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50W electronic halogen transformer

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nukie:
During lunch, I went to my local electrical distributor store today to see if they have any new+silly+expensive LED lighting, and I stumble upon a kit set of 12VAC 50W electronic transformer halogen set for approx AU$8. It can't be real... the kit consist of an 240v>12v electronic transformer that complies with electrical safety AS/NZ standards, a white two piece diecast aluminium fixture and a 50W halogen bulb. Well it was no brainer, the power supply was very small I wanted to see what's in it so I bought it and waited impatiently to knock off and get home.

I just renovated my 20 yr old home, and I have standard coil transformer for my halogen downlights. I want to see how these cheap power supply works and to my surprise, quite well. I wired it up and hooked up a kill-a-watt, with the standard 50W bulb, the kill-a-watt reports 53W. On the true rms Fluke is telling me output 12.0xxVAC which is very nice voltage regulation. Then I tried a bi-pin 20W bulb, the kill-a-watt says 23W.

Two high power TO-220 NPN for switching and a TO-92 NPN all I can see, and some caps and resistors, 7 diodes, lots of magnetics. 12V output is fully isolated via a toroid winding. I'll post some pictures of the internal tomorrow.

sacherjj:
Wow, I'd like to see that.  I'm trying to figure out if it is feasible to use LED lighting for the new closed in porch that I'm finishing.  It would be cool to be able to vary the voltage in the switching circuitry for using a dimmer without losing tons of efficiency.

mikeselectricstuff:

--- Quote ---the kit consist of an 240v>12v electronic transformer that complies with electrical safety AS/NZ standards,
--- End quote ---
.. if you believe the stickers... ;)

Zero999:
They're soo efficient because there so simple.

EMC filter -> rectifier -> oscillator - > ferrite transformer

The power factor will be very good because the current waveform is fairly sinusoidal due to the absence of a massive reservoir capacitor after the rectifier.

I doubt the regulation will be that great: the output voltage will vary in proportion to the primary voltage.

The output waveform will be a 20kHz to 200kHz AC square wave modulated with the mains frequency after full wave rectification. Have a look at the waveform with an oscilloscope and try varying the input voltage with a variac.

PetrosA:
You don't want regulated output in these transformers or else they wouldn't work with dimmer switches...

I just installed six 50W MR16 fixtures each with its own power supply (in the wall). They work very nicely:

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