Modifying an electronic transformer voids its warranty. Any attempt to perform this modification without the required skills is strongly unadvisable. You do this at your own risk. The device contains a lethal mains voltage. Proper safety precautions must be followed.
I recently needed electronic transformers for my renewed sauna lights, which consists of three 12V/20W halogen bulbs driving bundles of nicely distributed optical fibers both under the seats and in the ceiling. Additionally, I wanted to dim individually all three circuits so that the light levels can be nicely set. But why halogen and not LEDs? Because LED's are usually not a good solution for (Finnish!) sauna use, or in any hot and/or humid locations.
I also needed three separate dimmers. Electronic transformers with a dimming function are impossible to find. However, electronic transformers can be dimmed, but they require expensive phase cutting (MOSFET) dimmers. After adding up the costs I thought that there must be a clever (and cheaper) way of doing this.
It's a little known secret that those electronic transformers that are of the self-oscillating type (as most are), actually contain all components and the required circuitry for a very easy dimming addition/modification. I performed the same electronic transformed modification already in the mid 90'ties when installing a bunch of halogen ceiling lights.
Rod Elliott has a very nice and comprehensive page about the innards of electronic transformers, so I don' have to dig here into the details. Here is the link: https://sound-au.com/lamps/elect-trans.html
The incoming mains voltage is full-wave rectified and there is a high-frequency power self-oscillating push-pull high-frequency oscillator that pushes this AC-voltage via an isolating and downconverting transformer to the halogen bulbs. See Fig.2
The oscillator is started at about 1ms after each mains zero crossing, by an RC-circuit
Fig.1 The electronic transformer opened and after the modification.
Fig.2 The output voltage waveform in the original transformer. Note the high-frequency switching and the zero-activity area around each mains zero-crossing that occurs at the vertical centerline. The no-power area is compensated for by a slightly higher output voltage so that the effective output voltage is close to 12VAC.
Fig.2 The output waveform in a modified transformer when the output is dimmed to a very low value. The output voltage starting point is delayed, the stopping point is the same.
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