# Creating sparks with a mosfet and ignition coil

Started by May 22, 2014
```On Monday, July 7, 2014 6:31:40 PM UTC-7, Jim Thompson wrote:

*crunch*

> >> >L*I^2 = C*V^2... V is Vds across the mosfet, right?  So, the greater the C, the lower the V (which keeps my mosfet from frying), right?
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> >>
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> >>
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> >> Correct, And also limits the spark voltage.
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> >>
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> >>                                         ...Jim Thompson
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> >
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> >What is the governing equation for the spark voltage as a function of capacitor value?
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>
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> It's a function of primary/secondary/mutual inductance.  Can someone
>
> provide modern data?  Everything I have dates to 1973 when I last
>
> designed ignition systems for the automobile industry.
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>
>
>                                         ...Jim Thompson

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.

Michael
```
```On Monday, July 7, 2014 9:26:37 PM UTC-7, mrda...@gmail.com wrote:
> On Monday, July 7, 2014 6:31:40 PM UTC-7, Jim Thompson wrote:
>
>
>
> *crunch*
>
>
>
> > >> >L*I^2 = C*V^2... V is Vds across the mosfet, right?  So, the greater the C, the lower the V (which keeps my mosfet from frying), right?
>
> >
>
> > >>
>
> >
>
> > >>
>
> >
>
> > >>
>
> >
>
> > >> Correct, And also limits the spark voltage.
>
> >
>
> > >>
>
> >
>
> > >>
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> >
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> > >>
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> >
>
> > >>                                         ...Jim Thompson
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> >
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> > >
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> >
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> > >
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> >
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> > >
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> >
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> > >What is the governing equation for the spark voltage as a function of capacitor value?
>
> >
>
> >
>
> >
>
> > It's a function of primary/secondary/mutual inductance.  Can someone
>
> >
>
> > provide modern data?  Everything I have dates to 1973 when I last
>
> >
>
> > designed ignition systems for the automobile industry.
>
> >
>
> >
>
> >
>
> >                                         ...Jim Thompson
>
>
>
>
>
> 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.
>
>
>
> Michael

Oops, correction:  I tried manually touching the ignition coil primary to my 12V battery...
```
```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

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
```
```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.
```
```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?)

you used a polarized Electro didn't you?

Jamie

```
```its not a polarized cap. It is a metalized film cap.
```
```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 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
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```
```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.  :p

Michael
```