So I need a transformer for a small spark. The only one i have lying around is a hefty 5lb one about the size of a baseball that I think came out of a UPS unit. I understand the basics of a transformer but while reading up I got to thinking about the differences between a stepUP and a stepDOWN transformer. So I have a couple of questions:

1. if i take any transformer and I connect the power source on 1 side for steup and on the opposide for step down?

2. In any transformer that has a 2:1 winding, will 9V produce 18V and then 18 produce 9V the other way around?

 

In principle, yes to both questions: but transformer windings are designed for certain current and voltage maxima which have to be taken into account. For example, the insulation on a low voltage secondary winding may not be adequate to cope with mains voltage.
Stepping up voltage also steps down available current, since the transformer can handle only a certain power. Vin x Iin = Vout x Iout.

 

In principle, yes to both questions: but transformer windings are designed for certain current and voltage maxima which have to be taken into account. For example, the insulation on a low voltage secondary winding may not be adequate to cope with mains voltage.
Stepping up voltage also steps down available current, since the transformer can handle only a certain power. Vin x Iin = Vout x Iout.

Ok so if I take that transformer and connect a 3.7V or 9V battery I might get 7.4 or 18 V but the current would be super low right?

 

Yes if you pulse the Secondary side with 9v, the Primary will give out a larger voltage depending on the turns ratio.

 

Ok so if I take that transformer and connect a 3.7V or 9V battery I might get 7.4 or 18 V

No, you wouldn't. Transformers require AC energisation.

 

Right because you need the changing Magnetic field. I guess this is why this guy adds a switch to the battery:

http://www.instructables.com/id/Make-an-Ultra-Simple-High-Voltage-Generator/

 

So I need a transformer for a small spark.

While I do not know your location the simple way to go about this would be to find a junk yard and scavenge a automotive ignition coil. When we apply a voltage, say 12 VDC nothing will happen, the field lines of flux will extend outward from the transformer. When we remove power the magnetic field will rapidly collapse and the secondary will give you your wanted spark. Before automotive systems went to electronic ignition systems a set of points in the old distributor would open and close driven by a cam, all mechanical, and each time the points opened a spark plug would fire.

These coils could also be driven with a typical lamp dimmer. Applying a sine wave to these coils won't work as you want the field to collapse rapidly. This is why the points worked so well on the old automotive ignition coils, points would Make or Break as in right now.

Ron