"Electronic transformer" for an halogen lamp - Page 1
Forum section: electronics / projects

author: nuno
author_rank: Frequent Contributor
timestamp: June 07, 2016, 11:51:44 pm
content: So, was walking through a local "diy big store" and noticed these cheap (6.80€) "electronic transformers" (as they call it) for halogen lamps. It's basically an unregulated (output comes directly from the core transformer-coil) isolated DC-DC converter, mains to ~12V. Given the impressive specs for what it looks like physically, I thought I'd buy a pair to investigate.(has SMD components on the PCB's copper side)The output specs say "20 to 60W" so it should reach 4+A at the output (it even be seen "4.9A max" in the box). So let's make some tests...The output at low load (~50mA) is very noisy.Then I put a rectifier diode and a big capacitor and connected my dummy load and the conclusion is that I can only load it continuously to around 100mA. Higher than that and it quickly starts heating a lot (60ºC on the plastic case) and shuts itself off (positive points for that, hehehe).I was expecting it to not meet the rated specs, but really, not by that much! Unless I'm missing something on how these are used.What are your thoughts?

author: tautech
author_rank: Super Contributor
timestamp: June 08, 2016, 12:08:21 am
content: Look at the screenshot baseline.....you're using 10V/div and you're getting at least that either side of it.The spec listed at the output terminal makes no mention that's a DC supply.A halogen lamp won't mind this at all.

author: nuno
author_rank: Frequent Contributor
timestamp: June 08, 2016, 12:13:52 am
content: Yes, I'm aware of that. But if its output is rated 20 - 60W, if I rectify the output I should be able to get at least, say 1A out of it continuously instead of 0.1A, right? I'm doing single wave rectification, but I would not expect the output power to much more than double by doing full-wave rectification.

author: tautech
author_rank: Super Contributor
timestamp: June 08, 2016, 12:33:43 am
content: Quote from: nuno on June 08, 2016, 12:13:52 amUnits I've attempted to use to power LED downlights but were designed for Halogens didn't like low loadings, so much so that I had to replace them with LED compatible units. If you can load them close to their design parameters they might be OK, you'll need a 1.5A load or more.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 08, 2016, 12:47:18 am
content: No, it's probably a Royer converter, without a resistive load it will generate bad drive waveforms for the transistors and they'll toast.The "noisy" waveform is perfectly normal, you'll note the complete absence of bulk input capacitors, the thing just starts switching at 100Hz, twice your 50Hz mains.That is to say, every time the rectified mains gets to a certain level it starts switching at some KHz then it stops, 100 times per second.The integration is in the lightbulb, you won't notice the flicker.I may be entirely wrong.

author: nuno
author_rank: Frequent Contributor
timestamp: June 08, 2016, 01:14:13 am
content: I have now implemented a full-wave rectifier and I can get ~0.25A (@~15V) but at a case temperature of 90ºC.If I pull 2A it takes 10s to self turn off.It does a quick tick-tick-tick sound under load.Ok, "Royer converter", something to google, thanks.(what initially crossed my mind was using one of these to make a simple power supply)

author: Siwastaja
author_rank: Super Contributor
timestamp: June 08, 2016, 05:53:15 am
content: Those are unusable for making a DC power supply. They are specialized in drivinghigh frequency ACintoresistive loads only, with rather high minimum load requirement. For their task, they do exactly what needed with very high efficiency and considerably smaller BOM than a DC supply.

author: nuno
author_rank: Frequent Contributor
timestamp: June 08, 2016, 10:05:33 am
content: The 2 more interesting resources I found:http://sound.westhost.com/lamps/elect-trans.htmlhttp://www.st.com/content/ccc/resource/technical/document/application_note/e5/88/46/11/35/8c/4a/bf/CD00003902.pdf/files/CD00003902.pdf/jcr:content/translations/en.CD00003902.pdfFrom all I read only the 1st comments on the minimum load:Quote

author: oldway
author_rank: Super Contributor
timestamp: June 08, 2016, 10:12:19 am
content: What kind of diodes did you used as rectifier ? You must choose fast / ultrafast diodes.

author: Delta
author_rank: Super Contributor
timestamp: June 08, 2016, 11:45:08 am
content: Yep, these are finicky beasts. I learnt the embarrassing way that with only one lamp attached, their output is very low, 3.4VACish.

author: SeanB
author_rank: Super Contributor
timestamp: June 08, 2016, 05:51:53 pm
content: They are also extremely good RF noise generators. If you use them with a longish lead to the lamps they will blank out all radio within a good radius, including TV reception. Best use for them is as landfill, i prefer using the old iron core transformer, as these are perfect to drive LED replacement lamps and last decades with almost no issues.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 08, 2016, 06:05:26 pm
content: Proper ones do exist, and they are meant to be used directly at the lamp. Wiring at the 12V side doesn't make any sense because of resistive losses, either.This is also why they often exist at mere 20W rating: they are meant to drive a single bulb!Proper converters have almost infinite lifetime, and efficiency near 98%, which, coupled with doubled efficiency of the 12V bulb compared to a 230V bulb, made a lot of sense before LEDs, and still does when color temperatures near 3000K with CRI=100% (perfect) are wanted.50Hz iron transformer is so heavy that it cannot be located near the lamps in most cases, further increasing losses, but the electronic transformer can be integrated within the lamp fixture, or located very nearby.These should be considered as lamp driver components that need to be integrated in a complete lighting fixture by someone who knows what they are doing, and they are pretty much unusable for anything else. This is also why they are practically never supplied with a plug, but require an electrician for the installation.