How do Capacitor Discharge Ignition Circuits work?

[Russell_B]

I have been informed that such devices can produce large voltage sparks from low voltage power supplies, but i was always under the impression that capacitors discharge in a manner that is simply the opposite of the charge graph.

1)So how do these circuits work?
and
2)Are the US navy using similar circuits in their new railgun battleship weapons?

 

[Bob S]

As I write this, I am looking at an old working Mark Ten B CD ignition circuit, a kit made by Delta Products, Inc. It has a Dual NPN-driven push-pull oscillator driving the primary of an audio frequency toroidal transformer. The high voltage secondary is rectified and charges a capacitor. A series avalanch transistor, triggered by the points inside the distributor, discharges the capacitor into the primary of the ignition coil. See picture of the Mark Ten B on lower-left corner of page 155 of Popular Science Dec 1974

http://books.google.com/books?id=nd...resnum=8&ved=0CB8Q6AEwBw#v=onepage&q=&f=false

I have a schematic someplace.

Bob S

 

[vk6kro]

I think they had a circuit like this:

http://dl.dropbox.com/u/4222062/cdi.PNG [Broken]

When the SCR was not conducting, the capacitor charged up via the small resistor R and the primary of the ignition coil. The capacitors were of the order of 4 uF so quality oil filled capacitors could be used.

A trigger pulse on the gate of the SCR caused the capacitor to rapidly discharge through the SCR and the primary of the ignition coil.

R was there to avoid short circuiting the 300 volt power source when the SCR turned on.

One problem was that the output tended to fire adjacent spark plugs as well as the intended one due to coupling between the ignition leads and the very high voltages that were generated.

Is that how you remember them Bob?

 

[Bob S]

I think they had a circuit like this:

http://dl.dropbox.com/u/4222062/cdi.PNG [Broken]

When the SCR was not conducting, the capacitor charged up via the small resistor R and the primary of the ignition coil. The capacitors were of the order of 4 uF so quality oil filled capacitors could be used.

A trigger pulse on the gate of the SCR caused the capacitor to rapidly discharge through the SCR and the primary of the ignition coil.

R was there to avoid short circuiting the 300 volt power source when the SCR turned on.

One problem was that the output tended to fire adjacent spark plugs as well as the intended one due to coupling between the ignition leads and the very high voltages that were generated.

Is that how you remember them Bob?

I looked at my schematic, and it shows a common-emitter npn transistor firing a SCR, as you show. The output of the coil went to the center of the distributor cap (above the rotor). So for a V-8 running at 6000 RPM, the CD ignition would fire at about [STRIKE]600[/STRIKE] 400 Hz.

Bob S

 

[sophiecentaur]

I have been informed that such devices can produce large voltage sparks from low voltage power supplies, but i was always under the impression that capacitors discharge in a manner that is simply the opposite of the charge graph.

In answer to this bit of your question:
The rates of charge and discharge depend on the value of the series resistance. In this case, it is charged through a higher resistance than exists in the discharge path - hence it discharges in small a fraction of the engine cycle and charges up whilst the pistons are moving round into position. Not unlike a toilet cistern!
The rate of discharge is actually higher than you'd expect because there is a resonance between the C and the inductance of the coil.

Oh yes - the 400V, to charge it, is produced from the 12 V with an oscillator, a transformer and a rectifier.

 

[kva]

How do you discharge high voltage capacitor 6KV with a high voltage transformer ? how do you calculate the transformer needed ?

 

[davenn]

I thought the whole idea ( well major portion) of a CDI system was to do away with the points, thereby eliminating one of the main areas of wear and tear in the ignition system ?

Dave

 

[jim hardy]

old Evinrudes charge the capacitor to ~400 volts by a many-turn charging coil under the flywheel that is excited by a permanent magnet mounted in rim of the flywheel.
Usually there's one coil set per cylinder.

Cap is discharged through spark producing coil by SCR as shown, SCR is triggered by another magnet and smaller trigger coil. That magnet is on a moving plate that moves with throttle, for spark advance.

A nice system - if you're way out in the ocean and battery dies you can start it with a rope.

 

[Mike_In_Plano]

Ages ago, I performed the failure analysis / improvements for a furnace ignition that used SCR discharge. It would rectify the household AC voltage into a film capacitor and trigger an SCR to discharge the cap into about 8-10 turns of about AWG14 wire. This was wrapped about the high voltage winding, which would provide the ignition spark.

The discharge event put several tens of amps through a 1 amp SCR, which actually works pretty good.

Just remember, your SCR needs to fast - even if that means it's not so big.

 

[jim hardy]

Discharging into an inductance holds down "rate of current rise" , one of the SCR limits that must be observed.

The capacitor limits the current through the SCR.

It's a pretty nice system. A good designer values Mother Nature's help.