Re: Current limiting, newbie question

Hi all,

On 9 May 00, at 21:20, Tesla List wrote:

> Original Poster: "David Dean" <deano-at-corridor-dot-net> 
> Hi Ben
> A car ignition coil is an induction coil, designed to develop a high
> voltage when the primary current is interrupted. On older cars, before
> electronic ignition, there was only the coil and the points in the
> primary circuit. Of course there was a resister too, sometimes a block
> resister, sometimes a resistance wire between the switch and the coil
> (GM products mainly)to limit the current to the coil and prevent the
> core from saturating at low RPM or when ignition was on but motor not
> running, points from burning from excess current (the main reason).
> The resistor would be bypassed when the engine was cranking to give a
> better spark at the reduced voltage (current draw of starter *
> internal resistance of battery = voltage drop during cranking).
> Sometime in the '60s (when CDI started to catch on) some companies
> started marketing capacitive discharge ignition systems. In a CDI a
> cap is charged to a fairly high voltage (typically 400V) and then
> dumped into the coil when the points close (opposite of when the spark
> is made the old fashioned way) resulting in a "hotter" spark than
> could be achieved by the induction coil in the manner it was designed
> to be operated in. This mode is that of the disruptive discharge Tesla
> coil. When transistor ignition came out, coils were able to run
> without resistors because the transistor let more current flow to the
> coil than the points ever saw due to the forward current transfer
> ratio (Hfe). This was like having a coil run "hot" like in "start" all
> the time.
>  Nuff said.
> Some folks run car coils off dimmers. The SCR in the dimmer switches
> the coil like a set of points, more or less, based on the phase angle
> it happens to be set to fire at. Of course that is a gross
> simplification, but you get the picture. Best spark length would be on
> the order of a transistor ignition system. Still not quite that of the
> CDI.
> If you want to make a TC out of a car coil, you should look into using
> some sort of capacitive discharge type system. Check Terry's site for
> info, he is still the holder of the "GMHEI whatever cup thinggie" as
> far as I know and that is quite a feat, 13+ inches of spark from an
> ignition coil. I am sure you will get more and better responses on
> this subject so I will let it go.

Terry kindly sent me a pair of the coils to play with (read try and 
beat his record which I am sure will take an awful lot of luck). The 
feat mentioned is all the more impressive considering that the 
secondary winding height on these things must be all of 3/4"!!!!!!!!!