Re: ignition coil - aside reaction

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).