Re: Current limiting, newbie question

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
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.



----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, May 09, 2000 5:19 PM
Subject: Current limiting, newbie question

> Original Poster: "B Marshall" <bm-at-bens-stuff.fsnet.co.uk>
> I am planning on building a small tesla coil out of a car ignition
> Because the coil won't
> be able to handle a large amount of power (my guess is between 50
and 100
> watts), I will need to
> limit the current going in. I have measured the resistance of the
> which is 20 ohms.
> Therefor, at 240v it would draw 12 amps, using ohms law. If I wanted
> limit it to say 60 watts,
> I would need to limit it to 0.25 amps. 240/0.25 = 960 ohms, so I
> that I would put a 1k,
> 60w resistor in series with the primary. Would this work, or have I
> this totally wrong?
> Thanks,
> Ben