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Creating sparks with a mosfet and ignition coil

Started by Unknown 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? > > >> > > >> > > >> > > >> Correct, And also limits the spark voltage. > > >> > > >> > > >> > > >> ...Jim Thompson > > > > > > > > > > > >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
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. > > > > > > >> > > > > > > >> > > > > > > >> > > > > > > >> ...Jim Thompson > > > > > > > > > > > > > > > > > > > > > > > > > > > >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
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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.
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
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 Innovations | et | | 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