Re: ignition coil - aside reaction

From: 	Ken Pryor[SMTP:pryor-at-mmsi-dot-com]
Reply To: 	pryor-at-mmsi-dot-com
Sent: 	Thursday, July 24, 1997 3:34 PM
To: 	Tesla List
Subject: 	Re: ignition coil - aside reaction

Tesla List wrote:
> From:   William Noble[SMTP:William_B_Noble-at-msn-dot-com]
> Sent:   Wednesday, July 23, 1997 4:20 PM
> To:     Tesla List
> Subject:        RE: ignition coil - aside reaction
> allow me to comment on your "aside" -
> First. based on literature and some limited measured data, my recollection is
> that the actual primary voltage in an automotive ignition coil is about 200V,
> whether it's a 6 or 12 V ignition.  This is the voltage you measure when the
> points OPEN.  The CD ignitions I built typically drove the coil with 450V,
> whether it's a 12 or 6V coil (that just happens to be about what you get with
> 230VAC and a bridge as a peak DC voltage on the capacitor.  At high RPM this
> drops down to closer to 300V, but I would still get a 2 inch spark out of the
> coil - plenty for the cars I run.)
> Second.  The reason cars went to 12 was to reduce cost, specifically the cost
> of copper wire.  The ignition circuit has nothing to do with it.   The mass of
> copper is to support high currents, and at the time of the change (about 1956)
> this current was dominated by headlights and starter and the charging circuit.
>  Also, IR losses due to grounding and other problems are less significant at
> 12 volts.  The external resistance, called a ballast, is not a result of the
> change to 12V - some coils have internal ballast, some have external, some
> (for example GM products of the lat 50s to early 60s) use a ballast wire that
> is bypassed by contacts in the starter solenoid to compensate for lower
> voltage during cranking.  My understanding is that the ballast limits current
> through the points to prevent pitting.  I have run cars that have external
> ballasts with the ballast removed (sometimes by accident), and the coil does
> not overheat (but the car doesn't run quite right either).  I believe that
> earlier cars were 6V due to limitations on battery technology - specifically
> the insulation, that made the individual cells large.  My morgan uses 2 6V
> batteries to get even weight distribution.
> Aircraft are not 24 volts (sorry) - they are 28 volts.  Telephone circuits are
> 24 volts.  I will let some qualified aircraft mechaninc explain where 28 came
> from - my experience as a a lead designer working on passenger jet aircraft
> flight control systems only includes the 70s, and by then 28V was reserved for
> emergency functions and for the DC motors.  I believe that this is mostly
> abandoned now, but I haven't worked on a modern jet in the last 5 years (I did
> check on a DC9-80 and the DC-X, they both still had 28VDC circuits).  In the
> near future, 200VDC should come into play to drive high powered electrical
> actuators - I have seen a 300 HP DC motor that was about 6 inches long and 4
> inches in diameter (!) in magazine photos.
> -----Original Message-----
> From:   Tesla List
> Sent:   Wednesday, July 23, 1997 10:27 AM
> To:     'Tesla List'
> Subject:        Re: ignition coil
> From:   Jim Fosse[SMTP:jim.fosse-at-bjt-dot-net]
> [bill]  snip
>  aside: I believe this is the reason that automotive systems went from
> 6V to 12V, they kept the same coil and added external resistance, to
> improve the performance. Likewise, aircraft systems went to 24V;
> partially for the increase in RPM that the new spark system could run
> at (also lighter wireing for the same power).
> >
>         jim
Aircraft electrical systems are 24VDC the same way automotive systems
are 12 and 6VDC. The 28 volt is the charging voltage. The batteries are
24 volt.
Kenneth C. Pryor
Modular Mining Systems
3289 E. Hemisphere Loop
Tucson, AZ 85706
email: pryor-at-mmsi-dot-com
internet:  http://www.mmsi-dot-com