Ignition coils are WEIRD

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....

Reply to
tubeguy
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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.

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"I am a river to my people." 
Jeff-1.0 
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Reply to
Fox's Mercantile

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.

Reply to
Look165

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Reply to
philo

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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Reply to
tubeguy

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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Reply to
Fox's Mercantile

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

Reply to
etpm

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.

Reply to
+++ATH0

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