Yes, this is an autotransformer and does a true voltage reduction.
A solid state SCR dimmer does not really lower the voltage. It is a simple RC delay circuit that delays the SCR turn on somewhere into the sinewave cycle. This lowers the effective energy available to the lamp filament but doesn't lower the voltage per se. Of course turning the SCR on during the falling curve of the sine wave does in fact present an effectivily lower voltage.
The efficiency here is that the SCR is a switch and is either full on or off so no energy is wasted as would be in a resistive dimmer. There is a 0.7v drop across an SCR and that's why the need heatsinking. Do the math - 0.7v drop at several amps is still a bit of heat generated.
Autotransformers are somewhat less efficient buy very electrically quite. Solid state dimmers throw out both RFI and EMI. In recording studios, TV control rooms, etc you will often find Variacs on the lights. Solid state dimmers are still verboten wherever high quality audio is recorded.
*Note for purposes of this discussion an SCR and TRIAC are the same thing.
[This message has been edited by Gus99 (edited 05-08-2006).]
