# Building a Transformer with high leakage reactance???

Discussion in 'General Electronics Chat' started by electronice123, Mar 4, 2009.

1. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
304
0
I have been researching transformers lately as I am trying to figure out how to calculate leakage reactance to restrict current to an absolute minimum.

I am trying to build a transformer that will power a negative resistance (neon sign type) device at resonance.

From what I have read these types of transformers are built to have a leakage reactance of 100% of the full load impedance.

Does anyone know where I can find directions or calculations on this type of transformer.

From what I know it is wound in a stacked pancake style to get high voltages, it is also known as a high series capacitance transformer and it has a high leakage reactance to limit current to a minimum?

I have searched and searched and can't seem to find much about this type of transformer. They are used in Neon signs, bug zappers, and I believe flourescent lighting.

Any Help???

2. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
294
This is actually quite simple....just use VERY loose coupling between primary and secondary windings. You could wind a secondary around the OUTSIDE of the core to achieve this.

eric

3. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
335
The way it is done is to use a magnetic shunt in the transformer. This is mentioned here:

http://en.wikipedia.org/wiki/Neon_sign_transformer

I didn't find a picture with a brief search, but to give you an idea of what that means, imagine a transformer core stack made of E-I laminations. Normally all the windings are placed on the center leg.

But, if you placed the primary winding on one of the outer legs, and the secondary on the other outer leg (with nothing on the center leg), you can see how the magnetic path between the two windings would be shunted by the center leg. If you were to do it this way, the center leg shunt would be too good. In a real neon transformer, there is an small air gap in the shunt path (the center leg in this hypothetical example) so that the shunt isn't too good!

4. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
304
0
This is an interesting type of transformer. As I know it's secondary also has inductors on both the positive and negative sides for additional current limiting. All the coils have the same inductance.

The inductors are on the inside, the primary is wrapped in the middle, and the secondary is wound on the outside.

The transformer uses an E or EI core.

What effect will the core permeability have here. Do I need to get one with a lower permeability for a loose coupling?

5. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,047
294
This might present a bit of a Catch 22. Less permeability WILL reduce coupling, but it also lowers the Q. I think there are more subtleties to those "official" neon transformers than they're letting on.

eric