Silicon Chip Online

A balun IS a transformer

Thank you for including my recent letter in the October 2008 edition of SILICON CHIP magazine. However, the more I think about it, the more I’m sure that your definition of a balun is technically incorrect. As SILICON CHIP shows the diagram of a balun (page 40, August 2008), it clearly represents a simple coupling transformer with a primary and a secondary, as per conventional transformer definitions, with the core used for coupling energy.

In the "choke" balun, as used in typical RF antenna applications, the choke balun windings are closely coupled as a tight bifilar winding and the RF energy is transferred essentially in "transmission line mode" between the windings and not as in a conventional transformer through flux linkages. The core plays a significant role but not the critical one. It serves mainly to improve the low-frequency response of the balun. This analysis is the definition as explained in the book ‘Transmission Line Transformers’, Third Edition by Jerry Sevick W2FMI, a world authority on baluns for RF applications.

Felix Scerri, VK4FUQ,
Ingham, Qld.

Comment: by definition a "balun" is a "balanced to unbalanced" transformer. It serves to couple signals from a balanced circuit (eg, a dipole antenna) to an unbalanced circuit (eg, a receiver input stage with one side referenced to earth). You refer to "windings that are closely coupled". That describes a transformer.

A transformer does not need a core to be a transformer. Air-cored transformers are common in RF applications, apart from those referred to as baluns.

None of the material you quote from the textbook "Transmission Line Transformers" contradicts this. Arguably, it merely obscures the point. Any basic transformer consists of two or more windings coupled together. Flux is created by the primary winding and is coupled to the secondary, whether there is a core or not.

Furthermore, even where a balun is made from lengths of coax cable (transmission lines) and has no obvious windings, it can still be shown to be a transformer, with flux being coupled from one circuit to another.

Do not wire to the Active pin

I agree that it’s a very bad idea for D. J. to use a 240V power board for 12V. But the obvious simple change to make it a lot safer would surely be to rewire it from +12V on the Active pin and common on the Neutral pin to +12V on the Neutral pin and common on the Earth pin?

Gordon Drennan,
Burton, SA.

Comment: ostensibly that is a safer approach but if a plug wired in this way is connected to a power point which has Active & Neutral transposed (not an uncommon situation), you still have a potentially dangerous situation – both for the equipment and the user. There is really no way around it – it is just a very bad idea.