Malcolm Stebbins wrote: > At 08:56 AM 10/27/03 -0500, you wrote: > >> Once again exactly what I said. > > > On the contrary, you said 50,000 to 100,000 secondary volts was > induced by a primary pulse of 1,000 volts, which was in turn generated > by the collapse of a field generated by virtue of a 12 volt EMF. >
I admitted I was off by a factor of two or 3 on the voltage, but the principle is still the same. >About as "correct" as you have been so far . . . . . . in neither of these systems has the primary field collapse generated anything like the 1,000 volt pulse you posited. I believe the reference indicated it was 300 or 400 volts. I was off on the high side, I never measured it, I just know from experience it kicks like hell if you get a hold of the wire from the points when the field collapses. > > >> >> >> > >> > There are no automotive ignition systems running 50,000 to 100,000 >> > volts; not only is there no need for such a dangerous level, the >> > corona discharge initiated by such would be extremely and rapidly >> > destructive to the elements of the system, make the design of a >> > distribution system for the spark considerably larger, more >> > difficult and expensive, generate a great deal more RF >> > interference, and be an absolute bear to keep functional in damp >> > weather. >> >> OK, so I am a little high on the output voltage. That was not what >> the message was about, it was givin the information that you just >> gave above, that the turns ratio is only about 100:1, and that the >> spark is generated when the points break, not when they make. > > > Oddly enough, the message you wrote was about the information you > wrote, and the response I gave was about your errors in that message; > your reply to that was to state that I was incorrect. On the > contrary, both strictly regarding the elements in my response and your > description of the conventional ignition system generally, it was - > and is - you who are incorrect; - get it straight! > You have suggested that dropping 300 volts through this conventional > system is the same as dropping 12 volts through it, and by implication > that it would be perfectly all right to inflict the consequences of > this ignorant endeavor on oneself or another human - That was what > your message was about. Huh, I don't follow this at all. I never said anything of the sort. Both systems generate about 300 volts on the primary, a convention system when the field collapses, and a CD ignition when the SCR fires. The coil sees a very similar wave form with both. With a conventional system 12 volts is applied across the coil for a period of the dwell time of the points, and the current builds up to a point that when the points are opened, the cap across the points drops the current in a few micro seconds. This sudden collapse in the field generates about 300 volts across the primary, which is coupled ot the secondary and produces the high voltage. The CD ignition usually charges the capacitor through the coil during the time between the sparks, and then when the spark is due, a SCR shorts out the other side of the cap to ground, generating a 300 volt spike on the coil that gets coupled to the secondary. In both cases the coil sees a 300 volt microsecond duration spike which gets coupled to the secondary. If you don't believe this, then get an old spark coil, and connect it to a 12 volt battery and put a spark plug on the secondary. Then connect the battery and you will see that you get no spark. But when you break the connection, you will get the spark. i have done this many times. Dynamic and static voltages for capacitors and transfomers are totally different things. A capacitor conducts no current with DC, but will conduct a current that is proportional to the dV/dt of the waveform for AC. An inductor shows little resistance to DC, but will build up a current proportional to the integral of the voltage across it, ignoring it's dc resistance. The 300 volts in both cases is a short duration spike. Think about it. The reference indicates that there is a 100:1 turns ratio, and it generates about 30,000 to 50,000 volts. That means that the input MUST be 300 or more volts. 12 volts on the primary would only generate 1200 volts on the secondary, and even that is high in a conventional system due to the current limiting from the ballast resistor. The only significant difference between the two is that for a conventional system the coil is the energy storage device, and for a CD ignition the capacitor is the energy storage device. And yes, I built a CD ignition system from scratch back in the 60's and it worked fine on my old 1965 Impala. A strobe works the same was as a CD ignition, but the voltages may be somewhat higher, and the capacitor is likely larger, so the current in the secondary may be somewhat higher. > > I'd STRONGLY suggest that you try it on yourself first; perhaps the > mismatched parameters of the systems will limit the outcome to a > bearable level. > > I have done this myself. In fact the first fence charger I ever made was made this way. It had a heck of a wallop but worked for years! I used it when at UT to keep vandals from tearing down my TV antenna I had on top of the dorm. Marshall
