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marco cochet

Mon Aug 27 2007, 09:27PM

Registered Member #972 Joined: Mon Aug 27 2007, 09:02PM
Location:
Posts: 5

Hello all,

i have recently (today) become a member here but have looked at this site many times already.
i'm just a half year in high voltage and build schemes i can find in google,
since my knowledge for electronics is to limited to come up with own designs.
but now i'm facing a problem and i cant find the solution.
i build the following driver for a flyback;
Mod edit: oversized images: click to see originals and please RTFForumRules

this is working very good on my flyback with sparks 2 inch long but if i connect it to my ignition coil,
after a few sparks i blew my mosfet , which i replaced by an irfp450 because i have readely ( >5 )available.
so i placed some diodes like this;

the strange thing is the 555 timer or transistors never gets damaged, i just placed those diodes as a precaution. ( saw them on another very similar scheme)
i just liked this driver for its settings and it preforms very well according to my limited knowledge

i also build the zvs driver and get >10 cm arks from this setup with 48 volt input.
also i used the search engine on this forum and found someone with a similar problem but with a different driver.
he was adviced to put 10X 1.5KE350CP transils over the load.
this is the post :

over the load does mean parallel over the positive and negative connections from the coil right?

should this help in my case also?
i was also considering this circuit;

but the mj10012 is a bit costly if i destroy it on the first run

plus i like the idea to have one driver for multiple purposes.
Thanks in advance and sorry for my horrible englisch,

Marco

Sulaiman

Tue Aug 28 2007, 09:12AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Hello marco (#2), welcome to 4Hv.

I think it is the flyback spike on the primary that's destroying your switching transistor.
The transistor that you are using (irf840) is not very good for switching an ignition coil;
- Rds is about 0.85 Ohm, an ignition coil typically runs at 4A, so you are losing 3.4V across the transistor.
- Although the IRF840 is avalanche rated, the rating is too low (13mJ). e.g. 0.5 x 4^2 x 5mH = 30mJ
So you are heating the transistor a lot due to high Rds(on)
and the transistor cannot handle the flyback energy.

The simplest solution is to use a better switching transistor.

Alternatively you can divert the flyback energy away from the switching transistor, using circuits like you've posted.
FIRST, the two 1N4001 diodes are not helping, remove them.
SECOND, the UF4008 with 100 Ohms across the primary is not a good solution to the problem,
I would replace the 100 Ohm resistor with about 1.8 kOhm (between 1 and 5 kOHM will be best)
AND put a capacitor across this resistor, a few uF with 450V rating would be good.
Something like a motor-run capacitor would be ideal, but they are large and expensive.
You could start with one of the 330V electrolytic capacitors found in disposable camera flashguns
MAKE SURE that the positive side of the capacitor goes to the cathode (stripe) side of the UF4008.
You can put a voltmeter (dmm on volts range) across this capacitor,
make sure that the voltage does not exceed the voltage rating of your capacitor!
If the operating voltage across this capacitor is too high then reduce the value of the resistor
If the voltage is too low (poor output voltage/spark length) then increase the resistance.
Measure the operating voltage from capacitor positive to 0V to check it's less than the transistor voltage rating (500V)

The zener clamp will also work, I would put the zeners instead of R1, keeping U3 for best results.

P.S.
This is a common topic/problem and I'm sure that if you'd looked through the old posts you'd have found the answer.

marco cochet

Tue Aug 28 2007, 10:17AM

Registered Member #972 Joined: Mon Aug 27 2007, 09:02PM
Location:
Posts: 5

Thanks for the valuable advice Sulaiman.

i was looking through previous post but could not find this specific problem with the same components.
i already replaced the irf840 with an irfp450 but with the same results.
do you have any advice on which transistor or mosfet i should use?
or should i just buy a mj10012 ?
i did build the 555 circuit with a 2n3055 transistor but the biggest sparks i could get from my ignition coil where 5 mm.
if i was satisfied with that i better stick to a piezo ignition from a sigaret lighter

greetings,

Marco

uzzors2k

Tue Aug 28 2007, 11:34AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Keep trying with the IRFP450s and add a snubber (resistor and capacitor with diode) like Sulaiman says. If that doesn't work then try building the BICDFH (schematic 3), Blackplasma had good results with it and got 10 cm sparks with a two coil setup, IIRC.

Steve Conner

Tue Aug 28 2007, 11:47AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

marco cochet wrote ...
i was looking through previous post but could not find this specific problem with the same components.
i already replaced the irf840 with an irfp450 but with the same results.

Doesn't that prove that the problem is independent of the specific components used, then? So why complain that you couldn't find an instance of the problem with the exact same circuit?

It is in fact a common problem that virtually every noob to the single transistor flyback driver circuit complains of, irrespective of the details of how they built it. I personally favour using a small capacitor across the transistor to tune the flyback pulse down to a safe amplitude, which is how it was done in the first place, in the TV you got the flyback out of.

marco cochet

Tue Aug 28 2007, 11:56AM

Registered Member #972 Joined: Mon Aug 27 2007, 09:02PM
Location:
Posts: 5

Steve,

In fact when i use a flyback i dont have problems whatsoever.
its just when i hook up a ignition coil the transistor fails.
sinds the schematic was drawn with a flyback attached i thought that there was maybe a limitation in the design that prevented the use of a ignition coil.
this seems to be so because i got the advice to use another transistor.
i gonna try the many tips i received.
thanks guys,

Marco

Steve Conner

Tue Aug 28 2007, 12:43PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hmm, well, I apologize for not reading your post correctly.

The 8kHz minimum frequency of that circuit is probably too much for an ignition coil. Try making R4 much larger, so the circuit pulses fewer times per second. You might like to try making C3 larger too. If you get the frequency and on-time correct, an IRFP450 should drive an ignition coil well: it can pass plenty of current when on, and stand up to 500V when off.

If you add a resistance in series with the ignition coil primary to limit the current, you should be able to experiment without destroying too many transistors :/ If you're running it off 12v, something like a headlight bulb from a car would probably do.

You can try the various ideas for snubber networks that people have suggested here, at reduced power. The object is to get big sparks without the MOSFET heating up too badly. Once you're confident, you can try reducing the resistor or taking it out altogether.

I wouldn't bother buying the MJ10012. An IRFP450 or IRFP460 will probably work just as well.

marco cochet

Tue Aug 28 2007, 06:35PM

Registered Member #972 Joined: Mon Aug 27 2007, 09:02PM
Location:
Posts: 5

Hi guys,

It works it works!!!

i took the advice from sulaiman and replaced the 100 ohm resistor with an 5 Kohm i got from an old printboard out of a television.
that together with an 400 volt 11,5 uF capacitor across it that i took from a old electromotor did the job.
however if i do measurements on the circuit while it works, my voltmeter goes bananas

i suspect the emf

so i can only hope the voltage doesnt exceed the cap rating.
if it fails again on me now i just build the circuit with the mj10012.
again thanks for the help to all,

Marco

Sulaiman

Wed Aug 29 2007, 08:07AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Fixing the 'snubber' allows high voltage to be achieved
(because Vsec = (Nsec/Npri) x Vpri , where (Nsec/Npri) is about 100 for a typical ignition coil.

Now, if you want more POWER then you will have to reduce the frequency as Steve Conner wrote.
(I didn't notice the frequency)

With a 12V supply you need about 2.5ms or more on-time
so if you have 2.5 ms ON and 0.5 ms OFF the operating frequency will be around 333 Hz.

To pump even more power through you would need to increase the supply voltage for the ignition coil
I used 35V for my ignition coil supply, because that's what was handy when I made it.
A 50V 2A supply would probably be better.

P.S. My multimeter also goes 'crazy' near high-voltage at high-frequency, my old moving-coil meter is reliable though.

Steve Conner

Wed Aug 29 2007, 09:53AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hey, glad you got it working

I also find that digital meters go crazy around this type of circuit. They aren't too happy when it comes to measuring high voltage pulses.

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