Creating sparks with a mosfet and ignition coil

Change C4 to 1uF 100V, then use IRF530... should be OK.... no warranties, express or implied ;-)

Hard to know, the circuit is a mess of uncontrol. ...Jim Thompson

One microfarad! Huh. Ok!

Thanks!

Michael

A guess based on inductive energy, the original C4 value at 250V ;-)

...Jim Thompson

Should I add a Schottky snubber diode in parallel with the primary coil, for extra MOSFET protection? Or would that interfere with the cap?

Thanks,

Michael

That will interfere with the energy transfer. ...Jim Thompson

The link incorrectly states that "Energy flows back and forth resonantly between the magnetic field in the coil and the electric field in the capacitor, with a period given approximately by 2 pi x Sq. Rt. (LxC), the energy decaying rapidly as energy is transferred to the secondary side spark and to the wiring resistance."

This would certainly be the case if there was no spark at the secondary. As soon as there is a spark you have to switch in a new circuit model with a low resistance which rapidly absorbs the stored energy. So only the first part (less than T/4) of a sinusoid is seen after switching.

Pere

yes, it should be used for maximum effect!

It's a controlled release. In other words, it does not completely clamp the voltage, which is why you need a High Voltage fet.

Should work just fine.

Jamie

Okie doke! Thanks!

Michael

Back in the day (late '60s, early '70s) when I worked for the auto industry, the voltage across the breaker points in the primary of a mechanical-type system could easily go over

Best regards,

Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis

Ok, I will! It's time to place another order with Mouser anyway...

Thanks!

Michael

The voltage across the points (_and_ the effective spark voltage) is a function of the value of the capacitor across the points.

Here's my basic self-inverting CD ignition from the 1968-1973 time frame...

when I was designing chips for Ford, GM, Chrysler, and American Motors... you do remember American Motors don't you ?:-} (Mitt Romney's father, George W. Romney.) ...Jim Thompson

Bipolar transistor? Why?

Power/DMOS MOSFET's didn't exist in 1970.

To further complicate matters, the HV NPN's of the day had lousy beta, as in ~3 at IC=5.5A

So the tricky part, not shown, was how I developed a chip to calculate IC from IE measured at ground, to get the current trip point to stay right at 5.5A irrespective of temperature.

With today's Power MOSFET's it's a trivial task. ...Jim Thompson

I have a dual 60+10 uF, 440V capacitor I salvaged from the AC unit after installing a new cap. Would this work alright?

I finally have some spare time and maybe I can build this with parts I have lying around (irf530, dual cap, etc.)

Could you give the equations which would dictate the voltage across the spark gap and the voltage across the mosfet, as a function of capacitance?

Thanks!

Michael

For a typical automotive coil (from a point type ignition), ~1uF is the proper value... NON-electrolytic.

L*I^2 = C*V^2 dominates until the secondary "sparks"

And you need to time shut-off based on coil current.

I ought to write that up one of these days ;-) ...Jim Thompson

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