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/Proje...Car-Coil-1.png

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

Thanks,
Jim

[realmikel]

speed

high turns ratio coils ( spark coils) require time and current to build sufficient magnetic flux to generate high voltages. The orignal design used mechanical devices to operate them ( points). Have you insured that your solid state drivers are providing both? example 4 cylinder car 2000 rpm = 8000 firings per minute or 130 hertz ( roughly). In other words you could run a 12 volt 3 amp transformer to a single diode, no capacitor in circuit, hooked to the house mains in the usa and you would be in the normal range of this devices original use specifications. you could even double the speed. I am not sure it will operate at the speeds you mentioned.

[Farmhand]

Hi pha3z, I would try a few things.

First I would check the signal output of the 555 with the scope to see if it is
providing at least 10 volt pulses to the mosfet gate.

Then I would put a 0.1 uF to 0.5 uF non polarized capacitor directly across
the ignition coil primary the same as a car coil has in the car ( the standard
car one is usually about 240 nF or 0.24 uF ) .

The negative of the ignition coil primary should go towards the mosfet and the
positive to power. I not sure why the diode is between the primary and the
mosfet drain pin.

The other thing is the 6800 ohms from the mosfet gate to ground seems
excessive I would use 2 Kohm but I doubt that is the issue.

Then I would start with the frequency at about 1 Khz and raise it till
something happens, with only a few mm gap between the HV lead and a wire
connected to the negative of the coil you should see frequencies where the
sparks are good and some where the sparks are not so good.

With a 50% duty on the driving pulse I would expect over 1 or 2 amps through
the primary with 12 volts at reasonably low frequency. With 0.5 uF across the
ignition coil primary mine makes good sparks at a few spots between 1 Khz
and 5 Khz. If the current becomes excessive you might want to try to restrict
it somehow. A smaller cap across the coil should reduce the current through
the primary.

Cheers

P.S. Try not to shock yourself while holding anything in your hand I found the
recoil from a shock can cause things to be involuntarily thrown, I almost broke
a window with a screwdriver once.

..

[Aaron]

mostfet triggered ignition coil

These are my demos pulsing an ignition coil with a 555 and mosfet so you can definitely make it work.

The regular and peaking cap one are a mostfet between ignition coil and battery. The last two with CDI and plasma ignition are using 555 to trigger the trigger wire on a MSD unit.

Mosfet = IRFPG50
555 timer is only powered with a 9v rechargable radio battery, which is usually about 8.4-8.6 volts (if not re-chargable) instead of 9 and it works fine.

You can run it in a wide range through the khz range with no problems if you're simply powering the coil from a battery. If you are doing capactive discharge into the coil, you can get the higher speeds but the cap may not charge as high so the spark may get weaker the faster you go. Of course there are limits to the speed with just battery power too, just depends on what you're doing.

[Aaron]

555 mosfet ignition coil

Your circuit:



Try removing the diode between the mosfet and coil first and retest. I don't know what the point of that is and it defeats the intrinsic diode in the mosfet from sending some kickback towards the power supply on each off cycle - but there won't be much if any since you do have a secondary shuttling the spike over the gap to ground.

Also, does the IRF640 have the freewheeling diode inside?

If it does, you don't need the diode across source and drain unless you're bypassing it since they are usually junk. The diode you have from ground to drain, that is already built into the mosfet usually, you have to check your irf640 specs.

(I just checked and this is listed as the diagram for the irf640:



You see that diode? it is built in so you don't need the 1n4005 unless you are bypassing it with a better diode. 1n4005 is not what I would use to do that. The 1N4005 is a slow clumsy diode for this purpose and you should use a really high speed switching diode instead of what you are currently using. I can't remember the diode I use to bypass the intrinsic diode for reverse currents but you can get much better performance bypassing the internal one depending on what you're doing.

There are of course different variations of the 555 setup you can use. Mine I can adjust on time and off time separately so that it changes the frequency and duty cycle. Just scope the ground and output of 555 signal to see that you are even getting a real signal.

You have to make sure your mosfet isn't being locked open.

[Aaron]

comments?

Well, any results or comments?