A gap of nearly an inch?!?!

Good luck with that.

Start small.
Or did you mean 2mm or .020"

 

Ignition System 101:
Current passes from the battery through the ignition switch and goes through the ignition coil primary, through a switching mechanism to ground. A capacitor is placed in parallel with the switching mechanism and ground(provided by a distributor). As the magnetic field builds up in the primary (roughly 200 turns of heavy wire), The magnetic field is also crossing the secondary coil (wrapped around the primary and has roughly 20,000 turns of finer wire) at the same time. At some point depending on time, the primary coil's magnetic field becomes saturated and when the switching mechanism is opened, that magnetic field collapses. As it collapses, induction ramps up the voltage in the primary and induces a voltage in the secondary(mutual induction) according to the turns ratio of the two coils. This can produce 30-50,000+ volts on the secondary side. Once produced, it follows the ignition coil to the distributor cap, through the rotor, back through the cap to the appropriate cylinder's high tension wire, through the spark plug and jumps a gap of 0.035" - 0.060" (depending on vehicle) and back to ground. The capacitor used in this distributor is for absorbing voltage spikes caused by magnetic induction and protects the points from burning out prematurely.
Hope this helps.

 

<snip>The capacitor used in this distributor is for absorbing voltage spikes caused by magnetic induction and protects the points from burning out prematurely.
Hope this helps.

The capacitor also gives an extra ~400V to the primary of the coil, when the engine is running. This ~400V comes from the voltage spike stored in it. Due to the preceding cycle of the system.

 

Thank you shortbus. Thought I covered it but apparently not. Hope the OP got the jist of what I was trying to say.

 

Agreed.

Its other function is to prevent sustained arc over when the points open.
Max.

More to the point, it slows voltage risetime so the points have time to separate beyond the arc quenching distance.

You can make an excellent TAC with a high voltage MOSFET in grounded gate, it "opens" incredibly quickly. But its hard to get low Vgs-thr MOSFETs with high enough Vds, at low revs the improvement is hardly noticeable and cold starting a little worse. AFAIK the capacitor is still required to produce some resonant effect.

 

Good luck with that.

Start small.
Or did you mean 2mm or .020"

With a 555 driving a grounded gate MOSFET TAC and a "wasted spark" twin lead motorcycle coil, I was able to draw an arc several inches - quite handy for setting fire to bits of cardboard and other items with low thermal inertia.

 

With a 555 driving a grounded gate MOSFET TAC and a "wasted spark" twin lead motorcycle coil, I was able to draw an arc several inches - quite handy for setting fire to bits of cardboard and other items with low thermal inertia.

Agreed, but not with the single "strike" the OP was using.

 

Gap. > 20mm

You can destroy an automotive ignition coil if you don't have the proper size spark gap, as the spark energy has nowhere to go and voltage in the coil secondary will go sky-high so the result is that it might arc over internally in the coil secondary.

 

Okay, I haven't read all of the comments, but I just wanted to mention--

You do NOT need a condenser (capacitor) in order to fire the coil. Yes, in theory, if you connect the power and quickly remove it there should be a high voltage output. No, you will NOT get a spark 20mm long unless you are overdriving the thing, which is HIGHLY not recommended. I have burnt out coils doing that. I would expect approximately 10kv on the output, which will only give you about a 9mm spark in the best of atmospheric conditions. You can build drivers that generate a 12 volt square wave that can give you higher voltages, but I wouldn't try to get more than 20kv out of it. You also need to be able to supply a lot of current (usually about 5A) in order to get decent sparks out of it. Without a good driver and power supply, you're not going to be able to do that.

However, back to your setup, try adjusting the spark gap to about 5mm, and you should see a spark after connecting and quickly disconnecting the power as you described earlier.

Personally I use the light dimmer ignition coil driver circuit for most of my coil testing. It's cheap, easy to build, and works very well. I do NOT recommend going any higher than 5uF for the capacitor though. Any higher and you greatly increase the risk of burning out the coil. I suggest staying around one or two microfarads for regular use.

Hope this helps.
Regards,
Matt

EDIT: Please note the capacitor in this circuit IS NOT LIKE THE REGULAR AUTOMOTIVE CONDENSER. The automotive condenser is used to reduce sparking in the points (switches) that drive the coil in an actual vehicle application. The capacitor in this circuit (the light dimmer driver) is used to limit the current to the coil to prevent it from burning out. This is why you should keep it <5uF.

 

You can destroy an automotive ignition coil if you don't have the proper size spark gap, as the spark energy has nowhere to go and voltage in the coil secondary will go sky-high so the result is that it might arc over internally in the coil secondary.

Never had that problem with motorcycle coils, and I tried a variety of types when I was experimenting with my grounded gate MOSFET TAC.

But then motorcycle coils are usually potted - either a hard highly insulating polythene type plastic or epoxy like IC packages.

Car coils always used to be pretty much a bare bobbin with leyers of windings and insulation, cased up in an enclosure full of transformer oil.

 

EDIT: Please note the capacitor in this circuit IS NOT LIKE THE REGULAR AUTOMOTIVE CONDENSER.

For the most part, in the English speaking world the term Condenser became obsolete around the 1950's in favour of Capacitor in what was considered a more accurate description.
For some reason the automotive field did not follow suit and still uses the term to this day.
Max.

 

For the most part, in the English speaking world the term Condenser became obsolete around the 1950's in favour of Capacitor in what was considered a more accurate description.
For some reason the automotive field did not follow suit and still uses the term to this day.
Max.

Steam tractors have condensers, they also have wheels - you can see where some confusion might arise.

 

For some reason the automotive field did not follow suit and still uses the term to this day.

Actually, the automotive world does still use the term condenser. It is a heat exchanger used in an air conditioning system. Not sure what books you read or where the information comes from, but we use the term capacitor when reference to the electrical component is necessary. Condenser may be used by the really old techs, but since we don't use ignition points anymore, such does not exist. Capacitor it is!

 

Actually, the automotive world does still use the term condenser. It is a heat exchanger used in an air conditioning system.

I was not referring to anything non electric/electronic, the subject of the post was ignition coils, just pointing out it was continued to be used in the automotive industry long after it was dropped in other electric/electronic fields, in fact several previous posters used the term also!
Max.

 

For the most part, in the English speaking world the term Condenser became obsolete around the 1950's in favour of Capacitor in what was considered a more accurate description.
For some reason the automotive field did not follow suit and still uses the term to this day.
Max.

Yes, I've always wondered why they kept calling them condensers. But what I meant by that statement was that the capacitor in the dimmer circuit doesn't serve the same purpose as the capacitor (condenser) in the automotive circuit.

 

Yes, I've always wondered why they kept calling them condensers. But what I meant by that statement was that the capacitor in the dimmer circuit doesn't serve the same purpose as the capacitor (condenser) in the automotive circuit.

It condenses the spark at the points down to a smaller size.

 

It condenses the spark at the points down to a smaller size.

That seems like a stretch as far as being why they call it a condenser.

If sized properly, it prevents a spark from forming at the points at all -- upon opening anyway.

Since the voltage can't change across the capacitor (and if you buy a replacement in an auto parts store they are almost always labeled as condensers) instantaneously, the voltage across the points has to change continuously, starting from zero, as the points open. As the voltage builds in the capacitor, so does the voltage across the point gap. But the point gap is widening while, at the same time, the energy is being expended in the spark at the plug. If the capacitor is big enough, the voltage across the point gap never reaches the value needed to create a spark. If the capacitor is too big, it will rob too much of the energy and reduce the spark strength. As the points close, they are closing on a charged capacitor and that is when any spark happens and also when you get the greatest current through the points as it rapidly discharges that cap. The smaller the cap, the less energy has to be dumped, but the higher the voltage it will have charged to and, hence, the earlier in the closing motion the arc will strike. Its primarily this arc that pits the points in normal operation, as indicated by the much smaller pits compared to those seen when the condenser fails or is removed.

 

That seems like a stretch as far as being why they call it a condenser.

If sized properly, it prevents a spark from forming at the points at all -- upon opening anyway.

Since the voltage can't change across the capacitor (and if you buy a replacement in an auto parts store they are almost always labeled as condensers) instantaneously, the voltage across the points has to change continuously, starting from zero, as the points open. As the voltage builds in the capacitor, so does the voltage across the point gap. But the point gap is widening while, at the same time, the energy is being expended in the spark at the plug. If the capacitor is big enough, the voltage across the point gap never reaches the value needed to create a spark. If the capacitor is too big, it will rob too much of the energy and reduce the spark strength. As the points close, they are closing on a charged capacitor and that is when any spark happens and also when you get the greatest current through the points as it rapidly discharges that cap. The smaller the cap, the less energy has to be dumped, but the higher the voltage it will have charged to and, hence, the earlier in the closing motion the arc will strike. Its primarily this arc that pits the points in normal operation, as indicated by the much smaller pits compared to those seen when the condenser fails or is removed.

ITYMF the lack of pitting on points with faulty or removed capacitor is mostly due to the owner finding the engine runs so badly that its easier to walk!

 

...
As the points close, they are closing on a charged capacitor and that is when any spark happens ...

Did I mis-read that? The ignition spark happens when the points OPEN.

The points cap has little effect on spark intensity at all. It's primary function is to reduce points arcing when the points open, to increase points contact life and reduce RFI.

 

Did I mis-read that? The ignition spark happens when the points OPEN.

The points cap has little effect on spark intensity at all. It's primary function is to reduce points arcing when the points open, to increase points contact life and reduce RFI.

Yep, you misread it. I specifically stated many times that I was talking about the points gap.

And the points cap can have a very pronounced effect on spark intensity. If the cap is sized too big, then more of the coil energy gets dumped into the cap and NOT into the spark.