Using a mosfet with points as trigger for blaster

Bobbrumby · Post by **Bobbrumby** » Fri Nov 24, 2006 9:53 pm

Basically what i thought of doing is running a mosfet chip with the distributor points as a trigger so the bigger coil wont burn out the points. Well actually I really have the magnetic pickup setup.

Would I need the possitive triggering one?

These things even up to the job?

I did a bit of research and found nothing about homemade ignition mods but i did find mosfets open in like 50ns. Does decreasing time taken to break supply to the coil, increase the spark generated?

At 6000rmp this thing is only closing 400 times/second:
6000rpm * 4 = 24000 fires/minute
24000\60 = 400 fires/second
(that right?)

Can these things handle the inductive load or will I need a capacitor or something?

As you may have suspected my knowledge in the subject is vauge at best, Is this a pipedream? is mosfet alright? something else?

fredsub · Post by **fredsub** » Fri Nov 24, 2006 11:33 pm

yes, sure could do it with a mosfet,but don't forget a hi-speed diode for the back-emf of the coil.
Actually, i think its more common to see IGBT devices used for coil drivers, although I haven't acertained a reason why, because IGBT really get useful at higher voltages of >90v (rather than mosfets).

However, in either case you can't just drive them direct from a reluctor signal, or a mechanical points, you need some pulse shaping driver electronics....

Simpler,with all parts supplied would be the jaycar kit - called hi energy ignition or somehing like that....it also includes lengthing the spark.

On the USMB board i've read people commonly drive the blaster coil directly from the electronic points.

I use a cdi to drive mine.

Bobbrumby · Post by **Bobbrumby** » Sat Nov 25, 2006 1:03 pm

Whos cdi do you use?

BaronVonChickenPants · Post by **BaronVonChickenPants** » Sat Nov 25, 2006 4:56 pm

A 1990 brumby should have an electronic dizzy?

Bobbrumby · Post by **Bobbrumby** » Sat Nov 25, 2006 5:05 pm

Baron I just simplified the title so people would understand.

I saw this on a site:
http://www.dse.com.au/cgi-bin/dse.store ... iew/K3307W

Anyone tried this?

My electronic dizzy has the two wires coming out of it going accross the coil, one to + one to -. Does the electronic sensor ground the neutral of the coil at the distrubtor or does it short accoss the coil? Cause now I dont know for sure how it breaks the supply to the coil?

fredsub · Post by **fredsub** » Sat Nov 25, 2006 8:07 pm

You will find the + side leads to battery+ (via ignition sw & fuses). Coils are always switched to ground.
So points "closed", coil switched to ground => current flowing through coil => magnetic field in coil.
points "open" => magnetic field collapses, => voltage induced in secondary => spark.

You can still associate the electronic points with terms "closed" or "open" points, in that way the behaviour is similiar.
However there is some smarts in the electronic dizy - which also allows the coil to do without a resistor in series.
You see points are normally "closed" - electric current to build energy - but if they were closed all the time, the coil would get too hot. So the electronic dizy has a variable dwell. And it also "opens" the points when the engine is not rotating.

hmmm? hope i explained it clearly enough?

PeeJay · Post by **PeeJay** » Sat Nov 25, 2006 8:09 pm

I tried to hook up something similar to an electronic dizzy and I never managed to figure it out. The reluctor thingy is connected in parallel to the coil.

fredsub · Post by **fredsub** » Sat Nov 25, 2006 8:25 pm

haha, took me a bit to figure out and special PIC controlled driver to the CDI, otherwise, esp at startup, the cdi wouldnot fire regularly!! Now it hasnt missed a beat in years.

mattims · Post by **mattims** » Sat Nov 25, 2006 8:51 pm

i have used mosfets to drive coil on plug sparkers before and i could run the car no problem at all at revs but they got very hot at idle. I had a zener diode to stop back emf and they were the biggest mosfets i could find at jaycar.. about the size of a 20c coin with a big heatsink and in an oil bath.. and i still couldnt get it long term reliable so driving one big coil you might be better of as the time between firings is less.

Bobbrumby · Post by **Bobbrumby** » Sun Nov 26, 2006 1:21 pm

fredsub wrote:haha, took me a bit to figure out and special PIC controlled driver to the CDI.

What do you mean by that?

So did you make you cdi yourself?

fredsub · Post by **fredsub** » Mon Nov 27, 2006 8:58 am

yep sort of, original was a kit for a mech points car, didn't get it to work with the ea82 at first, but when engine replaced with a turbo, got re-interesed.
read the result.
here