Ignition coils are WEIRD

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This may be a litle off topic, but ignition coils are WEIRD, compared to
other transformers. I had to work on the ignition on an old farm tractor
last week. I always thought that the high voltage (secondary) was from
the tip (spark plug wire), to the coil's metal case. It turns out I was
wrong. (I am referring to the old coils which are about the size of a
slim beer can, and have a pointed top).

After a lot of research, I found that the secondary is measured from the
large center terminal, to the ground (-) side of the primary. Once I
learned that, it turns out that the secondary is around 500 to 600 ohms,
and the primary is around 6 ohms. (I compared this to several other
similar coils, and it is about the same).

These ignitions are very simple, but testing them is a challenge, since
they do not follow the (regular) transformer wiring. A regular
transformer will have 2 wires for the primary, and two or more on the
secondary. These transformers only have 3 connections, period....

But this has me puzzled. Since the minus primary connection is used for
the secondary high voltage, how can the circuit work????  
Ok, my point is this: The coil's minus terminal is connected to the
ignition points. The points are making a  ON-OFF surge repeatedly. (or
creating an AC across the primary). So, how can the coil deliver a spark
during the time the points are open, since the secondary is relying on
the primary (-) to complete the circuit, which is also being switched on
and off by the points. .

This still has me puzzled......

I did find the problem and get the tractor running though. There were
actually two problems. The points had welded together, so I replaced
them. But the spark was still weak which went back to the ignition
resistor. That resistor is supposed to read about 8 volts on the output
side, which feeds the coil. I was reading 4.5 volts, with the battery
fully charged at around 13.5 volts. So I replaced that resistor as well.
Works fine now....




Re: Ignition coils are WEIRD
On 2/11/19 12:32 AM, snipped-for-privacy@myshop.com wrote:
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Sigh. When you open the points,the magnetic field collapses
which creates a HUGE spike, not related to the turns ratio.
The "return" is through the primary back to the + terminal
on the battery.

<https://auto.howstuffworks.com/ignition-system3.htm


--  
"I am a river to my people."
Jeff-1.0
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Re: Ignition coils are WEIRD
wrote:

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This is proof that not all the videos on youtube are accurate. I watched
one and the guy insisted that the (-) terminal was the return path for
the secondary. Another video said that EITHER primary connector can be
used to test a coil for a spark.  

It does seem kind of bizarre that the (+) terminal is used for the high
voltage return, (knowing that spark plugs complete the circuit by being
connected to the engine block, which is connected to the battery
GROUND).

I guess this is why I think the circuits are weird. They do not seem to
match general electronic knowledge. But they work and have worked for
well over a half century, so I am not going to dispute it. Yet, it still
seems odd to me.... Especially when I see the high voltage is actually
grounding THRU the 12 volt battery.... (without damaging the battery).

Then too, I recall the time I connected a 12v battery charger to a
battery connected to a 12VDC electric fencer. It took about 3 seconds
for me to see a bright flash inside the battery charger, then smoke.
Every diode in that battery charger was instantly destroyed. I still do
not understand why the high voltage came thru the "primary" of that
fencer, but I now know to never connect a charger to a battery until the
fencers is turned off.

And on that same note, I know someone else who connected one of those
fencers to the battery in his car for a temporary livestock fence. Not
only did he get a nasty shock when he touched his car, but his
alternator was destroyed.  

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Re: Ignition coils are WEIRD
On 2/17/19 2:11 AM, snipped-for-privacy@myshop.com wrote:
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The internal resistance of a car battery is close to zero ohms.
When the points open, the magnetic field from the primary collapses.
This causes a huge inductive kick voltage spike. The condenser across
the points acts like a tuned circuit to sustain the high voltage by
acting like a tuned circuit. The secondary being in series with the
primary acts like a auto-transformer and increases the high voltage
oscillations to a value high enough to create a solid, hot, spark
across the electrode of the spark plug.

The fact that the return is via the positive terminal of the battery
doesn't mean shit.


--  
"I am a river to my people."
Jeff-1.0
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Re: Ignition coils are WEIRD
wrote:

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I was taught years ago that the condenser was to prevent arcing at the
points. It was not until recently I learned that its more important
function is to be part of a tuned resonant circuit. This makes so much
sense. About 40 years ago I had a motorcycle ignition problem that was
fixed by replacing the condenser and I never knew why. I still don't
know why Lucas ignitions used a rising field ignition instead of the
usual collapsing field. And I knew lots of guys who converted their
British bikes to rising field ignitions for the better spark. Me
included.
Eric

Re: Ignition coils are WEIRD
On 2019-02-17 15:53, snipped-for-privacy@whidbey.com wrote:
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Nonsense. This type of ignition is initiated by the points opening and
breaking the primary (LT) current.

The HT voltage achieved depends on the rate of change of primary
current, and failure of the condensor causes the interruption of the
primary current to be much slower because of arcing at the points, which
also wastes energy in the coil which should end up in the spark at the
plug, not to mention destroying the points in hosrt order.

The condensor functions by carrying the primary current for long enough
for the points to separate without an arc being initiated.

If the engine does start and run, the usual symptom of condensor failure
is misfiring at wide throttle openings (when cylinder pressure at the
point of ignition is higher, increasing the breakdown voltage across the
spark plug points) and difficulty starting.

Re: Ignition coils are WEIRD
A large wire (primary) and a small one (secondary) are connected  
together to the ground.
So it makes 3 connections.
The turn ration can easily be about 70 100.
The transformer is generally in an oil can so the isolate either  
termally and electrically.
Take care, sparks are above 20,000 or 30,000V.
For old engines, a capacitor was connected in // to the ruptor and it  
was this cap which was making such sparks.
At low speed, the ruptor was actuated at about 25 Hz, so when ruptor was  
making OFF-ON-OFF, ther was only one spark duting the OFF-ON transition.



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Re: Ignition coils are WEIRD
On 2/11/19 12:32 AM, snipped-for-privacy@myshop.com wrote:
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https://en.wikipedia.org/wiki/Autotransformer

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