Creating sparks with a mosfet and ignition coil

[Jasen Betts]

On 2014-07-08, mrdarrett@gmail.com <mrdarrett@gmail.com> wrote:

Ah, ok.

I built it with an astable multivibrator (with discrete transistors, haha) and at 50% duty cycle at around 100 Hz, it kinda sorta sparked. I'm guessing the 10uF (440V) dual A/C cap was too big.

I wanted to just take the cap off, but I know I risk barbecuing my irf530.

I tried just manually touching the 12V wires to the cap, and got
nice sparks (but also got impressive sparks just on the 12V wires
too).

I need 1 uF, huh.

yeah, that's the normal ignition system capacitor size.

energy in a capacitor is 0.5V^2/C

So reducing the capacitance by a factor of 10 has the potential to
boost the voltage by a factor of sqrt(10) which is about 3


if you measured the peak voltage seen across the MOSFET that may give
some clue as to how much lower you can go in capacitance.

Michael

--
umop apisdn


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I bought a 1uf cap from Radio Shack and at first I got beautiful sparks. Then after I felt my MOSFET to see if it was hot, it didn't work anymore. I disconnected the coil and replaced it with a 12v auto light bulb which worked fine. Kind of puzzled.

Oh and my DMM said I get voltages of hundreds, sometimes 1000+ volts across the cap. (Maybe my cap is dead?)

 

Oddly, the sparks are nice without any cap at all. My MOSFET doesn't even get hot, at least during my 5-second trials.

 

its not a polarized cap. It is a metalized film cap.

 

[Maynard A. Philbrook Jr.]

In article <46db8dfe-9d27-4f58-b9f4-a1df76f4c257@googlegroups.com>,
mrdarrett@gmail.com says...

I bought a 1uf cap from Radio Shack and at first I got beautiful sparks. Then after I felt my MOSFET to see if it was hot, it didn't work anymore. I disconnected the coil and replaced it with a 12v auto light bulb which worked fine. Kind of puzzled.

Oh and my DMM said I get voltages of hundreds, sometimes 1000+ volts across the cap. (Maybe my cap is dead?)

sounds about right.

you used a polarized Electro didn't you?

Jamie

 

Since I am having so much trouble with a cap, would a Schottky in parallel with the coil primary be enough to protect my irf530 mosfet?

 

On Friday, July 11, 2014 7:27:52 AM UTC-7, Jim Thompson wrote:

On Thu, 10 Jul 2014 22:48:16 -0700 (PDT), mrdarrett@gmail.com wrote:



Since I am having so much trouble with a cap, would a Schottky in parallel with the coil primary be enough to protect my irf530 mosfet?



That'll make a single spark. In ignition systems ringing is

desirable.



...Jim Thompson

Oh I won't mind a single spark too much, since I'm generating them at 100 Hz anyway.

Michael

 

[Jim Thompson]

On Thu, 10 Jul 2014 22:48:16 -0700 (PDT), mrdarrett@gmail.com wrote:

>Since I am having so much trouble with a cap, would a Schottky in parallel with the coil primary be enough to protect my irf530 mosfet?

That'll make a single spark. In ignition systems ringing is
desirable.

...Jim Thompson
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