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Re: New Coil - Barney was (half) right!

Subject: Re: New Coil - Barney was (half) right!
Date: Sun, 16 Aug 1998 18:41:54 EDT
In a message dated 98-08-16 00:52:19 EDT, writes:

>  As an electronics tech and field engineer with 35 years in the gun and
>  missile testing game at White Sand and McGregor Ranges and as a sometime
>  professional auto tech, I've stayed out of this until now.  However,
>  this looks like the last post on the matter of breaker-point ignition,
>  so I thought I'd clear up a few points.  


We're getting close, but we ain't there yet.

>   When the points open current doesn't instantly stop flowing in the
>  primary windings of the coil.  Current now flows through the condenser
>  and coil primary as the condenser charges up toward the 12 volt battery
>  voltage.  As the condenser charge nears the battery voltage the current
>  thru it and the coil primary drop to the point where the magnetic field
>  in the primary collapses. 

I have no argument with the first half of this statement, but a little
clarification of the second half is needed. The current decay through the
coil, which is charging the condenser, is exponential, dropping off very
rapidly at first, then decaying slower as the condenser is charged. There are
primarily three things determining the voltage at the output of the secondary
- turns ratio, maximum current in the primary, and rate of collapse of the
magnetic field in the primary. The rate of decay of the magnetic filed is
directly proportional to the rate of current decay, so the output voltage will
be maximum at the begining of the primary current decay.

>    Yup, the condenser now discharges thru the coil secondary and the
>  spark plug, "powering" the spark. 

I don't think so - the secondary voltage peaks while the condenser is still
charging. At least it appears to in the waveform I'm looking at from one of my
text books, and based on the statements I made above, it seems reasonable.
>    Nope.  The condenser discharge current and the back EMF off the coil
>  primary are opposite in polarity to and greater than the battery voltage
>  and block any primary current from the battery.

Yes, the back EMF of the coil primary is opposite to the battery, and this
polarity is what keeps the primary current flowing in exactly the same
direction it was before the points opened.  If you draw out the circuit (with
the points open), you will see the it consists of three parallel legs. The
first leg consists of the battery (and the remaining electrical circuits in
the car) in series with the primary, the second leg consists of the condenser,
and the third leg consists of the secondary in series with the plug. When
viewed this way, the circuit certainly becomes interesting!

Looking at this circuit, it is obvious that the primary current MUST also flow
through the battery, and the secondary current either flows through the
battery and the primary, the condenser, or a combination of both.

This is the point at which I throw up my hands and holler HELP!
Unfortunately, in the thirty years since I got my degree, the theory I need to
cope with all this has long since escaped me. I know just enough to be

>    I've tried to state things for the layperson to get the drift.  No
>  flames pls, 'cause of minor imperfections.  ;^)

Certainly no flames intended. I just want to know the answers.

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74

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