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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.... |
Ignition coils are WEIRD
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Ignition coils are WEIRD
On 2/11/19 12:32 AM, wrote:
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.... https://en.wikipedia.org/wiki/Autotransformer |
Ignition coils are WEIRD
On Mon, 11 Feb 2019 00:43:10 -0600, Fox's Mercantile
wrote: On 2/11/19 12:32 AM, wrote: 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. . 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. 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. https://auto.howstuffworks.com/ignition-system3.htm |
Ignition coils are WEIRD
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Ignition coils are WEIRD
On Sun, 17 Feb 2019 16:20:16 -0600, Fox's Mercantile
wrote: On 2/17/19 2:11 AM, wrote: 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). 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 ****. 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 |
Ignition coils are WEIRD
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