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
Felix Scerri, VK4FUQ,
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?
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.