Hi all,
I purchased an "electronic transformer" today - the sort intended for use with domestic 12 volt lighting. Despite the very modest price, the unit is labelled " Made in Australia " ! The "12 volt" output from these devices is in fact a 100% amplitude modulated 50 kHz wave - modulation is at 100/120 Hz.
Ratings marked on my unit include 11.5 volts, 60VA & pf = 0.95 - plus it carries the double insulation symbol.
It needed to replace a similar, failed unit inside a lighting effect device as used in discos etc. The effect employs a 12 volt, 100 watt dichroic lamp plus an 80mm, 12 volt fan blowing across PSU and lamp ( the plastic case is removed from the PCB for my app).
Initially, I imagined that the 60 VA rating would be inadequate for the 100 watt lamp load, but on test it proved to work fine with very little heating of any of the parts - even with no fan cooling. Efficiency must be 95% or better.
The circuit design is astonishingly simple, basically just 2 mosfets, a few power diodes & low value plastic caps and a couple of small, ferrite transformers.
The puzzle is how exactly the maker arrives at the VA rating.
Why is it considered "60VA" when nothing on the PCB rises more than 10 degrees C with a 100VA load in free air ?
I can only guess it relates to voltage regulation with varying load as actual output voltage follows the AC input.
........ Phil