How to specify, find a transformer, given circuit schematic?

[Charlweed]

I'm an electronics neophyte, looking for a transformer, to generate a spot of plasma at the end of two leads. In particular, I am trying to build the circuit here:


This circuit is published in a couple of magazines. The last component I need to finish the circuit is the transformer that converts 12vDC to “a few kv” at kilohertz frequencies. The author calls the transformer a Tesla Inductor, and suggests obtaining one by pulling one out of a furnace ignition system. I have had no luck finding a used furnace ignition system that has a transformer.

I would buy a transformer new, but neither the schematic nor the articles detail the specifications of the transformer, and I do not know enough to infer them.
I have figured out that I need a low current, high voltage transformer. This may be known as an ignition, or spark, transformer. I think I need to know at least the impedance and operating frequency, as I have inferred the input voltage (12) and input current (10 ma) from the 555 timer. Some vendors seem to want me to specify windings, winding ratios, resistance etc.

Thanks for any help!

 

[crutschow]

Perhaps a standard car ignition coil would work. I would start with a higher value resistor for R7 (say 10 ohms) and work down from there until you have the spark you need.

 

[MikeMl]

Yeah, look for one out of a '66 Chevy or a '70 Ford...

 

[Charlweed]

Thanks guys, 'Ill try that. I may have a 1959 Triumph coil in the garage. But can you give any guidance on what characteristics of a car ignition coil make it suitable? In terms of transformer parameters? And what should I use as a power source? I was thinking D cells. Would a 12v car battery fry my circuit?

 

[tcmtech]

An automotive ignition coil wont run at a few kilohertz though. Maybe a few hundred at best.

If you are trying to get a good high frequency high voltage output a common flyback transformer from a old CRT type TV or computer monitor is what you would be wanting. Those can easily be ran at frequencies in the 20 - 50 KHZ range which will give you a nice smooth quiet arc.

For a 12 volt primary you may only need 3 - 5 turns of wire on the open half of the core which is a easy hand wind operation.