Ignition Coils in stompboxes

[Fret Wire]

Weird idea keeps popping into my head about using an ignition coil from an auto or small engine (Briggs and Stratton) in a pedal. How would a coil react if it was feed a standard 9v supply? Maybe at lower current it can be a constant supply. Might be interesting in a Morley type wah ckt, or a rocker type arrangement where the treadle could trigger voltage bursts. But I was thinking in terms of distortion. I can picture it in my head, but don't have the tech background to pull it out. I keep seeing clock drivers and photocells.

http://www.bgsoflex.com/igncoil.html

http://www.electrical-engineering-forum.com/ee/Ignition_Coil_Question_110416.html

http://members.misty.com/don/igcoilhv.html

http://www.geocities.com/CapeCanaveral/Lab/5322/coildrv.htm

http://www.infodotinc.com/basae/41.htm

Well, at least I found a bionic spud gun using coils. :twisted:
http://www.geocities.com/ResearchTriangle/Lab/4772/spudintro.html

[R.G.]

I = V / R

V = L dI/dt

Any questions?

[Fret Wire]

Vl = L di/dt = L d2q/dt2
Vr = Ri = R dq/dt
L di/dt + Ri + q/C =V(t).

How would you apply the above if you tried to use a coil for a stomp ckt? Next lesson or am I wasting my time? Would it necessarily have to be used the same way as an inductor? What about the tone or clipping section of a ckt?

[R.G.]

Vl = L di/dt = L d2q/dt2
Vr = Ri = R dq/dt
L di/dt + Ri + q/C =V(t).
How would you apply the above if you tried to use a coil for a stomp ckt?

I personally would measure the coil inductance, voltages, capacitances, etc. and then try to sub in the numbers and figure out what is happening with the voltages and currents. Then I'd apply that to figure out if it did a useful change to a waveform for a stomp circuit. Different people have different approaches, of course. Some people can just swap things in and then some kind of magical convergence happens and everything is great.  I've never had much luck with magical convergences, so I always want to mess with numbers and try to see what Mother Natures is telling me.

How would a coil react if it was feed a standard 9v supply?

As I said,
V = L di/dt, so I(t) = V/L until it's limited.

The limit, as I said is I = V/R, where the coil's resistance stops the current there, and from then on, until the current changes, it acts just like a resistor.

Then when you stop the current,
V=L di/dt. The voltage goes to any voltage necessary to keep the same current flowing. If there is a parasitic capacitance, there may be some ringing as the cap eats some of the flowing current and then rings the energy out as  a damped sinusoid, but you get a spike.

Standard ignition coils have low R, so the current limit is pretty high, maybe a few amperes. A 9V battery won't last long unless externally limited.

You'd have to measure the resistance, inductance and capacitance or natural frequency of the coil to be able to do some real numbers, of course.

An auto ignition coil is (commonly) a transformer with about a 100:1 step up ratio. That is, the 12V step on the primary is about a 1200V negative step on the secondary. When you let the primary inductance go, the primary flies to about 150-300V. The secondary is then usually 15kV to 30kV.

Would it necessarily have to be used the same way as an inductor?

No. It makes a pretty good 100:1 transformer too. Low frequency response sucks because it's designed to be a high current flyback converter core, but at higher frequencies, it's OK.

What about the tone or clipping section of a ckt?

Well, an inductor is an inductor. The primary inductance could be useful in some tone circuits. The secondary inductance is much higher, of course, and it could also be used for an inductor.

The 100:1 transformer ratio might be good if you wanted to step signal up to maybe 100V with limited frequency range for some special clipping applications.

Might be interesting in a Morley type wah ckt, or a rocker type arrangement where the treadle could trigger voltage bursts.

The voltage bursts are typically large enough to kill components. Short term operation?

But I was thinking in terms of distortion. I can picture it in my head, but don't have the tech background to pull it out.

Can you give me more to work with? Voltage spikes are easy to do at any smaller voltage without going to something as large, heavy, power hungry, and occasionally dangerous as an ignition coil.

[Fret Wire]

Thank you for the explanation and lack of laughter. I'm going to think on it tonite, and make sure I can clearly explain what I see/hear in my head, and what I want to achieve sound wise.

In the meantime, is there a way to have a diode clipping arrangement where one set of lower voltage diodes handles clipping until a larger voltage is injected, where a second higher voltage set takes over. Then switches back and forth as needed. And have similar, or more intense clipping characteristics, but at a higher voltage. Can a higher voltage/current charge be introduced post amp, but pre-clipping?