Ignition Coil Won't Spark from Mosfet but DOES spark By Hand??? HELP!?

[pha3z]

I'm trying to get an ignition coil to create sparks running it with a Mosfet powered by a 555 timer.   The Mosfet is definitely switching.  I checked it with resistor, ampmeter, and o-scope.  Everything is behaving predictably but when I use it to power a car Ignition Coil, I don't get any sparks.  I can tap the terminals of the Ignition coil by hand -- Even with it connected up to the mosfet circuit -- and get sparks.  But when I try to let the mosfet drive it, there are no sparks.  There is definitely current flowing.  And if I change the frequency to super low (like a pulse every second), the ampmeter is registering the pulses.  If I change the frequency to super high (like 100khz), I get averaged current flow on the ampmeter.  But I don't get sparks!!!!!  I also switched out mosfet with other kinds (different ratings).  Didn't fix it.

Anyone else ever run into this problem???

Here's a schematic similar to my setup:
http://www3.telus.net/chemelec/Projects/Car-Coil/Car-Coil-1.png

Hope someone else has had this problem before and can help!!

Thanks,
Jim

[Rafael Ti]

I would advise you to use a driver between pin 3 of 555 and the gate of MOSfet. It looks like the output resistance of 555 is to big to drive the MOSfet correctly due to capacity G - S, and this is why u can't get sharp enough pulses. Check the TC4422 for instance... You can also try to increase the capacity of 1uF tantalum capacitor shown on diagram to say 100uF,or more, but it rather won't work.
The other reason could be that the Rds on of the transistor is to high comparing to impedance of coil and the curent you have is not enough to spark...

[TinselKoala]

A mosfet driver will help, but I think the turn on- turn off time of the 640 is too slow anyway. Try a 510, and try the same circuit with a 2n3055 on a heatsink.
I used a 2n2222a to drive the gate of the mosfet in the flyback version of this circuit. Be careful !! The ignition coil can startle you but the flyback version can kill.

http://www.youtube.com/watch?v=VNL8QTS0sM0

[TinselKoala]

Ignition coils don't work like that they are too of the flyback type just as regular CRT flyback transformer meaning they spark on field collapse.
So you slowly charge up the coil/magnetic field and when you switch it off the field suddenly collapses and this generates the high voltage for the spark.
In older setups a capacitor was charged and then discharged into the 0 ohm primary from the ignition coil which are configured as auto transformers and this worked very well.
It's called CDI for capacitor discharge ignition so you might want to take a look at that type of circuitry.

Ignition coils don't work like that?
Well, then..... I must have made some mistake.... because in my video above I show an ignition coil WORKING LIKE THAT. Don't I? And I even give the schematic so anyone with an ignition coil and a Radio Shack can build one and see whether or not ignition coils work like that.

The main differences between the ignition coil version and the flyback version is that the ignition coil has a heavy iron core and generally is driven at a frequency far from its resonant frequency, so all you get is voltage amplification by turns ratio. The flyback version uses a user-installed 10-turn primary, has no heavy iron core, and if done properly works at or close to its resonant frequency. So you get voltage amplification by standing wave resonance, like in a Tesla coil, and can make more dramatic sparks... until you fry the flyback transformer.

A capacitive discharge ignition system doesn't even HAVE an ignition coil. That's the whole point.
http://en.wikipedia.org/wiki/Capacitor_discharge_ignition

[TinselKoala]

The transformers used in CDI systems are very different from ignition coils.

The OP of this thread asked for help with an ignition coil spark system using a 555 timer.  Are you being helpful?

Let's see your video demonstration of your circuit.