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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