On a typical automotive points type ignition, the scope pattern shows condenser / capacitor oscillations when the points open. How could I do that intentionally to drive a transistor and get an amplified 12 volt, 20 amp copy of the + or - half of that signal. I want to use this like PWM for heavy duty stepper motor coils at low speed so each step would start strong and diminish to nothing. I would like to use this on the high side if possible. I have been playing with FETs but this looks like a job for analog transistors. What strong PNP or NPN would you recommend?

Ignition info from this site: http://www.users.bigpond.com/ergoff/othersc1.htm

Picture below
Condenser Oscillations : The dissipating energy from the collapsing magnetic field of the coil is discharged to the condenser. This energy is stored briefly in the condenser, then pushes it back to the coil. Each time the condenser gives up the stored energy, the energy weakens. This repeats until all of the energy is used up.
Coil Oscillations : The energy in the primary winding is decaying due to the collapse of the magnetic field. These oscillations show the energy dissipating.
Points Close : The contact breaker distributor points close, dwell begins and current flows through the coil. There should be a sharp, clean 90 degree drop in the wave form at this point. "Dwell" is the time the points remain closed.
Points Open : At this point the contacts have opened and this causes the magnetic field in the primary to collapse, inducing a high voltage in the secondary winding. The current flow has stopped and spark begins. The upward spike is a representation of the inductive "kick" in the primary winding when the coil current is interrupted.

In my trusty Forrest Mimms Engineer’s notebook, I found an electronic bell circuit using a 741 op-amp and a few Rs & Cs to make a signal like that. What do you think?

Attached Images

Primary ignition.jpg (43.0 KB, 13 views)

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Not exactly sure I understand yorur question. Have you considered SCR/thyristors? You may want to check an application note from On Semiconductor #HBD855/D. It is 240 pages, includes DC motor control, and was revised in 2006. John

Edit: I would give the URL, but I saved it as a pdf and don't have the URL handy.

 

Thanks John
http://www.onsemi.com/pub/Collateral/HBD855-D.PDF
That will keep me busy for a while!

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If the motor is single stepping, I don’t want it to sit there cooking until it gets the next step. I might put a parallel circuit to each coil in the milliamp range just to keep the brakes on.
What I had in mind is something like this.

Attached Images

Diminishing pulse.jpg (5.5 KB, 3 views)

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