during cranking and discharge when the overdwell protection is triggered. The cranking timing is crazy advanced and the engine will not start and fires back against the starter.

The coils are fed a direct 12v source to charge but are triggered by a separate logic level input. The dwell time the coils auto fire at are different depending on the voltage supplied. The stock gm dwell tables show 30+ ms at 6v vs 3-4 ms at 12v. So I think the overdwell citcuit is amperage based or somehow directly detecting core saturation, i'm not really sure but I am fairly positive it is not based on time alone.

My idea was to set a relay active during cranking with a resistor in line to reduce the current feeding the coils to slow the time it takes them to charge to combat the crazy dwell times during cranking with this ems that I cannot change.

This did work but the resistor cut the voltage down along with the amperage. Ohms law duh, I know. The coil is just a transformer and simply multiplies the input voltage to get the high voltage spark. With less than 6v going into the coils the spark is very weak or it just doesn't fire at all.

I am looking for a way to limit the amperage charging the coils but keep the voltage to 12v. My idea is to use a resistor then a voltage booster to put the lower current back to 12v after the drop.

I have tried a simple booster circuit using a 555 timer.

This worked well but under load the useful amount of voltage boost was small. So then I began looking for an actual dc transformer circuit

The first part of that circuit was the next thing i tried. Stopping after the transformer output. I used a small 10:1 transformer and it works very well. I can really cut the input voltage pretty low 1-2v and still get over 100v output. I can tune the output with the resistor values but again the output voltage tanks to less than 1v with any kind of load on it. I am using a 0.14 amp 12v case fan as a test load.

Am i doing something wrong with these circuits that they cannot really drive a load at higher voltages or am I just using the wrong tool for the job altogether?

I know a good bit about computers and about mechanics. The electronics bits in between I am not so experienced with. If there is more I need to learn or experiment with please make recommendations or suggestions about what I need to research. I don't mind facing what I don't know and need to learn but I'm not really sure what direction to even go from here.

I know the simplest thing would be to use different coils, but they are wired up and work perfectly and the voltage booster idea and the circuit seems to work too. I feel like it is close but I am still missing something and if someone with some real electronics knowledge would look at it they could figure it out fairly easily.