Fig.1. Original unmodified circuitry that triggers the oscillator in the electronic transformer. Only the triggering components involved are shown for clarity.

Background

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 benches and in the ceiling. Additionally, I wanted to dim individually all three circuits so that the light levels could be nicely set. But why halogen and not LEDs? Because LED's are usually not a good solution for (Finnish!) sauna's, or when used in any hot and/or humid locations.

Low-cost electronic transformers with an integrated dimming function are hard to find. However, electronic transformers can be dimmed, but the majority of them require external expensive phase cutting (MOSFET) dimmers. After adding up the costs I thought that there must be a clever (and cheaper) way of doing this. What if you could completely avoid a separate dimmer?

It's a little known secret that 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 similar electronic transformer modifications already in the mid 90'ties when installing a bunch of halogen ceiling lights. Worked like a charm!

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 another site that explains somewhat precisely the electronic transformer operation, but in a nutshell:

The incoming mains voltage is full-wave rectified. A high-frequency self-oscillating push-pull high-frequency power oscillator pushes this AC-voltage via an isolating and downconverting transformer to the halogen bulbs. See Fig.2 for the output waveform.

The oscillator starts at about 1ms after mains zero crossings when the voltage across C2 has reached about +30VDC (charged by R2) when the Diac DB1 fires and gives a starting pulse to the lower transistor Q2's base, see Fig.1. After the oscillator has started D5 keeps C2 discharged to keep this trigger from disturbing the oscillator for the rest of the cycle.

The oscillator stops when the mains voltage approaches zero due to insufficient operating current. This process repeats once for every mains voltage cycle.

If the starting point is delayed and made adjustable,  then it's possible to reduce the effective output power, thus dimming the load. Fortunately, halogen bulbs do not mind this kind of chopping at all.

Simply by adding a linear potentiometer in series with R2 does the trick, see Fig2. This circuit is now almost identical to the circuitry used in phase angle Triac dimmers!

The modification

Reverse engineer the unit you inted to use to find the components.

Find the values for R2 and C2. Select the pot accordingly. C2 and be slightly modified.

Make sure there is room for the potentiometer to be assembled in the casing (do not use long external wires)

Cut the end of the resirso

Assemble the pot and lflexible wires.

Fig.2. Modified circuitry for triggering the oscillator. Only by cutting one wire and adding a potentiometer, you can dim the electronic transformer directly.

Fig.3 The electronic transformer after the simple modification, and before reassebly.

Fig.4 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.5 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. Thus power is delivered to the load bulb for a much shorter period and the light is dimmed.

Fig3 For electrical safety use onlypotentiometers with a 6mm plastic axle (cut here to length). This potentiometer has also a plastic thread for maximum isolation safety. Some metal threaded potentiometers do not have enough internal creepage distances for safe mains voltage use. Be sure to use a safe type.