Hello everybody,

I would like to dump a capacitor into the primary of a step-up trigger transformer's primary (let's say an ignition coil) to obtain a spark. The voltage (around 300 volts) is obtained from a main circuit through a resistive divider and it is a given, I cannot change it. Now, on what basis should the capacitance of the trigger capacitor be selected? Does the capacitance affect the characteristics of the spark (power, length etc.)? How?
From what I learnt so far the capacitance plays no role, as it can be in the range of 0.01 uF to an unspecified value...

Many thanks.

 

The field developed in the secondary coil is proportional to the rate of change of the current (and thus field) in the primary. A capacitor helps sharpen the dI/dt curve delivered to the primary. For a single pulse, I don't think there's an upper limit on useful capacitance. If the pulses are frequent, I think the capacitor cannot be too large or it will hinder the downward side of the pulse.

 

For a given voltage the energy available from a capacitor is directly proportional to the capacitance. What the transformer does with that energy depends a lot on the design of the transformer and its load.

What more can you tell us about the transformer and the load?

 

Thank you wayneh and DickCappels.

I cannot say much about the transformer because it is an old, salvaged trigger coil. All I know is that the turns ratio is 100:1 and the input voltage is 12V. I plan to use the circuit to trigger a xenon flashlamp, which is bigger than the average, for photographic uses. Therefore I should keep the charging time of the capacitor as short as possible to have a good repetition time. I plan to use a small electrolytic 1uF, 450V capacitor.

One other question: should I mount the capacitor in parallel or in series with the transformer?

 

Isn't what your asking, the same thing a CD(capacitive discharge) ignition system does?

 

You should connect it like this:

http://www.repairfaq.org/sam/strbfaq.htm

The capacitor is across the lamp terminals and the trigger pulse is delivered to a trigger electrode. If the flashtube does not have a trigger electrode, you can make one my wrapping some wire around the tube of attaching a piece of conductive tape to the tube and using that as a trigger terminal.

 

You should connect it like this:

If C4 is the capacitor in question, then I retract my comments in#2. C4 is there to couple a limited signal pulse to the trigger. A brief "short" that quickly opens when the capacitor charges.