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Ignition coil driver problems

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Mads Barnkob

Sat May 03 2008, 11:12AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Hey all

I sat down this morning to improve my ignition coil driver abit, and eventually i could draw 20mm sparks from a 12V/1A, when suddenly as im turning the potmeter between pin 6 and 7 on the 555, the current raises to 3.5A and it draws real long sparks only to die seconds later.

Ive changed the 555, a C547B that drives my mosfet, and tried with a 2n3055 instead of my mosfet. still no output from the coil.

Ive meassured the resistances on the coil, 2.8R on the primary and 6K on the secondary, its an old metal cylinder ignition coil, can anyone compare these numbers to a similar coil if you have it?

I dont think its the coil thats broken, atleast I hope it isnt, but I only had one spare 555 so cant really double check :(

and ofcause this happends just as im about to take some pictures of it

Berni

Sat May 03 2008, 12:40PM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

Its easy to test the ignition coil.

Have the ground realy close to the HV output conect the 12V ground to it
then tap the other contact on it with the + from the 12V evry time you disconect it it should make a spark.
If there is no spark your coil is most proabobly dead.

Oh and your resistances seam about right.Primary is tipicaly a few ohms and the secundary somewhere from 5 to 15 KOhm

As for testing the MOSFET, just meshure resiatance over drain and source.Some of the times they short out.

Antonio

Sat May 03 2008, 12:41PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The resistance values seem reasonable. Ignition coils are famous for destroying unprotected electronic drivers. The primary coil stores a lot of energy (10 times more than a TV flyback), and if something is not done to deal with the stored energy, it is dissipated on the driving transistor, and after it is destroyed in the remaining circuit. A "snubber" circuit is essential, although it does not appear in most of the schematics found in the Internet (just look for comments saying that the circuit died soon, as in your post...). Test the transistors with the ohmmeter, and assemble a low-frequency oscillator driving a LED to test the 555 (or listen to its output with an earphone connected trough a resistor). Be careful with the connections, and notice that the return of the current coming from the high-voltage terminal is one of the terminals of the primary coil. This terminal must be connected to the power supply, and sparks shall be drawn between the output and this terminal.

Mads Barnkob

Sat May 03 2008, 03:48PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I were aware of the risc of kickbacks that could kill my components, luckily it lasted for about a total of 1 hour over the days I worked it and I had fun :D

I tested my spare 555, it seems fine in the circuit i made, led is happily blinking.

transistors are not shorted, and I can beep them when they are on, and not when off, so they seem to work too.

resistors and capacitors all measured and seems to work fine, the 555 is oscilating out on pin 7.

I can measure 13vdc out on pin 3, but I cant get any picture on the scope of the squared signal as I used to.

so all my components seems to work when alone, but not together :/

also checked board for shortcircuits etc, nothing found.

EDIT: the schematic

Dr. Dark Current

Sat May 03 2008, 04:57PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

MadsKaizer wrote ...

I can measure 13vdc out on pin 3, but I cant get any picture on the scope of the squared signal as I used to.

If you get no square signal and your pin 3 is at 13V (ie. at Vcc) all the time, then your 555 is toasted and the transistor is ON all the time.

Mads Barnkob

Sat May 03 2008, 05:04PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Plasmaddict wrote ...

MadsKaizer wrote ...

I can measure 13vdc out on pin 3, but I cant get any picture on the scope of the squared signal as I used to.

If you get no square signal and your pin 3 is at 13V (ie. at Vcc) all the time, then your 555 is toasted and the transistor is ON all the time.

In my test circuit the LED is blinking with 1.65 second on/off time, but in the offperiod it glows faintly, and cant either measure it very good on the scope.

im off to order some more chips, better get a bunch this time :D

Berni

Sat May 03 2008, 05:54PM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

Did you also test the coil by taping the pin on the coil with your 12v supply?Ignition coils do produce a fwe cm spaks when used like this(Its not like a flyback that wont do anythyng if you tap the input with the power wire)

Mads Barnkob

Sat May 03 2008, 06:07PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I just took out the 555 of its socket to run it on the test circuit and my fluke aint too fast, but according to output is pulsing between 10 and 2 Vdc.

I also discovered that the ignition coil is ok, i accidently turned on the driver and it began to hum as it normally would, and i could draw some 5mm flyback like arcs, but the mosfet were almost glowing after a few seconds. this was without a 555 in the socket, so guess the voltage can run backwards when no control from the 555 is going and the mosfet just opens up with the frequency from C1?

Berni

Sat May 03 2008, 06:32PM

Registered Member #1132 Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84

Now that is pretty strange.

Lets try testing the cirucit under a load.Replace the ignition coil with some kind of resistive load at like 3-10Ohms you could also try a speaker and see if the mosfet is sqitching to the 555 timer corectly.

I think i tryed a simalar cirucit once but i used some modern NPN power transistor.And the transistor did not fry. Meaby its because it was suposed to be used in switchmode suplys so it stands high voltages.

I have sugesions to put some kind of zener oner the coil so it clamps the inductive kickback a bit.Thing here is clamping the kickback completly will reduce the output a lot since ignition coils rely on the principle of inductive kickback.Thats why they are aslo caled a induction coil.

Mads Barnkob

Sat May 10 2008, 09:57PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Only the 555 was fried, got a new batch today and it once again works like a charm. :)

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