CDI ignition - gross vs nett spark energy

[Circlotron]

... it's just more false claims and misunderstandings.

The oscillator runs in the KHz frequencies

First false claim or misunderstanding.
The MSD inverter charges the cap in one single pulse in a similar principle to an inductive ignition discharging into a spark gap.

where a V8 at 6000RPM fires single sparks at 200hz

Second false claim or misunderstanding. It is 400Hz.

[Keith Morganstein]

I had to look up the tung sol

http://worldphaco.com/uploads/THE_TUNG_ ... N_Inc..pdf

[Fred Winterburn]

The Tung Sol that I own is the same one as in the article. I bought it from the good doctor. This was an NOS positive ground unit that he repaired the coil on (probably defective from new) and converted to negative ground. I converted it back to positive ground just for originality and have it in my collection. Every time the capacitor discharges, a flash of blue light is emitted from the thyratron. Really cool museum piece but I would never install it on a car. I've only run it through its paces on my test rig. Nice piece of history, Fred

I had to look up the tung sol

http://worldphaco.com/uploads/THE_TUNG_ ... N_Inc..pdf

[user-23911]

The oscillator runs in the KHz frequencies

First false claim or misunderstanding.
The MSD inverter charges the cap in one single pulse in a similar principle to an inductive ignition discharging into a spark gap.

Yes it's 400Hz with an 8 cylinder and 200Hz with a 4 cylinder.


Next........waiting for someone to provide the mathematics to back up the ....."charges the cap in one half cycle"........because it can't.

Not unless the pulse transformer is the same size as an inductive ignition coil , which it isn't.
If it was........then you may as well just use inductive ignition because it's better.

[Fred Winterburn]

Joe,
The basic MSD circuit was posted on page 3 of this thread and from memory is more or less correct compared to the circuit I traced some years ago from my MSD6A. Notice a couple of things that are different than a typical inverter type power supply used in most battery powered CDIs. First, note that there are two power transistors in parallel configuration with the emitters and collectors tied together. The emitters are both connected to the same winding, not separate windings as in a typical push-pull type oscillator. Now I know that a single transistor or even two transistors in parallel can be made into an oscillator type supply pulsing a single winding, but why then would the two transistors be configured the way they are as that would require a larger transformer to handle the power which then reduces the frequency? If using two transistors why not take advantage of that to increase the frequency of the oscillation. The answer here is that the power supply is not an oscillator. The biggest piece of evidence other than testing and scope images, is that both paralleled transistors have their bases tied to the same side of the feedback winding, but more crucially, the feedback winding (which is really a feedback and control winding in this circuit) is in turn controlled by the trigger circuit (VTRIG on the schematic). The squeaking one hears from the old MSD6 spark directly mimics the power supply squeak which is not from a free running oscillator, but from the distinct sound that several short duration CD sparks make separated by 1mS. I have measured up to 5 sparks in the series at low rpm. It sounds like a squeak because the frequency is very low, IE spark rate X extra sparks at low to medium rpm.
And here's the clincher: Note that both transistors are permanently connected between the battery terminals. The emitters through the main winding of the transformer, and the collectors directly to ground. In a free running oscillator of the type more commonly used in CDIs and many other power supplies, that configuration would result in a power supply that never turned off. This one does, because the bases of the transistors are controlled directly by the trigger circuit.There is no other switch in the power supply to turn it off. Fred

The oscillator runs in the KHz frequencies

First false claim or misunderstanding.
The MSD inverter charges the cap in one single pulse in a similar principle to an inductive ignition discharging into a spark gap.

Yes it's 400Hz with an 8 cylinder and 200Hz with a 4 cylinder.


Next........waiting for someone to provide the mathematics to back up the ....."charges the cap in one half cycle"........because it can't.

Not unless the pulse transformer is the same size as an inductive ignition coil , which it isn't.
If it was........then you may as well just use inductive ignition because it's better.

[Circlotron]

Also the MSD6A inverter transformer has a laminated iron core, not a ferrite one like you might expect if it was running in the tens of kilohertz range. Iron gives higher inductance per turn of wire but starts to fizz out at high frequencies. You could still use some grades of ferrite at several kilohertz but would need a lot more windings to get enough inductance.

[user-23911]

2 transistors are used to double the power handling ability and heat dissipation. The same principle was used in some colour TVs some 40 years ago.
It's a free running oscillator but it shuts down then restarts when the SCR fires.


Easy to see if you put a scope on it.

[Circlotron]

Not unless the pulse transformer is the same size as an inductive ignition coil , which it isn't.

Well, I just went and measure both of them.
The cross sectional area of the outer legs was easier to measure, and one would expect that they are each exactly half the area of the centre leg of the core. That being the case, an E-core coil centre was 18x18mm = 324 sq mm. The MSD inverter transformer centre was 19.5x19mm = 370 sq mm. So it is in actual fact bigger. And don’t you start telling me rulers can’t be trusted.