Halogen lamp electronic transformer modification

How to Add a Dimming Function to an Electronic Transformer

(for Halogen Lamps)


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.

What is needed

  • Any suitable 115/230VAC to 12VAC electronic transformer intended for low voltage halogen lamps
  • A plastic shaft 6mm potentiometer with a plastic knob (see text)
  • A small capacitor of 1nF/50V (see text)
  • Reasonable mechanical and electronics skills


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.

Operating principle

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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