----- Original Message -----
From: wizardz <wizardz@maxinter.net>
To: Charley & Peggy Robinson <ccrobins@ktc.com>; <mgs@autox.team.net>
Sent: Thursday, January 20, 2000 7:09 PM
Subject: Re: Changing condensers/capacitors - Claculate it
> Interesting thought there Charlie...
> yes the capacitance does make a difference in it's usefulness
> at limiting points wear and tear. (electrical arch energy absorbed
> to minimize the spark across the points as they open)
> It should be simple to calculate the proper RC (resistance/capacitance)
> timing constant F=1/(RC) ( or better stated as RPM/2 =1/(RC)
> to best fit a particular ign.coil resistance with
> the best condenser for the RPM you drive most often.
Interesting indeed, and because of my own interest in this subject I have
recently been using an oscilloscope on an ignition test rig. My apologies
for the length of this post, but if you have a look at the web site below
(select 'Spanners' and 'Ignition') you can find an even longer description!
Firstly, it is the L/C (inductance i.e. coil/capacitance) tuned circuit of
the coil and capacitor that is the critical thing as far as a good spark
goes. This results in a waveform and pulse shape and length that shows
hardly any variation with rpm. Indeed, at higher rpms you find the next
pulse 'catching up' with the end of the previous pulse.
The effect of the capacitor is two-fold:
1. It causes the spark to occur at the plug within about 20uS (20 millionths
of a second) of the points opening. Without the capacitor the spark occurs
at the plug about 2ms after the points have opened. Not much, you might
think, but at 2000 rpm it has the effect of retarding the spark by about 48
crankshaft degrees!
2. It cause a spark duration of about 2mS (2 thousanths of a second).
Without the capacitor the spark duration reduces to about 0.2mS, even though
its peak voltage is about 70% higher..
There is a high-frequency 'ringing' (the effect of the L/C tuned circuit) in
the primary of 15kHz, 200v peak about the 20v point immediately the points
open. This decays over about 0.5mS then there is a steady 20v for a further
1.5mS, and these two events coincides with the duration of the spark at the
plug. When
the spark ceases a lower frequency ringing commences of about 2.25kHz, 20v
peak about the 12v point, and this decays over about 3ms, then the 12v
line is maintained until the points close again.
Because of the high-frequency ringing the instantaneous voltage across the
points falls from 200v to zero in about 20uS, this nips the arcing off very
rapidly, and it is this effect that protects the points. Without the
capacitor there is nothing to interrupt the current until the 200v 'spike'
reduces sufficiently - and the points gap opens far enough - to extinguish
it, at which time the flux in the coil finally collapses. It is this delay
in the final collapse that delays the spark at the points.
There is a clear audible and visible difference in the spark at the plug,
and the distance the spark will jump (barely a plug gap without, at least
1/4" with) between no capacitor and capacitor.
The really interesting thing is that even though the voltage on the
'primary' of the coil is oscillating and passing through zero twice each
cycle, the 'secondary' voltage exhibits a single, square-wave negative
voltage pulse. Why isn't it oscillating in sympathy? The other interesting
thing is that these oscillations die away in 0.5mS, and then a steady 20v is
maintained on the primary for another 1.5mS, and yet the single output
voltage pulse is still being maintained. Maybe there is a third, VV high
frequency oscillation present that is beyond the capacity of my oscilloscope
to display it. If the HT lead is not connected to a plug there is no
high-frequency waveform and no steady 20v. The low-frequency oscillation
starts immediately the points open and decays over about 5mS, and this
waveform is repeated at the HT lead i.e. the coild is acting exactly as we
would expect a transformer to act.
I then tried different values of capacitor (like half, and double) and
although both audibly altered the spark,.the changes in the visible spark
and on the oscilliscope were much more subtle, consisting of changes to the
higher and lower frequency components of the main HT pulse, its basic
voltage and duration did not change..
PaulH.
http://freespace.virgin.net/paul.hunt1/
(or if that URL doesn't work try )
(http://194.168.54.52/paul.hunt1 )
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